Index of content:
Volume 35, Issue 6, June 2008
- Therapy Scientific Session: Auditorium B
- Brachytherapy I
MO‐D‐AUD B‐01: An Analytical Approach to Account for Shielding, Anatomical Heterogeneities and Patient Dimensions for 192Ir High Dose Rate Brachytherapy Applications35(2008); http://dx.doi.org/10.1118/1.2962342View Description Hide Description
Purpose: Based on a separate primary and scatterdose calculation technique and pre‐computed Monte Carlo (MC) data, we have developed an analytical approach to account for shielding, anatomical heterogeneities and patient dimensions for high dose rate (HDR) brachytherapy.Method and Materials: Using the PTRAN_CT MC code, primary and scatterdose kernels of an HDR source in water were generated. Separate 3D kernels for rectal treatment with a tungsten‐shielded applicator were also created. Photon attenuation and scatter in tissue heterogeneities were corrected for via ray tracing. To quantify the reduced backscatter close to the skin,MC simulations were performed with an isotropic point source placed at various distances from the center of a 30 cm diameter water sphere. Scatter correction factors were derived, which vary as a function of distances between (1) the source and the surface, (2) the point of interest and the source, and (3) the point of interest and the surface. We applied this analytical method for three clinical cases and compared the results with PTRAN_CT calculations. Results: Our technique accurately accounted for the effects of tungsten shielding and anatomical heterogeneities for a rectal patient plan. The reduced backscatter close to the skin was also calculated correctly for a base of tongue and a breast case. Around bony structures several centimeters away from the active dwell positions, there was a minor discrepancy due to softening of the spectrum. Differences in the lung due to reduced scattering in this low density region were also observed. Conclusion: Making use of pre‐computed 3D scatterdose data, our analytical technique is capable of calculating dose around metal shielding and the patient skin with high accuracy. Its validity and limitations have been studied for HDR applications. Conflict of Interest: Research sponsored by Nucletron BV.
MO‐D‐AUD B‐02: Dose Rate Constants Determined by a Photon Spectrometry Technique for 20 Different Models of Low‐Energy Brachytherapy Sources35(2008); http://dx.doi.org/10.1118/1.2962343View Description Hide Description
Purpose: To perform a systematic and independent determination of the dose rate constants (Λ) of available low‐energy interstitial brachytherapy sources using a recently developed photon spectrometry technique (PST). Method and Materials: A total of 60 low‐energy interstitial brachytherapy sources (20 different models with 3 sources per model) containing either (14 models), (5 models), or (one model) were included in this study. A recently developed photon spectrometry technique (Med. Phys. 34, 1412–1430, 2007) was used to determine the PSTΛ for each source. Source‐dependent variations in PSTΛ were analyzed systematically against the spectralcharacteristics of the emitted photons and the AAPM consensus values (CONΛ) when available. Results: The PSTΛ determined for the , , and sources had values of 0.661 to 0.678, 0.959 to 1.024, and 1.066 cGyh−1U−1, respectively. The variation in PSTΛ among the 5 source models was less than 3%; due mainly to the variations in spatial distribution of radioactivity. The variation in PSTΛ among the 14 source models was larger and the maximum difference was over 6%. These variations were caused primarily by the presence of silver in some source models and, to a lesser degree, by the variations in activity distribution. When silver was present, the PSTΛ exhibited strong dependence on the silver content with values varying from 0.959 to 1.019 cGyh−1U−1. When silver was absent, the PSTΛ was less variable and had values within 1% of 1.024 cGyh−1U−1. The PSTΛ was found within 2% (14 models) and 2.6% (one model) of CONΛ for 15 models current have such a value. Conclusion: Excellent agreement between PSTΛ and CONΛ was observed for all source models that currently have an AAPM consensus value. These results demonstrate that the PST is an accurate and robust technique for the determination of Λ for low‐energy brachytherapy sources.
MO‐D‐AUD B‐03: An Enabling Technology for Creating Sculpted Brachytherapy Dose Patterns with the Xoft Axxent™ System35(2008); http://dx.doi.org/10.1118/1.2962344View Description Hide Description
Purpose: Study a potential means of partially attenuating X‐rays from the Xoft Axxent™ system over controlled spatial areas, while minimizing changes to depth‐dose characteristics. This would be the basis of an enabling technology to sculpt brachytherapydose patterns to, for example, spare critical structures such as skin in breast brachytherapy treatments. Method and Materials: Measurements of output were made from the Xoft 50 kVp source attenuated by thin (0.001″) dot‐shaped layers of silver, with diameters of several mm. Measurements were made with azimuthal scans around the source at distances from 2 to 7 cm. The dots were attached to the source cooling catheter, approximately 2.6 mm from the source center. Attenuation calculations and Monte Carlo studies of the effect were performed using EGSnrc. Results: Partially attenuating dots of silver create shadows in measured dose that agree with attenuation calculations in proportion, and in the critical behavior of how the attenuation varies with distance. Monte Carlo studies were consistent with measured results. Materials other than silver (or elements nearby in atomic number) will harden the beam, and so the attenuation will lessen with distance. Silver has much less of a hardening effect, because the K absorption edge reduces the higher energy portion of the spectrum at a rate similar to the losses in the lower energy region. Conclusion: It is possible to create predictable, directed shadows in dose around the Xoft 50 kVp x‐ray source using practical thicknesses of silver foils. The shadows have soft edges owing to penumbral effects when the dots are placed on the cooling catheter, within a few mm of the source. Such shadows could be used in future applications to spare healthy tissue during brachytherapy. In a simulated breast treatment plan, using a simple model of the attenuation, isodose lines were shifted by several mm.
MO‐D‐AUD B‐04: Parameter Optimization for Brachytherapy Robotic Needle Insertion and Seed Deposition35(2008); http://dx.doi.org/10.1118/1.2962345View Description Hide Description
Purpose: To investigate influence of different needle insertion and seed deposition techniques for roboticbrachytherapy. To find optimal sets of low, normal and high translational and rotational velocities of the needle for decreasing insertion force, needle deflection and OR time, and increasing seed placement accuracy. Method and Materials: We have developed EUCLIDIAN — a fully automatic robotic prostate brachytherapy system. Robotic system parameters were optimized via preclinical experiments using two types of polyvinylchloride and tissue phantoms, cannula and stylet single‐axis force sensors, and six‐axis force‐torque sensor. Cannula sensor measures the force on the cannula during insertion, withdraw, and axial force exerted by tissue at rest. Stylet sensor measures the force while seed is expelled from the cartridge, during seed travel through the cannula, and at the moment when seed is deposited into tissue. Position of the needle tip and consequently deposition depth into the phantom was measured using optical encoders on the cannula and stylet motors. Cannula and stylet translational velocity range was 5–120 mm/s, and cannula rotation range was 0–30 rev/s. Force patterns were analyzed based on the experimental data. Results: According to the criteria for minimizing insertion force and OR time while maximizing seed deposition precision, it was found that best performances were achieved when cannula and stylet normal speed was 70 ± 10 mm/s and optimal high speed was 100 ± 10 mm/s. Optimal cannula rotation speed range was 15–25 rev/s. In order to avoid seed jam in the cartridge, optimal speed for pushing seed out of the cartridge was 2–5 mm/s. Conclusion: Optimal parameters were programmed in the EUCLIDIAN configuration files. Seed deposition techniques have significant influence on reduction of insertion force, needle deflection and seed deposition accuracy. Future investigation will be on adaptive parameter tuning for specific clinical encounters.
Acknowledgement: Supported by NCI‐R01‐CA091763.
35(2008); http://dx.doi.org/10.1118/1.2962346View Description Hide Description
Purpose: To develop and verify a method for determining source dwell position for implementing a MammoSite® treatment procedure with a Nucletron remote after‐loader. Method and Materials: When delivering partial breast brachytherapy using an implanted balloon device, a 2 mm error in source dwell position produces a 15% error in dose delivered to the prescription point. Therefore, the procedure for determining a Reference Length entered into the after‐loading device produced by one vendor (Nucletron Corporation, Veenendaall, The Netherlands) for the dwell position at the geometric center position of a MammoSite® balloon provided by another vendor (Hologic, Marlborough, MA) must be determined and verified.. Devices with scales and indicators, connecter between the two systems, dummy wires, transfer tube and software provided by both vendors need to be used together to plan and deliver the treatment. A planning and delivery procedure was developed and tested by means of: 1). phantom measurements using images of a dummy source to compare with film dosimetry of the dose pattern produced by the active source. These measurements established and verified the procedure, and 2). a patient planning and treatment procedure with appropriate imaging QA steps. Results: The interpretation and accurate use of the scales and indicators and dummy wires was established. A 2mm offset was confirmed for determining the source Reference Length. It was found to be important to verify the Reference Length value using fluoroscopic and radiographicimages acquired with the patient at a conventional simulator. For approximate 10% of the tested cases, adjustments on the order of 1mm were needed based on the simulation procedure. Conclusion: The method of determining the Reference Length for Mammosite® treatment planning should be established with images and a phantom. The Reference Length measured for each patient should be verified with an imaging procedure using a conventional simulator.
MO‐D‐AUD B‐06: Source Motion in Permanent Implant Prostate Brachytherapy Due to Ultrasound Probe Deformation35(2008); http://dx.doi.org/10.1118/1.2962347View Description Hide Description
Purpose: To determine the amount of seed motion in the prostate due to ultrasound probe deformation during permanent implant prostate brachytherapy.Method and Materials: A C‐arm was used to take variable angle images of clinical implants immediately after the last needle was delivered with the patient and ultrasound remaining in the treatment position, after the ultrasound probe was lowered, and after it had been removed with the patient remaining in the treatment position. Three dimensional seed coordinates were calculated and corresponding seed coordinates were compared to determine the motion induced by the ultrasound probe. A rigid body registration was performed and deformational effects were evaluated using the residual seed motion. Results: Seed positions over all patients moved, on average, 6.6 mm posterior, 1.6 mm caudal, 1.5 mm patient right and the mean total motion was 7.1 mm (range 2.1 mm – 12.3 mm). The mean for a single patient ranged from 5.3 mm (2.4 mm – 8.3 mm) to 9.7 mm (8.1 mm – 12.3 mm). The rigid body registrations showed rotation about an axis perpendicular to a sagittal plane in each patient (mean 4.2°, range 2.9° – 5.9°). The mean residual seed motion was 1.1 mm (0.2 mm – 4.4 mm) and showed non‐random deformational patterns. Conclusion: Final seed positions are significantly different from those delivered due to the ultrasound probe. Non‐random residual motion within the implant can be associated with deformation and may have dosimetric consequences.
MO‐D‐AUD B‐07: Analysis of Rectal Dose Variability Due to Inter‐Fractional Variations of Rectal Marker Positioning in Film‐Based HDR Cervical Brachytherapy35(2008); http://dx.doi.org/10.1118/1.2962348View Description Hide Description
Purpose: In film‐based intracavitary brachytherapy for cervical cancer, rectal dose is usually computed using rectal markers. Position of the markers may not accurately represent the anterior rectal wall. The study is to retrospectively analyze the variability of rectal dose due to variations of marker placement in a multi‐fractionated HDR treatment regimen. Method and Materials: A cohort of five patients, total 18 applications, treated with multiple‐fraction tandem/ovoid HDR brachytherapy was studied. To correlate the rectal points from different fractions to the same coordinate system, the cervical os point and the orientation of the applicators were manually matched. With the applicator matching, rectal points obtained from other fractions were input into the original treatment plan for each application. A rectal dose was then calculated from all the possible rectal points. The fractional rectal doses were summed as the new cumulative rectal dose for each patient, which was compared with the original cumulative rectal dose. The reproducibility of the results was also analyzed by repeating the matching procedure. Results: The maximum inter‐fractional variation of distances between rectal dose points and the closest source positions was 1.1 cm and the corresponding maximum variability of fractional rectal dose was 65.5%. The percentage difference in cumulative rectal dose estimation for each patient was 5.1%, 16.4%, 25.7%, 18.9%, 12.2%, respectively. Overall reproducibility of the results was within 1.8%. Conclusion: Our results show underestimation of the rectal dose caused by variations of rectal marker positioning relative to the anterior rectal wall, which should be taken into consideration in film‐based HDR cervical brachytherapy. By manually matching the rectal points into the same treatment plan, one may minimize the possibility of underestimating the rectal dose. We will also anticipate a more accurate approach for evaluating rectal doses in HDR intracavitary brachytherapy with the emerging 3‐D volume imaging based treatment planning.
MO‐D‐AUD B‐08: Treatment Planning for Complex Brachytherapy Dose Distributions Using High‐Z Shields and Conventional Software35(2008); http://dx.doi.org/10.1118/1.2962349View Description Hide Description
Purpose: Certain brachytherapydose distributions, like for LDR prostate implants, are readily modeled by treatment planningsoftware using the superposition principle of individual seeds to replicate the total dose distribution. However, dose distributions for brachytherapy treatments using high‐Z shields are currently not well‐modeled using conventional software.Method and Materials:Dose distributions from complex brachytherapy plaques determined using Monte Carlo methods were used as input data, and included COMS‐based eye plaques using , , and ; 4–8cm diameter AccuBoost peripheral breast brachytherapy applicators from Advanced Radiation Therapy; and the 2 and 3cm diameter Valencia skin applicators from Nucletron Corp. Radial dose functions, g(r), and 2D anisotropy functions, F(r,θ), were obtained by positioning the coordinate system origin along the dose distribution cylindrical axis of symmetry. Origin: tissue distance and effective active length, Leff, were chosen to minimize g(r) and F(r,θ) interpolation.Dosimetry parameters were entered into the Pinnacle treatment planning system, and dose distributions were subsequently calculated/compared to the original Monte Carlo‐derived dose distributions. Results: The planning technique was able to reproduce complex brachytherapydose distributions for all three plaque types. Agreement improved as distance from the coordinate system axis decreased, 1% errors on the axis were attributed to g(r) interpolation. Agreement was best for the Valencia applicator and worse at the plaque edge for COMS eye plaques and the AccuBoost applicator. Agreement between input and planned dose distributions improved as the spatial resolution of the fitted dosimetry parameters improved. For agreement on the order of 1%, dosimetry parameter spatial resolution of 1mm was required, and the F(r,θ) dataset included over 1,000 datapoints. Conclusion: A new technique was developed to simulate complex brachytherapydose distributions in tissue using conventional treatment planningsoftware. These results should be generalizable to other source types and planning systems. Conflict of Interest: Research sponsored by Advanced Radiation Therapy.
MO‐D‐AUD B‐09: The Role of the Radiotherapy Physicist in Intraoperative Partial Breast Irradiation Using a Low Energy X‐Ray Source, Based On 10 Years Clinical Experience35(2008); http://dx.doi.org/10.1118/1.2962350View Description Hide Description
Purpose: To describe the role of the radiotherapyphysicist in the clinical implementation of the Intrabeam™ system for intraoperative radiotherapy (Carl Zeiss, Germany), based on 10 years experience at University College London Hospital. Method and Materials: On delivery of the 50kVp electronic X‐Ray system, the Radiotherapy Physics Group undertook acceptance and commissioning. Half‐value layer measurements were made using a PTW 23342 0.02cc ion chamber. A dedicated water phantom was employed to measure variation of dose rate with radial distance from the X‐Ray source in 5 orthogonal directions and in one direction for each of 8 spherical applicators. These measurements were compared with the manufacturer supplied QA peripherals and with radiochromic film to assess radial isotropy and output constancy and stability. A radiation protection survey and risk assessment was performed for unshielded operating rooms (OR) prior to clinical introduction. The routine physics requirement comprises: pre‐treatment QA and calculation of applicator treatment times; in the OR: actively delivering and monitoring the radiation treatment, monitoring and enforcement of staff radiation protection in and around the OR and measurement of patient skindose by TLD.Results: UCLH physicists have commissioned 4 such X‐Ray sources for clinical use. We have treated 134 patients over a period of 10 years. To date, dose rate surveys during treatment have demonstrated the safe usage of the system under controlled conditions and no member of staff has had a recordable radiationdose.Conclusion: The Intrabeam™ device has been shown to be very stable dosimetrically and also practical within a standard clinical environment. The involvement of the radiotherapyphysicist in the commissioning and clinical implementation of this intraoperative radiotherapy system is imperative to ensure safe treatment delivery and radiation protection of staff and patients.
35(2008); http://dx.doi.org/10.1118/1.2962906View Description Hide Description
Purpose: Intensity‐modulated radiation therapy(IMRT) provides excellent lateral dose conformity while energy‐ and intensity‐modulated electron therapy (MBRT) can spare distal critical structures for shallow targets. This work aims to combine IMRT and MERT for advanced mixed beam therapy (MBRT) of breast and head and neck cancers.Method and Materials: We have acquired a motorized electron‐specific multileaf collimator (eMLC) for accurate beam delivery for both conventional electron therapy and MERT. The eMLC is retractable to provide large apertures for efficient photon and electron beamdelivery for MBRT. Extensive measurements were performed to verify dose distributions collimated by the eMLC and to validate MBRT treatment plans. Monte Carlo based dose calculation, treatment optimization and leaf sequencing algorithms were investigated for efficient and accurate beam delivery. This technique is being implemented clinically for scalp, head and neck, and breast treatment through pilot studies that are specially designed for dose escalation and hypofractionation. Results: The eMLC provides similar electron beam characteristics to that obtained with a conventional electron applicator/cutout. The leakage is 1.8% for 16MeV electron beams and is less than 1% for other lower electron energies. A typical MERT has a 2–3 modulation‐scaling factor, which results in a maximum 5% leakage dose that is similar to that from IMRT. The measurement results agreed with the planned dose distributions to 3%/3mm for both uniform and heterogeneous phantoms. Conclusion: We have developed a MBRT system for the treatment of shallow targets that consists of hardware tools and software tools for accurate and efficient beam delivery. The technique is being implemented clinically for partial breast, scalp and head and neck treatments.
TH‐D‐AUD B‐05: Electron Intensity Modulation for Mixed‐Beam Radiation Therapy with An X‐Ray Multi‐Leaf Collimator35(2008); http://dx.doi.org/10.1118/1.2962907View Description Hide Description
Purpose: To explore the use of mixed‐beam therapy (MBT), i.e., combined intensity‐modulated electron and x‐ray beams using the x‐ray multileaf collimator(MLC), in the treatment of head and neck and breast cancers. For shallow head and neck and partial breast targets, the addition of electrons has the potential of improving target coverage and sparing of critical structures and reduction of integral dose due to rapid dose falloff with depth and reduced exit dose.Method and Materials:Dose calculations for electron beams collimated by the MLC were performed with Monte Carlo methods and integrated into a commercially‐available treatment planning system. Energy and intensity modulation of the electron beams was accomplished by dividing the electron beams into 2×2‐cm2 beamlets, which were beam‐weight optimized along with intensity modulation of x‐ray beams. MBT treatment plans were created for 8 shallow head and neck and 9 partial breast irradiation cases, and optimized to obtain equivalent target dose coverage compared to the original IMRT plans. MBT treatment plans were evaluated with respect to target conformality and integral dose in comparison with original clinical plans. Results: The shallow head and neck targets were shown to have reduced integral dose with MBT, on average 8% less than IMRT, as well as reductions in doses to critical structures distal to the target along the electron beam direction. MBT partial breast plans had an average reduction in integral dose of 14%, and also showed improvement in target conformality by an average of 24%. Conclusion: MBT shows promise for improving normal structure sparing and reducing integral dose, while maintaining target coverage and increasing target dose conformality.
TH‐D‐AUD B‐06: Variability of Low‐Z Inhomogeneity Correction in IMRT/SBRT: A Multi‐Institutional Collaborative Study35(2008); http://dx.doi.org/10.1118/1.2962908View Description Hide Description
Purpose: The dosimetry of IMRT beamlets with low‐Z inhomogeneities is a difficult problem and its accuracy is highly uncertain due to lateral disequilibrium. Various inhomogeneity correction algorithms: pencil beam (PB), collapsed cone convolution (CC), anisotropic analytical algorithm (AAA), Monte Carlo(MC), and combination of them are employed in different treatment planning systems (TPS) for dose calculations. This multi‐center collaborative study evaluates the accuracy and suitability of these algorithms for inhomogeneity correction in clinical trails. Method and Materials: A simple lung phantom was constructed with cork sheets (0.25 g/cm3) sandwiched between two 3 cm thick solid water slabs. The CT data of this phantom was sent to 8 institutions employing different TPS. Dose calculations were carried out at various depths over the field sizes: 0.5×0.5–10×10 cm2 for 6 and 15 MV beams with grid size (2×2 mm2). The calculated inhomogeneity correction factor (CF) was compared with measured data using a micro‐chamber. Results: The calculated CF with various algorithms showed marked variability and can be categorized in two classes; pencil beam (PB) and MC based collapsed cone (CC). The CF calculated with PB rises steadily in the lungtissue whereas CC exhibits the effect of electron transport. The differences between measured and calculated CF values varied from 70% to −10% for 6 MV from small to large fields. For 15 MV beam, the differences are even larger for small fields but reduce significantly for large field sizes (>5×5cm2). Conclusion: It is concluded that PB based algorithms should be avoided for dose calculation in small fields with low‐Z inhomogeneities. The MC derived kernel based algorithms such as CC and AAA produce similar results to each other within ±10% and should be preferred for patient treatment in IMRT/SBRT. This study raises serious concerns in dosimetric variability in lung cancers possibly impacting the clinical trials.
35(2008); http://dx.doi.org/10.1118/1.2962910View Description Hide Description
Purpose: Inhomogeneity corrections are commonly employed in computerized treatment planning. Among the methods of validating the accuracy of these corrections are measurements made in phantoms containing slabs of inhomogeneity using static fields. IMRT fields, because of presence of small segments, require a more robust inhomogeneity correction algorithm. The AAPM task group 65 report discusses the degree of accuracy of different algorithms employed in treatment planning systems in correcting for inhomogeneity. We have evaluated the accuracy of Philips Pinnacle3 system in modeling the dose within and at the interface of inhomogeneities for IMRT plans. Method and Materials: The phantom used for evaluation is a modified IMRT QA phantom made by Civco. The phantom was modified by milling slots for TLD chips within the lung and bone‐equivalent inserts, and near the interfaces with solid water. A seven‐field IMRT plan was generated by optimizing to a spherical target within the phantom, between the lung and bone inserts. Three additional avoidance structures where added in order to achieve a realistic IMRT plan with significant modulation. The inhomogeneities were not used as constraints in the optimization. The planned IMRT was then delivered to phantom with calibrated TLD chips placed in a number of slots. The TLD readings for each location were then compared to the dose predicted at that point by Pinnacle. Results: The preliminary results of these measurements indicate satisfactory agreement between the measured and Pinnacle‐predicted doses with some differences between the results for lung and bone. Conclusion: This work demonstrates the degree of accuracy of inhomogeneity correction for highly modulated fields in Pinnacle3 system. More measurements are planned for other TLD points and additional work undertaken will compare the homogeneous and inhomogeneous plans in actual patients for different sites.
35(2008); http://dx.doi.org/10.1118/1.2962911View Description Hide Description
Purpose: To evaluate the commercial CMS Monaco IMRTtreatment planning system which employs a Monte Carlo(MC) based dose calculation engine, biological motivated cost functions, multi‐criteria optimization, and an efficient sequencing algorithm. Method and Materials: For a head and neck, a liver, a prostate and a rectal cancer patient, step‐and‐shoot IMRT plans were designed using Monaco. The plans were compared to ones generated by the established CMS XiO treatment planning system. The plans were optimized to achieve the same clinical objectives concerning dose to the tumor and to the relevant organs‐at‐risk. However, whereas the XiO plans were formulated using DVH and minimum/maximum dose constraints, the Monaco plans utilized the biological cost functions offered by the system. DVHs, EUD, mean‐ and maximum‐doses were compared, as well as the number of beam segments and MUs. Finally the plans were delivered on a MapCheck device to verify the agreement between the MC calculated dose distributions and measurements to be less than 3% and 3 mm. Results: Plans optimized with Monaco achieved at least similar and in some cases superior dose distributions. The multi‐criteria optimization tools and the sensitivity analysis helped to reduce the time needed to optimize the plan. The Monaco plans resulted in fewer segments and lower number of MUs and therefore reduced delivery time. All calculated dose distributions passed the dose verification with the MapCheck device. Conclusion: The commercially available Monaco system produces clinical relevant plans, which are dosimetrically equivalent or superior to plans from the conventional XiO system, feature shorter delivery times, and can easily be verified with normal QA procedures using MapCheck.
TH‐D‐AUD B‐09: Comparative Analysis of Peripheral Doses for Base of Tongue Treatment by Linear Accelerator and Helical Tomotherapy IMRT35(2008); http://dx.doi.org/10.1118/1.2962912View Description Hide Description
Purpose: To determine which Intensity Modulated Radiation Therapy(IMRT) modality, either linear accelerator or helical tomotherapy, delivers higher peripheral (out‐of‐field) doses to various structures from a typical tongue treatment.Method and Materials: The principal investigator developed a method of placement of human organs onto the limited‐organ anthropomorphic phantom. Multiple human CT data sets were segmented for 18 critical structures and organs at risk, and fused to the anthropomorphic CT data set. The GTV and PTV of a base‐of‐tongue plan were overlaid on the phantom CT for clinically acceptable tumor delineation and nodal involvement. Eighteen contours, distributed throughout the phantom, were designated for thermoluminescent device (TLD) placement; and using geometry and fluoroscopy each TLD location is established in the phantom. Following the RTOG IMRT Protocol 0522, treatment of the primary tumor and involved nodes (PTV70) and subclinical disease sites (PTV56) was planned using IMRT to 70Gy and 56Gy. Two comparable, clinically acceptable treatment plans were produced: one utilizing a linear acceleratortreatment planning system and one using a helical tomotherapy planning system. Each treatment plan was delivered to the anthropomorphic phantom four times. Results: Assessment was based on total treatmentdoses. Within 2.5 cm (one helical tomotherapy field width) superior and inferior to the field edges, normal tissue doses were on average 45% lower using linear accelerator based treatment. Beyond 2.5 cm, the average helical tomotherapy treatmentdose was 52% lower. The majority of points were proven to be statistically different using the Student's t‐test with p>0.95. Conclusion: Helical tomotherapy IMRTdelivers one addition field width of radiation above and below the treatment field, contributing to the higher peripheral doses adjacent to the field. At distances beyond one field width, where leakage is dominant, helical tomotherapy doses are lower than linear acceleratordoses.
- IMRT — Clinical Applications
35(2008); http://dx.doi.org/10.1118/1.2962573View Description Hide Description
Purpose: Volumetric Modulated Arc Therapy (VMAT) is an arc‐based technique that utilizes dynamically modulated arcs to deliver intensity‐modulated radiation therapy(IMRT)treatments on a conventional linear accelerator. In this work, we evaluate VMAT as a treatment technique for patients with carcinomas of the head‐and‐neck. Method and Materials: Five complex head and neck cancer patients, with multiple prescription levels, were selected for this study. Fully inverse planned VMAT plans were optimized for these patients using our homegrown arc‐sequencing software. The software uses simulated annealing to optimize the aperture shapes and weights while minimizing the differences between the optimized (ideal) and sequenced intensities. The optimized plans were compared with step‐and‐shoot IMRT plans generated using the Pinnacle3treatment planning system. VMAT plan verifications have also been performed using Elekta's Precise Beam Infinity™ control system which has been installed on an Elekta Precise linear accelerator in our clinic. Results: Using our arc‐sequencing tool, VMAT can be used to create highly conformal head‐and‐neck treatment plans. As compared with traditional fixed‐field plans, VMAT was able to reduce the average parotid dose from 85.3 cGy to 73.6 cGy per fraction. Additionally, the average number of monitor units was reduced from 1058.3 to 502.3 per fraction. Initial delivery tests demonstrate that using VMAT complex head‐and‐neck deliveries can be completed in under 6 minutes. Conclusion: VMAT should serve as an important tool in the delivery of radiation therapy for head‐and‐neck carcinomas. By utilizing the dosimetric advantages of rotational IMRT, VMAT can provide more uniform target doses and reduced critical structure doses as compared with fixed field IMRT.Conflict of Interest: Research partially sponsored by Elekta Corporation.
TU‐D‐AUD B‐02: Volumatric Modulated Arc Therapy: Implementation and Evaluation for Prostate Cancer Cases35(2008); http://dx.doi.org/10.1118/1.2962574View Description Hide Description
Purpose: Volumetric modulated arc therapy (VMAT) (Otto K, Medical Physics, 2008) is an emerging treatment paradigm which modulates MLC aperture and dose rate during gantry rotation. The purpose of this study is to implement and evaluate VMAT relative to the standard IMRT approach. Method and Materials: A gantry rotation up to 360° is modeled as 360 evenly divided beams. Beam apertures and dose rate are optimized with respect to gantry angles under a DVH constraints based objective. Differences between our VMAT implementation and previous VMAT are: using gradient search for dose rate optimization rather than random search, and sampling multiple MLC leaf positions within the allowed leaf speed constraints rather than a single one in each iteration. A planning study including 5 prostate patients with a prescription dose of 86.4Gy was performed to evaluate VMAT verse the standard 5‐field IMRT approach. VMAT treatment plans are normalized such that certain critical organ dose limits are met, and are comparable to IMRT plans. V95, D95 and mean dose of PTV are used to evaluate plans, while monitor unit (MU) and delivery time are used to assess delivery efficiency. Results: The PTV V95, D95 and mean dose in VMAT plans are 97.0±0.8%, 96.4±0.5%, and 101.7±0.4%, respectively, vs 97.1±0.8%, 97.5±1.0% and 103.0±0.7% in IMRT plans. VMAT and IMRT plans are indistinguishable measured by these dose indices. The advantage that VMAT presents is it reduces MU by 49.8±7.4%. Secondary scatter dose to patient is reduced accordingly. A typical prostate treatment is shortened from about 5 minutes for IMRT to 2.6±0.2 minutes for VMAT. The better delivery efficiency of VMAT is accomplished by having a larger time averaged beam aperture: 49.1±5.2cm2 vs 19.5±1.5 cm2 in IMRT.Conclusion: VMAT technique can reduce treatment time by up to 50% while maintaining comparable dosimetric quality to standard IMRT approach.
TU‐D‐AUD B‐03: MVCT‐Guided Partial‐Breast Irradiation in Prone Position: Daily Setup Uncertainty and Dose Verification35(2008); http://dx.doi.org/10.1118/1.2962575View Description Hide Description
Purpose: We analyzed the daily setup uncertainty and dose verification for partial‐breast irradiation (PBI) in prone position using helical tomotherapy. Method and Materials: According to an in‐house protocol, early‐stage breast cancer patients received PBI treatments in the prone position on the TomoTherapy Hi‐Art system using megavoltage‐CT guidance (TomoTherapy, Inc., Madison, WI). For planning, kilovoltage‐CT scans were obtained with the involved breast suspended downward. Treatment plans were generated based on criteria from the NSABP B‐39/RTOG 0413 protocol; the PTV_eval was to receive 3.85 Gy per fraction over 10 fractions administered twice daily. Before each fraction, an MVCT scan was acquired and compared with the planning kVCT images to refine the patient position. Along each shift direction, a margin to estimate the setup uncertainty for treatments without MVCT guidance was calculated from the average and standard deviation among the daily shifts. The dose actually delivered in each fraction was reconstructed based on the daily MVCT, accounting for the daily shifts. Among all fractions, the average and standard deviation for specific DVH points were compiled for comparison with the plan DVH. Results: Among the MVCT‐guided shift data, the random setup uncertainty in general exceeds the systematic difference from the plan. The overall margin was as large as 24.5 mm among the cases analyzed. From the MVCT‐based recalculations, the reconstructed doses differed little from the planned doses for each breast, each lung, thyroid, and heart. Average reconstructed doses for PTV_eval were slightly lower than the planned dose, attributable to increased breast thickness for some fractions. Yet, at least 90% of PTV_eval received over 90% of the prescribed dose. Conclusion: The estimated margin to account for setup uncertainty motivates improvements for PBI positioning. With MVCT guidance for prone‐positioned PBI, the deviation of the delivered target and organ‐at‐risk doses from the planned dose is minimal.
TU‐D‐AUD B‐04: A Dosimetric Comparison of Simultaneous Integrated Breast Radiotherapy Using 3D Conformal, IMRT and Tomotherapy Techniques35(2008); http://dx.doi.org/10.1118/1.2962576View Description Hide Description
Purpose: The dosimetric characteristics of simultaneous integrated boost (SIB) breast radiotherapy techniques utilizing 3D‐conformal (3DSIB), linac‐based intensity modulation (IMRT) and Tomotherapy (TOMO) were compared. The SIB techniques were compared to conventional whole breast irradiation (WBI). Method and Materials: Fifteen patients were included in the study. The SIB treatments (3DSIB, IMRT and TOMO) delivered 45Gy(25×1.8Gy) to the whole breast minus the boost bed, PTV(Breast‐Boost), and concurrently delivered 60Gy(25×2.4Gy) to the tumor bed, PTV(Boost). The WBI treatment delivered 45Gy(25×1.8Gy) the whole breast, PTV(Breast), followed by 20Gy(10×2Gy) to PTV(Boost). The WBI and SIB treatment were planned to deliver equivalent dose to the targets. Statistical significance between dosimetric parameters was determined using Wilcoxon signed‐ranks. The dosimetric data were compiled to form population dose volume histograms which display the mean value and 1σ uncertainty. Dose homogeneity was quantified by the parameter D95%−5%, representing the percent difference in dose covering 95% of the target minus the dose covering 5% of the target. Results: Using the D95%−5% parameter the PTV(Breast‐Boost) volume received a more homogeneous dose using TOMO(14.3%) compared to 3DSIB(25.1%), IMRT(27.1%) and WBI(44.7%). The PTV(Boost) volume received a morehomogeneous dose using non‐intensity modulated techniques, WBI(6.9%) and 3DSIB(7.0%), compared to IMRT(11.6%) and TOMO(12.0%). The mean percent volume of ipsilateral lung that received ⩾20Gy was smaller using IMRT(15.8%) compared to 3DSIB(17.7%), TOMO(17.1%) and WBI(19.0%). For left‐sided breast cancer patients, the mean percent volume of heart that received ⩾35Gy was less for intensity modulated SIB techniques, IMRT(0.1%) and TOMO(0.2%), than 3DSIB(4.4%), and WBI(6.2%). The contralateral breast received a smaller mean dose using IMRT(0.39Gy) compared to WBI(0.81Gy), 3DSIB(0.67Gy) and TOMO(1.28Gy). Conclusion: IB techniques delivered a more homogenous dose to the PTV(Breast‐Boost) volume, while non‐intensity modulated techniques delivered a more homogeneous dose to the PTV(Boost) volume. Intensity modulated SIB techniques (IMRT and TOMO) decreased the heartdose relative to 3DSIB.
TU‐D‐AUD B‐05: Quantifying the Dosimetric Trade‐Offs When Treating Targets with Concavities Using IMRT35(2008); http://dx.doi.org/10.1118/1.2962577View Description Hide Description
Purpose: To quantitatively assess the relationship between intensity‐modulated radiotherapy (IMRT)‐driven sparing of dose to normal tissues located “within” concave targets with (1) heterogeneity in dose delivered to the target and (2) redistribution of dose to normal tissue beyond the concavity. Method and Materials: Idealized volumes resembling mesothelioma and prostate cancer were considered, both with concave targets containing normal tissue. Multiple IMRT plans were generated with progressive dose restriction to the normal tissue within the concavity. We quantified the impact of such sparing on the heterogeneity of dose within the target tissue itself, and the dose received by normal tissues beyond the concavity. Results: As the dose to the normal tissue within the concavity is reduced, the heterogeneity in dose delivered to the target increases in an exponential fashion. Further, there is a rapid increase in redistributed dose to other normal tissue regions, particularly to the other normal tissues immediately adjacent to the outside edges of the concavity (i.e. adjacent to the “free margin” of the target). Conclusion: For targets with concavities, there are dosimetric consequences associated with reducing dose to normal tissue located within these concavities. There appears to be a trade‐off between the heterogeneity in dose delivered to the target, the mean dose to the concavity, and the redistributed dose to other normal tissues. Understanding the interaction between these dosimetric factors can aid the planner in assessing the feasibility of a desired dose distribution.