Index of content:
Volume 36, Issue 6, June 2009
- Therapy Scientific Session: Ballroom B
36(2009); http://dx.doi.org/10.1118/1.3182515View Description Hide Description
Purpose: The AAPM Eye Plaque Dosimetry Task Group 129 presents an update of the dosimetry calculations for the Collaborative Ocular Melanoma Study (COMS) plaques. Results from a multi‐center comparison using several brachytherapytreatment planning systems (BTPSs) are presented. Method and Materials: Dose distributions around 16‐mm diameter COMS plaques loaded with I‐125 (model 6711) or Pd‐103 (model 200) were determined using three TG43‐based BTPSs (Pinnacle v8.0d, BrachyVision v6.1 and v8.1, PlaqueSimulator v5.3.7), and two Monte Carlo codes (MCNP5 and BrachyDose). TG‐43 plans assumed an unbounded homogeneous medium. Monte Carlo plaque simulations included the Modulay (gold alloy) backing, Silastic (silicone polymer) insert, and interseed interactions. Doses along tumor central axis (−1 to 20 mm) and to defined critical structures such as fovea, optic disc, lens center, and lacrimal gland center in a standard eye model were calculated. Results: Agreement among all TG‐43 based plans on the central axis was within ±2% for both point‐ and line‐source approximations. However, for off‐axis points, BrachyVision v 6.1 had a truncation error in coordinates, which resulted in dose deviation of about 5% relative to other plans. As expected, the doses at off‐axis points were lower for the line source approximation than the point source approximation. The largest deviations were found at the lacrimal gland center, where the line source model resulted in 10% and 20% less dose than the point source model for I‐125 and Pd‐103 sources, respectively. Monte Carlo simulations predicted dose values are about 20–30% lower than the average of TG‐43 plan values, due to full plaque geometry; with the exception of few off‐axis points up to 90% lower. Conclusions: This multi‐center comparative analysis of BTPSs dose results indicated the importance of careful selection of TG‐43 parameters, source model assumptions, and the BTPS coordinate resolution limits, and the value of complete Monte Carlo calculations.
WE‐D‐BRB‐02: Comparing a Grid‐Based Boltzmann Solver with Monte Carlo Simulation for Voxel‐Based Therapeutic Radionuclide Dose Calculations36(2009); http://dx.doi.org/10.1118/1.3182516View Description Hide Description
Purpose: To compare the accuracy and speed of a deterministic grid‐based Boltzmann solver (GBBS) with Monte Carlo(MC) simulations for calculating voxel‐based absorbed dose rates from SPECT/CT imaging.Methods: A SPECT/CT image of a breast cancer patient with metastatic osteosarcoma was obtained using a tracer administration of EDTMP. The therapeutic activity administered was determined from MIRD estimates. DOSXYZnrc and the GBBS Acuros were used to calculate dose rate maps from the activity distribution over the full CT grid. The GBBS photondose rate was calculated using the collisional KERMA approximation rather than explicitly transporting the generated electrons; an energy cutoff was used to neglect spatial transport of electrons below a threshold of 200 keV for Acuros and 189 keV for DOSXYZnrc. The photon, beta‐particle, and total absorbed dose rates calculated using GBBS were compared with the gold standard MC simulations using the gamma index. Gamma index parameters evaluated were 3%/3mm and 5%/5mm; both used a step size of 0.5mm with a 10 mm search distance. A patient mask was created from the CT to report pixels within/near the patient. The gamma failure points (γ>1.0) within the patient mask were viewed overlaid on the activity map and on the CT for the calculated beta, photon, and total dose rates. Results: For total dose rate, 90.1% and 99.6% of pixels within the patient mask had γ⩽1.0 for 3%/3mm and 5%/5mm respectively. γ failures were noticed at the edges of the activity distribution for beta‐particles, and at various interfaces for the photons. For Acuros, the beta‐particle and photon transport required about 10 minutes each. DOSXYZnrc took approximately 15,000 times longer. Conclusions: GBBS has the potential to provide accuracy similar to MC in a much shorter time; this could be useful for voxel‐based radionuclide absorbed dose estimates in a clinical setting.
36(2009); http://dx.doi.org/10.1118/1.3182517View Description Hide Description
Purpose: To calculate three dimensional dose distributions in the eye region for novel plaques used in the treatment of iris melanoma at the Mayo Clinic and compare these with dose distributions for other plaques used in these treatments.Method and Materials: The EGSnrc user‐code BrachyDose is used to perform Monte Carlo simulations. Plaques and seeds are fully modeled. The Mayo Clinic plaques are based on the Collaborative Ocular Melanoma Study (COMS) 22 mm plaque design with a gold alloy backing, outer collimating lip, and silicone polymer insert. An inner collimating lip surrounds a 10 mm diameter cutout region at the plaque center. Plaques span 180, 270, and 360 degree arcs. Three‐dimensional dose distributions in the eye region are calculated and are compared via depth‐dose curves, tabulation of the dose at critical structures (cornea, sclera, lens, macula, optic disk), and isodose contours. Results: The inner lip reduces dose to the cornea and surrounding region by 30–45%. Doses at some points of interest (e.g. cornea) differ by as much as 60–70% compared to those calculated with the TG‐43 protocol. The outer lip collimates radiation and significantly reduces doses to neighboring tissues. Catering plaque arc length to tumor extent reduces doses to the anterior portion of the eye outside the treatment area. For the same prescription dose, Pd‐103 offers a lower dose to critical structures than I‐125, with the exception of the sclera adjacent to the plaque. Conclusion: The Mayo Clinic plaques offer a number of advantages compared to other plaques used in the treatment of anterior eye tumors.Dose is significantly reduced to regions outside the treatment area. Calculations achieving 2% statistical uncertainty on the prescription dose take a few minutes on a single CPU, making BrachyDose sufficiently fast for routine clinical treatment planning.
WE‐D‐BRB‐04: An Evaluation Study of Treatment Planning of Brain Tumor Using Implanted Neutron Brachytherapy and Compared with Photon IMRT36(2009); http://dx.doi.org/10.1118/1.3182518View Description Hide Description
Purpose: The objective of this study is to perform a plan evaluation using Californium‐252 neutronbrachytherapy for the treatment of malignant gliomas and compare the neutronbrachytherapy planning with photonIMRT planning. Materials/Methods: After a phase I trial of neutronbrachytherapy (Cf‐252 implant) for the treatment of malignant gliomas, there is renewed interest to evaluate the effectiveness of radiation therapy using neutronbrachytherapy compared with photonIMRT. Here we performed a dosimetric comparison of two treatment plans based on the same patient using equivalent prescribed dose. Isodose lines and dose volume histogram of braintumor and adjacent critical structures were used for plan evaluation. Dose of neutronbrachytherapy was calculated using CT‐converted Monte Carlo model and simulated by Monte Carlo code MCNPX. The tissue component of Monte Carlo model was adapted from the sectioned images of human cadavers of the Visible Human Project of NLM. Dose of IMRT plan was calculated using Pinnacle3 TPS. A relative biological effectiveness of 6 was used to determine the neutron equivalent dose (ncGy) for central nervous system (CNS) tissues. An equivalent dose of 6000 cGy was prescribed for both the IMRT plan and the neutronbrachytherapy plan. Results: The targets were well covered by the 95% isodose line in both IMRT and neutronbrachytherapy plans. Comparing the IMRT and Cf‐252 neutronbrachytherapy, the mean dose was 6102 cGy and 6708 ncGy for target, 403 cGy and 177 ncGy for brainstem, 1022 cGy and 311 cGy for chiasm, 311 cGy and 275 ncGy for pituitary, and 705 cGy and 420 ncGy for brain, respectively. Conclusions: Cf‐252 Brachytherapy provided conformal dose distribution to the braintumor and reduced the dose to the surrounding critical organs compared to IMRT. The implanted Cf‐252 source provides high dose to braintumor and reduces the radiation exposure of normal brain.
WE‐D‐BRB‐05: Novel Post‐Lumpectomy Breast Cancer Brachytherapy with the Capability of Simultaneous Focal Lymph Node Irradiation36(2009); http://dx.doi.org/10.1118/1.3182519View Description Hide Description
Purpose: A new technique of post‐lumpectomy breast cancerbrachytherapy using beta‐emitting therapeutic radionuclides, rhenium‐188 (Re‐188) and rhenium‐186 (Re‐186), carried within lipidnanoparticles(liposomes) was investigated. This therapy strategy is advantageous regarding: 1) mm‐range focal radiation by beta‐particles ensures localized brachytherapy; 2) sustained high local retention within the lumpectomy cavity and accumulation in associated lymph nodes enables simultaneous cavity and lymph node focal radiation therapy; and 3) ease and flexibility of this modality to treat various locations and cavity shapes in the breast with minimal invasiveness. Method and Materials: Breast cancer surgical model in nude rats, gamma camera imaging, and animal organ dissection were used to study local and lymph node retention of radioactive liposomes. EGSnrc Monte Carlo code system was used to compare dose point kernels (DPK) for Re‐188 and Re‐186 in breast tissue, water, and other soft tissues. The DPK were then used for dose calculations of uniformly distributed liposomal Re‐188 or Re‐186 injected to the lumpectomy site within ellipsoid and spherical cavities. The dose calculation for lymph nodes modeled as a disk shape with exponentially decreased activity from the cortex was also performed. Results: High and sustained intracavitary retention of radioactive liposomes accompanied by lymph node accumulation was observed. Different human tissues showed very similar dose curves for Re‐188 and Re‐186. The therapeutic ranges in the cavity could be about 5 and 2.5 mm with Re‐188 and Re‐186, respectively, while radiationdoses beyond 1 cm sharply dropped to minimal. Also, the entire lymph node could receive therapeuticdose.Conclusion: The promise of a new post‐lumpectomy breast cancerbrachytherapy technique is presented. A rapid fall‐off in radiationdose with distance allows this treatment modality to spare substantial healthy tissue with reduced complications. It is also favorable through simultaneous focal treatment of microscopic tumor cells spreading to downstream lymphatics.
36(2009); http://dx.doi.org/10.1118/1.3182520View Description Hide Description
Purpose: To compare dosimetry characteristics of Nag‐eye plaque with COMS eye plaque using I‐125 and Cs‐131 brachytherapy sources. Material and methods: Monte Carlo technique was used to generate 3D dose distributions of a 16‐mm Nag eye plaque and 16‐mm COMS eye plaque loaded with the I‐125 and Cs‐131 brachytherapy sources separately. The Nag eye plaque is a comparatively simpler eye plaque that uses fewer seeds than the COMS and does not require a Silastic seed carrier. The simulations were carried out with gold alloy plaques. A water equivalent seed carrier was used instead of the Silastic trough designed for the traditional COMS eye plaque. The Nag eye plaque used only eight sources forming two squares; the COMS eye plaque was loaded with thirteen sources forming three isocentric circles. A spherical eyeball 24.6 mm in diameter and an ellipsoid tumor 6 mm in height and 12‐mm in diameter were used to evaluate the doses delivered. Results: The doses along the eye plaque axis and the DVHs of the tumor were calculated. Our results indicated that, to achieve a prescription dose of 85 Gy at 6 mm from the inner sclera edge, the Nag eye plaque required 6.156 U/source for I‐125 and 6.82 U/source for Cs‐131; the COMS eye plaque required 4.015 U/source and 4.433 U/source for the same source types. The doses from two types of eye plaques on the central axis were almost the same at distances greater than 5 mm; the OSU‐Nag plaque gave slightly lager doses than the COMS at distances less than 5 mm. The DVHs of the tumor showed that the NAG plaque tended to create slightly more hot dose regions than the COMS. Conclusion: The dosimetric characteristic of Nag‐eye plaque is comparable to COMS in using either I‐125 or Cs‐131 source.
WE‐D‐BRB‐07: Photon Spectrometric Determination of the Dose Rate Constant of Advantage™ Pd‐103 Brachytherapy Sources36(2009); http://dx.doi.org/10.1118/1.3182521View Description Hide Description
Purpose: Although several dosimetric characterizations using thermal luminescent dosimetry(TLD) and Monte Carlo simulation have been reported for the Advantage™ Pd‐103 source introduced recently for prostate brachytherapy, no AAPM consensus values have been established for its dosimetry parameters. The aim of this work was to perform a photon spectrometry based determination of this source's dose rate constant (PSTΛ), independent of the TLD and Monte Carlo techniques. Method and Materials: Three Model IAPD‐103A Advantage™ sources were obtained from the source manufacturer. The relative photon energy spectrum emitted by each source was measured along the radial direction in the source's bisector using a high‐resolution Germanium detector designed for low‐energy photon spectrometry. The PSTΛ of each source was determined from the measured energy spectrum and the activity distribution in the source. Inter‐source variations in the measured spectra and in PSTΛ were investigated. Comparison of PSTΛ with those determined by TLD (TLDΛ) and Monte Carlo (MCΛ) techniques were performed and a likely consensus value was estimated. Results: The energy spectrum emitted along the radial direction in the bisector was similar to that emitted by the well‐established Model 200 source. The PSTΛ in water was 0.676±0.026 cGyh−1U−1 similar to 0.678±0.026 for Model 200 source. Inter‐source variation in PSTΛ was < 0.01%. The PSTΛ was close to MCΛ of 0.690±0.021 and 0.687±0.002 determined by PTRAN (Appl. Ratiat. Isotopes. 64:881–887,2006) and EGSnrc (Med. Phys. 35:4228–4241,2008), respectively. It was 3.4% lower than TLDΛ of 0.700±0.056 (Appl. Ratiat. Isotopes. 64:881–887,2006). A likely consensus value, determined by averaging the values of the three techniques, [〈PSTΛ〉+〈TLDΛ〉+ 〈MCΛ〉]/3, was 0.688 cGyh−1U−1. Conclusions: The PSTΛ obtained in this work provides an independent determination of the dose rate constant (Λ) for Advantage™ Pd‐103 source. More accurate consensus value of Λ can be established by combining the TLD, Monte Carlo, and photon spectrometry techniques.
WE‐D‐BRB‐08: Effects of Shielded Ovoids in HDR 192Ir Cervical Brachytherapy: A Monte Carlo Study Using Cone‐Beam CT Images36(2009); http://dx.doi.org/10.1118/1.3182522View Description Hide Description
Purpose: To investigate the effects of the Fletcher‐Williamson applicator set on the bladder and rectal dose for high‐dose‐rate (HDR) treatment of cervical cancer.Method and Materials: Twenty HDR cervical treatment plans were calculated using an applicator‐based dose superposition method. Pre‐calculated Monte Carlo dose data were used to account for the effects of the tandem and ovoids. A library of 3D dose distributions around all possible dwell positions in water were generated using the PTRAN code. Both the plastic ovoids and the ones with bladder and rectal shields made of tungsten alloy were modeled. The patient calculations were based upon the treatment plans previously created using cone‐beam computed tomographyimages.Results: The stainless steel tandem together with either shielded or unshielded ovoids reduced the dose to Points A and B by 2.5% and 2.2%, respectively. The dose to the hottest 0.1 cm3 and 2 cm3 of the bladder and rectum was reduced by 3.5%–4.5% with plastic ovoids. When shielded ovoids were used, the maximum dose reduction to the hottest 0.1 cm3 of the bladder was 19.5%, while the ICRU bladder point was reduced by up to 24.5%. However, the dose to the bladder point was reduced by over 10% in only 3 out of the 20 plans. The bladder point dose was decreased by 6.6% on average. On the other hand, there was a 1% additional decrease in the rectal dose compared to the unshielded case. The ICRU rectal point did not correspond to the part of the rectum receiving the highest dose.Conclusion: The ovoid shields in the Fletcher‐Williamson applicator set were found ineffective in shielding the high‐dose region in the rectum. The orientations of the ovoids also strongly influenced the dose to the bladder.
Conflict of Interest: Research sponsored by Nucletron.
36(2009); http://dx.doi.org/10.1118/1.3182523View Description Hide Description
Purpose: In a recent ABS survey, over 90% of those surveyed used a single channel vaginal cylinder for endometrial cancerbrachytherapy. Here, we establish the clinical and dosimetric advantage of using the Mammosite™ balloon in an intravaginal delivery of radiation to the vaginal apex as compared to a standard cylinder. We have called the use of the balloon in this manner “GynSite®.” Method and Materials: Twenty patient treatments were analyzed for each delivery method. Each patient was imaged either with CT or MR with the device in place. Using Brachyvision v8.2, the prescription dose was optimized to the surface of each device. The Dose Homogeneity Index (DHI) and the FWHM of the differential DVH were analyzed for each delivery. The dose to the 1 mm vaginal lymphatic channel was determined and the V100 and V150 reported. Results: The GynSite® delivers a dose distribution similar to that of the standard cylinder. However, it allows more patient‐specific delivery of the radiation to the vaginal apex because of the flexibility and variable size of the balloon. Cylinders only come in discrete sizes (diameter of 2.0, 2.5 cm, etc.) whereas the balloon has variable shapes (spherical or elliptical) and fill volumes (35cc – 125cc). This flexibility allows the device to better conform to the patient's anatomy thus reducing air gaps between the applicator and the target vaginal mucosa at 1mm depth. Additionally, it is more comfortable for the patient because it is inserted and removed in a deflated state. The DHI and FWHM were similar between the cylinder and the GynSite® deliveries. Conclusion: The GynSite® is clinically superior to a standard cylinder since it conforms better to the patient's anatomy and therefore delivers a better dose to the lymphatic channel and possibly improves tumor control.
- IMRT I
36(2009); http://dx.doi.org/10.1118/1.3182211View Description Hide Description
Purpose: During the delivery of volumetric modulated arc therapy (VMAT), random errors exist in both the MLC leaf positions and the gantry angle. In this work, we investigated the impact of such random errors on VMAT plan quality and delivery accuracy. The impact of system calibration errors was also examined. For comparison purposes, we performed a similar study on step‐and‐shoot IMRT plans. Material and Method: VMAT plans for three treatment sites (prostate, pancreas and head‐&‐neck) were created using a home‐grown arc sequencer. Next, random and systematic leaf position errors were introduced into these plans with the random errors sampled from Gaussian distributions of varying widths ranging from 1 to 3mm. Two types of systematic errors, including MLC bank shifts in the same direction (Type I: leaf gap unchanged) and MLC bank shifts in opposite directions (Type II: leaf gap increase/decrease). The plan quality variations were compared in the Pinnacle3 planning system. Plans with systematic errors were verified using the MatriXX ion chamber array with gamma evaluation criteria of 3%/3mm. Results: The plan degradation observed for VMAT plans was slightly less as compared to that for fixed‐field IMRT plans when random errors up to 3mm to the leaf positions were introduced. With type I systematic errors of 3mm on leaf positions, the average standard deviation of PTV dose increased by 10.2%. This value increased to 18.4% for the corresponding fixed‐field IMRT plans. A larger impact on the IMRT plans was also observed when type II systematic errors were introduced. The above results were confirmed by plan verification measurements with higher gamma passing rates for VMAT plans when systematic errors were applied. Conclusion: The VMAT delivery technique has better tolerance to random and systematic errors in gantry angle and MLC leaf position errors as compared with step‐and‐shoot IMRT.
Research supported by Elekta.
MO‐D‐BRB‐02: Using Total‐Variation Regularization for IMRT Inverse Planning with Field‐Specific Numbers of Segments36(2009); http://dx.doi.org/10.1118/1.3182212View Description Hide Description
Purpose: Currently, there are two types of treatment planning algorithms for intensity modulated radiation therapy(IMRT). The beamlet‐based algorithm generates fluence maps with high complexity, resulting in large numbers of segments in the delivery. The segment‐based direct aperture optimization (DAO) algorithm uses a small number of segments. However, the number of segments is typically pre‐fixed, and the optimization is computationally intensive. In this work, a regularization based algorithm is proposed to overcome the drawbacks of the DAO method. Method and Materials: Instead of smoothing the fluence maps, we include a total‐variation term in the optimization objective function to reduce the number of signal levels of the fluence maps and therefore the number of deliverable apertures. As compared to the DAO algorithm, our method has an efficient form of quadratic optimization, with an additional advantage of optimizing field specific numbers of segments based on the modulation complexity. Results: The proposed approach is evaluated using two clinical cases. Provided that the clinical acceptance criteria of the treatment plan are satisfied, for the prostate patient, the total number of segments is reduced from 61 using the Eclipse planning system to 35 using the proposed algorithm; for the head and neck patient, the total number of segments is reduced from 107 to 28. The head and neck result is also compared to that using an equal number of 4 segments for each field. The comparison shows that using field‐specific numbers of segments achieves a much improved dose distribution. Conclusion: A total‐variation based inverse planning method is proposed in this work. As compared to other existing methods, the proposed algorithm is derived using different principles and implemented efficiently. The patient studies show that the proposed algorithm significantly reduces the total number of segments used in the treatment without compromising the delivered dose distribution.
MO‐D‐BRB‐03: Using a Database of Patient Geometric and Dosimetric Information for Quantitative IMRT Plan Quality Control36(2009); http://dx.doi.org/10.1118/1.3182213View Description Hide Description
Purpose: To empower IMRTtreatment planners with the ability to judge new plans against the performance of similar past plans. Method and Materials: This is accomplished by searching a database of treated patients using the geometric relationships between targets and organs at risk (OARs). We introduce the concept of a shape relationship descriptor to quantify this intuitively important relationship. The overlap volume histogram (OVH) simplifies the complex 3D relationship between a target and an OAR. The OVH is the normalized 1D histogram of the OAR volume within a distance of the target. The OVH descriptor was used to search a patient database, providing a patient specific set of dose volume histograms (DVHs). These DVHs were then presented to the planner to aid their decisions. Results: The method was applied to both parotids of 32 treated head‐and‐neck patients. The 17 parotids that promised the greatest reduction in D 50 were selected for re‐planning. These 17 parotids came from 13 patients. Our method indicated that the doses of the other nine parotids of the 13 patients could not be reduced, so they were included in the re‐planning process as controls. Re‐planning with an effort to reduce D 50 was conducted on the 26 parotids of these 13 patients. Average reductions of D 50 were 6.6Gy for the 17 improvement candidates and 1.9Gy for the controls. Originally, several parotids violated the RTOG planning goal of V(30Gy)⩽50%. Eleven of these were improvement candidates, and re‐planning reduced this number to three. Re‐planning had no impact on the five control parotids that were violating the RTOG planning goal. According to the physician reviews, re‐planning did not degrade target coverage or OAR sparing. Conclusions: Our method offers a patient specific DVH evaluation for OARs, providing an effective mechanism of quantitative IMRT plan quality control.
MO‐D‐BRB‐04: Mixed Integer Models for Elucidating the Tradeoff Between Treatment Time and Plan Quality in VMAT Delivery36(2009); http://dx.doi.org/10.1118/1.3182214View Description Hide Description
Purpose: Rotational IMRTdelivery techniques, such as volumetric modulated arc therapy (VMAT), are alternatives to standard IMRTdelivery, claiming faster delivery and equally good dose distributions. In reality there is a cost to faster delivery times. The purpose of this investigation is to understand the tradeoff between plan quality and treatment time in rotational delivery by use of optimization models. Method and Materials: We study a simplified VMAT model for a 2D phantom and construct an exact mixed integer optimization model for it. The model includes treatment time as a constraint, and otherwise does not specify how the delivery is to be done, allowing for either VMAT or IMRT solutions. The model is too large to solve, but we trap the optimal solution between tight upper and lower bounds with additional models. The lower bound solutions are used to warm start the upper bound solutions. We construct Pareto surfaces for the joint tradeoffs of treatment time, tumor maximum dose, and organ at risk mean dose. Results: Upper bounds are within 10% of lower bound solutions. This allows us to conclusively demonstrate that 1) the treatment time constraint becomes more important when high intensity modulation is required to achieve optimal dose distributions, and 2) finding a good feasible VMAT plan is benefitted (by ∼5% in objective value) by warmstarting using the fluence modulation required at each angular location. Conclusion: For problems where the optimal dose distribution has non‐convex iso‐dose lines (to avoid nearby critical structures), the high modulation necessary to achieve such plans translates into longer treatment times. In these cases, when allowed treatment time is small, the optimal solutions show fluence being delivered at every angle — VMAT. If treatment time is allowed to be larger, IMRT plans are found to be optimal.
MO‐D‐BRB‐05: A Novel Dynamic MLC Leaf Sequencing Algorithm for 4D Treatment with Deformable Target Motion Correction36(2009); http://dx.doi.org/10.1118/1.3182215View Description Hide Description
Purpose: To investigate a dynamic MLC (DMLC) leaf sequencing method for the 4D IMRT delivery. Materials and methods: A 4D CT was obtained and 10 phase CTdata sets were created retrospectively. Individual IMRT plan was generated for each phase on Pinnacle treatment planning station, and a 4D plan was created using a deformable image registration technique. Based on the DMLC leaf sequences generated from the plan of each phase, the non‐rigid motion corrected 4D leaf sequence was created by matching the time indexes of the leaf sequences and the breathing cycles of each phase. The 4D leaf sequence was iteratively adjusted for the maximum leaf speed to avoid any beam hold‐offs. At the same time, a rigid motion corrected 4D leaf sequence was calculated by adjusting the leaf center corresponding to the center of mass position of the target in each phase. The opening density map (ODM) of the 4D plan was used to create a traditional no‐motion corrected leaf sequence. All three leaf sequences were delivered by Varian 2100 C/D with Millennium 120 leaf DMLC. The MatriXX (IBA Dosimetry) was used to measure the delivered fluence map every 40 ms. The delivered fluence maps were registered back to the patient's coordinate system. Results: Using the 4D plan ODM as reference, the fluence map from the non‐rigid motion corrected 4D leaf sequence showed a maximum dosimetric difference of 3%. However, the fluence maps from the rigid motion corrected 4D leaf sequence and the no‐motion corrected leaf sequence showed maximum dosimetric differences of 30%. Conclusion: For 4D plan DMLC delivery, the non‐rigid motion corrected leaf sequencing, the rigid motion corrected 4D leaf sequencing and the no‐motion corrected leaf sequencing were investigated. Results showed that the non‐rigid motion corrected leaf sequencing is the one that closest deliverers the 4D plan.
MO‐D‐BRB‐06: Comparison of Dosimetric Inaccuracies Introduced by Intra‐Fraction Respiratory Motion in IMRT and VMAT36(2009); http://dx.doi.org/10.1118/1.3182216View Description Hide Description
Purpose: To compare the dosimetric inaccuracies introduced by intra‐fraction respiratory motion in equivalent IMRT and VMAT plans. Method and Materials: First, a commercial respiratory motion platform was adapted to support the 3D dosimetric phantom. A slit‐field IMRT and equivalent VMAT plan were delivered to the phantom with one‐dimensional (S‐I direction) respiratory motion (cos4 pattern, peak‐to‐peak amplitude = 1.5cm, 15 BPM). A clinical patient IMRT plan and equivalent VMAT plan were also delivered to the moving phantom. The plans were delivered to the phantom with the motion platform both static and dynamic with a range of different starting phases in the respiratory cycle. Measurements were compared using the percent differences (PD) of the dynamic measurements relative to the static measurements. Results: The mean dynamic versus static target PD for non‐clinical IMRT and VMAT fractions were −6.4 ±11.9% and −4.2 ±6.7%, respectively. The mean dynamic versus static target PD for clinical lungIMRT and VMAT fractions were ‐1.1 ..4.2% and ‐0.9 ..3.3%, respectively. The PD of the clinical IMRT plan showed sensitivity toward the relative starting phase of the target, while the clinical VMAT plan did not. This suggested that the IMRT plan experienced a greater degree of the MLC interplay effect than the VMAT plan. After several fractions, IMRT and VMAT dosimetric inaccuracies due to respiratory motion became less distinguished, most notably for the IMRT plan. Conclusion: In the context of stereotactic radiosurgery, in which fewer fractions are delivered, VMAT may be particularly advantageous to reduce discrepancies introduced by intra‐fraction motion.
MO‐D‐BRB‐07: Retrospective RapidArc Dose Reconstruction Based On MLC Dynamic and Delivery Log Files Recorded During Treatment36(2009); http://dx.doi.org/10.1118/1.3182217View Description Hide Description
Purpose: To develop a methodology for retrospectively reconstructing the dose delivered to head‐and‐neck (HN) patients in RapidArc treatment based on dynamic log‐files which record the actual leaf positions, gantry angles, and delivered monitor units (MUs) during the RapidArc delivery. Method and Materials: After a RapidArc treatment was finished, two dynamic log‐files were retrieved from the linear accelerator: (1) MLC log‐file which recorded the actual leaf positions and respective gantry angles every 50 ms and (2) delivery log‐file which recorded the actual delivered MUs and gantry angles at the control points defined in the RapidArc plan. Through the common parameter of gantry angle recorded for both dynamic log files, the actual delivery status such as leaf positions, delivered dose indices, and gantry angles for every control points were re‐constituted. This data was compiled and converted into a DICOM radiotherapy plan (RP) file using in‐house developed software written in MatLab code (Mathworks, Natick, MA). The DICOM RP file was then imported into Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA) and the actual delivered dose was reconstructed on the on‐treatment CBCT acquired for the patient. Results: A retrospective dose reconstruction procedure has been established for RapidArc and applied to a phantom and two dummy HN cases. For the case in which the tumor shrinkage is minimal, the reconstructed and planned doses were consistent to within 3–5% in high dose region. The DVHs of the target and other organs do not have significant differences. However, large dosimetric changes (10–15%) were observed for the case with tumor shrinkage, indicating the need for re‐planning or adaptive measure to be taken. Conclusion: RapidArc dose reconstruction provides a pragmatic way to probe the actual dose delivery at a particular fraction and represents an indispensable step toward adaptive radiotherapy for this highly conformal treatment.
MO‐D‐BRB‐08: A Hybrid Strategy of Offline Adaptive Planning and Online Image Guidance for Prostate Cancer Radiotherapy36(2009); http://dx.doi.org/10.1118/1.3182218View Description Hide Description
Purpose: In online image‐guidedradiotherapy of prostate cancer, the setup error and inter‐fractional motion is eliminated through pre‐treatment imaging and couch correction at each fraction. However, the rotation and deformation is not corrected and only accounted for in planning margin. In this study, we propose a hybrid of online and offline adaptive image‐guidance strategy for both low risk patients (LRP, CTV=prostate) and intermediate risk patients (IRP, CTV=prostate+seminal vesicles). The benefit of margin reduction is evaluated geometrically. Method and Materials: Planning and treatment helical CTimages from 25 patients over 412 fractions were used. Online image‐guidance was simulated by matching the center of mass of CTV in treatmentCT to planning CT. Offline replanning was performed by constructing the internal target volume (ITV) from the union of position‐corrected CTVs from the first 5 treatment CTs. The volume overlap index (OI) of ITV and CTVs of the remaining fractions were compared with the OIs between PTV from planning CT and the treatment CTVs. Margins from 0 to 10 mm were investigated. Results: The mean ITV volumes are 62.5 and 90.1 cm3 for LRP and IRP, respectively, equivalent to 0.8 and 1.0 mm uniform margin to CTV0 (mean volumes of 55.7 and 78.0 cm3). The margins needed for 99% OI for ITV (V99% = 67.9 and 105.5 cm3) are 1.7 mm and 2.6 mm less than those for the planning CTV (V99% = 84.5 and 144.7 cm3) for LRP and IRP, respectively. Conclusion: The hybrid of online and offline adaptive radiotherapy protocol can effectively account for the patient‐specific interfraction organ motion and setup errors for prostate cancer patients. The planning margin can be reduced further using the hybrid strategy compared with online image guidance alone. Reduced irradiated volume will also lead to decreased toxicity in critical organs.
36(2009); http://dx.doi.org/10.1118/1.3182219View Description Hide Description
Purpose: A planning study was performed to investigate the geometry‐based adaption of a step and shoot IMRT‐plan. Method and Materials: Six cases with large rectum and prostate deformations were selected. A 9 field IMRT‐plan (A) was planned on a first CT(CT1). The plan fulfilled all requirements for prostate IMRT in our clinic and its quality was comparable to a conventional high‐quality step and shoot IMRT plan. For a second CT(CT2), three plans were considered: the original plan with optimized isocentre position (B), a newly optimised plan (C) and the original plan, adapted using optimization rules (D), based on a geometry‐based concept called “2‐Step IMRT”. Several DVH‐parameters were utilized for quantification of plan quality: CTV D99, central PTV D95, V95 for an outer PTV, V80 and V50 for rectum and bladder. Results: Unlike B, D achieved almost the same target coverage as plan C. For the OARs, the rectum V80 was slightly increased for the original plan. The volume with more than 95% of the target dose was 1.5 ± 1.5 cm3 for C, compared to 2.2 ± 1.3 cm3 for A in CT1 and 7.2 ± 4.8 cm3 in CT2. D resulted in 4.3 ± 2.1 cm3, an intermediate dose load to the rectum. All other parameters were comparable for C and D in contrast to the results from B. Conclusion: The first results for adaptation using the 2‐Step IMRT algorithm are encouraging. The plans were superior to plans with optimised isocentre position B and only marginally worse than a newly optimized plan C. Computerisation is needed to accelerate the procedure, which is currently performed manually. Checks have to be developed to allow an ad‐hoc application of the adapted plan.
- IMRT II
36(2009); http://dx.doi.org/10.1118/1.3182561View Description Hide Description
Purpose: In this work, the dosimetric consequences of respiratory motion on helical tomotherapy (HT) treatment delivery are comprehensively investigated. The dependence of dose errors on respiratory‐motion parameters: amplitude, period, phase, and motion direction is examined. The impact of characteristic breathing‐motion induced offsets and drifts on dose errors is also scrutinized, and the influence of target margin and field size on the results evaluated. Method and Materials:Treatment delivery into a moving patient was simulated by incorporating synthetic motion data in the HT dose calculation engine, which includes all relevant treatment dynamics such as MLC opening times, gantry rotation, and couch translations. This allowed for a realistic evaluation of the dosimetric sensitivity of over 12 HT plans (3 margins, 2 field sizes, two clinical lungcancer cases), retrospectively. The prescription dictated a minimum dose of 50 Gy delivered to 95% of the planning target volume (PTV). Results: For PTV equal to the internal target volume (ITV) (which, takes only breathing motion amplitude into account), the results show that mean regular breathing amplitudes engender GTV D95% errors less than 1.5 %, regardless of period, phase or motion direction. Meanwhile, average practical offsets generate less than 3.6 % GTV D95% errors. Results also show that for SI breathing motion, the GTV D95% dose error is virtually the same regardless of margin size. If the gantry period to breathing period ratio is an integer, GTV (or PTV) dose errors may sometimes be sensitive to the breathing phase. Overall, the dose error magnitude is plan specific, increases with increase in amplitude, offset or drift, and depends on motion direction and field size.Conclusion: The dosimetric impact of respiratory motion on HT treatment delivery is plan specific. However, for non phase‐sensitive plans with PTV equal ITV, average regular breathing motion and offsets engender insignificant dosimetric consequences.
36(2009); http://dx.doi.org/10.1118/1.3182562View Description Hide Description
Purpose: Important limitations for dose painting are due to treatment planning and delivery constraints. The purpose of this study was to determine the sensitivity of dose conformity to optimization parameters using the clinical TomoTherapy Hi‐Art treatment planning system (TPS). Materials and Methods: Uptake data from a patient who underwent a Cu‐ATSM PET/CT (surrogate of hypoxia) scan was retrospectively extracted for treatment planning. Extracted data was reformatted from voxel‐based to level‐based for compatibility with the Hi‐Art TPS. Optimized treatment plans were generated with physical objectives for each prescription sub‐volume modifying pitch, jaw width, modulation factor and iteration number. Effects of variations in plan parameters were simulated varying one physical parameter while keeping the other parameters unchanged. Avoidance structures were not used. The conformity of treatment plans to their non‐uniform prescriptions was evaluated via quality‐volume histograms (QVH) and percent receiving planned dose within 2 percent of prescription (Q0.98–1.02). Results: In general, the conformity of treatment plans to dose prescriptions was found to be adequate for delivery of dose painting plans. The conformity was better as the jaws decreased in width from 2.5 cm to 1.0 cm (Q0.98–1.02:64% vs 73%) and as the pitch increased from 0.122 to 0.443 (Q0.98–1.02:71% vs 73%). The effects on dose conformity of modulation factor and iteration number seem to be insensitive to change. Treatment delivery times varied from 5–11 minutes for changes in field width (2.5 cm to 1.0 cm) and from 11–25 minutes for changes in pitch (0.443 to 0.122). Conclusions: This investigation demonstrates the ability of the helical tomotherapy to create and deliver plans with non‐uniform dose distributions. Results indicate that agreement in prescription dose and planned dose distributions for all treatment plans are sensitive to physical parameter changes such as pitch and field width but insensitive to changes in modulation factor and iteration number.