Volume 33, Issue 6, June 2006
Index of content:
- Therapy Scientific Session: Room 224A
- Stereotactic, Single and Hypofractionated Treatment II
TU‐E‐224A‐01: Evaluation of Heterogeneity Corrections Algorithms Through the Irradiation of a Lung Phantom33(2006); http://dx.doi.org/10.1118/1.2241622View Description Hide Description
Purpose: To evaluate the impact of applying heterogeneity corrections to the calculation of prescribed doses to a target located within the lung.Method and Materials: The Radiological Physics Center's (RPC) anthropomorphic lung phantom was sent to institutions nationwide. This phantom simulates a patient not only in dimensions but also in densities for imaging and treatment purposes. This design includes two lungs with density of 0.33 g/cm3 and a target centrally located in the left lung with density near 1 g/cm3. TLD and radiochromic films were used as dosimeters within and near the target region. Institutions that received the phantom were requested to image, plan and treat the phantom as if a patient. The prescription dose, based on a stereotactic plan, was 20 Gy to the target, calculated without applying heterogeneity corrections. The institutions were asked to submit both the homogeneous and heterogeneity corrected treatment plans using the same number of monitor units. Results: Twenty‐one irradiations, mostly with 6 MV x‐rays, were analyzed from 7 different Treatment Planning Systems (TPS). The ratio of dose to the target from the plan with to the plan without heterogeneity corrections was calculated and analyzed based on the algorithms used for the heterogeneity correction. A comparison of corrected dose given by the TPS and dose given by TLD was performed. The average ratio between dose with to dose without the heterogeneity correction was 1.18 with values ranging from 1.12 to 1.21. The superposition convolution algorithms agreed better with measurements than the other algorithms studied. The average TLD/Inst dose ratio in the target was 0.97 ranging from 0.92 to 0.99. Conclusions: There continues to be a differences in the heterogeneity corrected tumordoses within the lung from different planning systems.
Work supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.
TU‐E‐224A‐02: Dosimetry of Very Small (1.5 and 3 Mm Diameter) Photon Beams: Diode and Film Measurements Versus Monte Carlo Calculations33(2006); http://dx.doi.org/10.1118/1.2241623View Description Hide Description
Purpose: To measure the dosimetric parameters of 10 MV photon beams in 1.5 and 3 mm diameter fields with a small‐field diode, radiographic XV film, and radiochromic HS film, and to compare measured data with the same parameters calculated by Monte Carlo(MC) simulations. Methods and Materials: A 10‐MV Clinac‐18 linac was used as the radiation source and the very small diameter fields were set up with radiosurgical collimators. PDDs, profiles, and output factors of the very small photon beams were measured with diode/water tank and film/scanner techniques. The measured parameters were compared against those calculated by MC simulations using an experimentally determined circular source diameter of 1.5 mm in beam modeling. For improved accuracy, the PDDs were measured with the diode and the water tank using beam profile scans. A document scanner was used as film densitometer to offer high spatial resolution (254 lpi) required in very small photon field dosimetry.Results: PDDs measured by the diode agree well with the MC‐calculated results, within ± 2% and ± 3% in 3 and 1.5 mm fields, respectively. Lateral profiles measured by the diode and film generally agree with MC calculations, but significant discrepancies are observed in the tail portion of the 1.5‐mm beam profile. Relative dose factors obtained by averaging the diode, HS film, and MC results are 0.22 ± 0.01 and 0.43 ± 0.01 for the 1.5 and 3 mm fields, respectively. Conclusion: Based on the good agreement between the measured and MC‐calculated dosimetric parameters for the 1.5 and 3 mm diameter, 10 MV beams, we conclude that MC calculations can be used in general for dosimetry of very small photon fields, provided that the physical source size of the linac is correctly measured and used in the MC simulations.
TU‐E‐224A‐03: Image and Dosimetric Verification of Positioning Accuracy for Helical Tomotherapy Intensity Modulated Stereotactic Radiosurgery33(2006); http://dx.doi.org/10.1118/1.2241624View Description Hide Description
Purpose: To investigate the accuracy of Helical Tomotherapy for intracranial radiosurgery using Mega‐voltage CT (MVCT) image fusion. Method and Materials: Tomotherapy generates a set of MVCT images which are fused to the treatment planning KVCT to facilitate the patient setup. For intracranial lesions, mutual information matching program using bony anatomy automatically generates the lateral, longitudinal, vertical, pitch, roll and yaw adjustments that are necessary to align the MVCT to KVCT images. In our study, a pReference head phantom immobilized by Nomos Talon system was used to verify the positioning accuracy of MVCT fusion. Three gold‐filled titanium markers inside the head phantom and the localization tip in the Talon system were used as the independent imaging makers to verify the MVCT positioning accuracy. In addition, dosimetric analysis was performed with a 0.015cc pinpoint ion chamber placed inside the phantom during KVCT simulation. A tomotherapy SRS plan with ion chamber sensitive volume as the target was generated and delivered. After MVCT to KVCT auto registration, dosimetric profiles in three directions were measured by stepping the couch away from the registered position. If the registration was perfect, the dose profile peak position should correspond to the registered position. The distance from the maximum dose to the registered location provides the ultimate accuracy of the system, including MVCT imaging accuracy and radiation dose delivery accuracy. Results: The average localization differences between the MVCT and KVCT were 0.92mm in lateral, 0.82mm in longitudinal and 0.60mm in vertical directions. Dosimetric measurements showed 1.3mm offset laterally and 0mm offset in the other two directions. The relative large setup error in the lateral direction is partially due to the manual couch adjustment with mechanical scale in that direction. Conclusions: The MVCT image fusion can be used to setup SRS patients within accuracy comparable to the current SRS standard.
TU‐E‐224A‐04: Determination of Total Scatter Factors for Stereotactic Radiation Fields by Optimized Fitting of Readings From a Small Ion Chamber and a Mini‐Diode33(2006); http://dx.doi.org/10.1118/1.2241625View Description Hide Description
Purpose: To determine the total scatter factors (TSF) for small radiation fields on the BrainLab micro‐MLC (M3) and cones by fitting the readings from two detectors: a small ion chamber and a mini‐diode. Method and Materials: Outputs were measured for 6MV beams using a small ion chamber (Wellhofer IC3, active volume 28mm3) and a stereotactic diode (Scanditronix DEB050, active volume 0.017mm3). It has been reported that ion chambers (IC) underestimate output for small fields (<20 mm) and diodes overestimate output for large fields (>50mm). Herein outputs at intermediate field sizes (20–50 mm) were used to combine data from the two detectors. The output from IC was normalized to a reference 10×10cm field; the output from diode was scaled to match the output from IC at intermediate field sizes. The scaling was determined with modified least square optimization. The final TSF was composed of scaled diode output for small fields, IC output for large fields, and their averages for intermediate fields. Results: For square fields formed by M3 and secondary jaws, the scaled TSF from diode matched the TSF from IC to within ±0.6% for field sizes 24–60 mm. The diode overestimated TSF of the reference field by 3.2% at depth of 5cm. The IC underestimated TSF by 23%, 5%, 1.4% for 6, 12, 18 mm fields, respectively. Similar results were found for the cones. TSF from diode was 0.702 for the 5mm cone, an improvement of 29% from IC measurement. Conclusion: We have demonstrated that neither IC or diode alone provides accurate TSF for stereotactic fields of all sizes. The diode is more accurate than IC for fields <20 mm, however, we show that the diode should be cross‐compared with an IC using radiation fields of intermediate sizes. A modified least square optimization method is presented for this purpose.
TU‐E‐224A‐05: Intermediate Energy X‐Ray Photons (0.2 – 1.0 MeV) for Radiosurgery: Producing a Beam and Measurement of Radiological Penumbra33(2006); http://dx.doi.org/10.1118/1.2241626View Description Hide Description
Purpose: The main advantage of stereotactic radiosurgery is the steep dose gradient associated with the intracranial dose distribution. We are examining the effects of x‐ray energy on penumbra and dose gradient. Specifically, we are exploring the reduction in radiological penumbra for intermediate energy x‐ray photons (0.2–1.0 MeV) or so called IEP's. The purpose of this work is two‐fold: 1.) to produce an IEP beam using a medicallinear accelerator and 2.) to examine the radiological penumbra associated with this beam. Method and Materials: A Siemens medicallinear accelerator was adapted to produce IEP's. PDD measurements versus depth (SSD=100cm,FS=2×2cm2) were done in solid water using a Markus parallel plate ionization chamber (PTW Freiburg). These were compared with Monte Carlocomputer simulations (MCNP‐4C). Monte Carlo involved generating x‐ray spectra that impinged upon a SW phantom. A penumbra measurement device (consisting of a half‐beam block) was constructed to examine radiation beam edge profiles using film (Gafchromic EBT) at SSD=100cm,FS=1.1cm2 and depth=2cm. In a separate experiment, film irradiations were done collimating a 3×3mm2 beam using a 10cm thick brass block flush with the SW surface. In all cases, the geometric penumbra (due to the finite source size) was made negligible by having the collimation very close to the phantom. A high‐resolution digital microscope (Axiomat) was used to acquire film profiles. Results: Measured PDD values were 55.4%(surface), 62.6%(5cm), and 34.7%(10cm). Monte Carlo PDD's compared with measurement suggest a nominal 800kV x‐ray beam. For the half‐beam block, the 80%–20% film edge profiles were 0.345mm (IEP) and 2.10mm (6MV). For the 3×3mm2 field, there was a 5‐fold reduction in radiological penumbra (IEP vs. 6MV). Conclusions: A novel intermediate energy photon beam (of nominal energy 800kV) has been produced using a conventional linear accelerator. There is a substantial reduction in radiological penumbra when using IEP's with small fields.
TU‐E‐224A‐06: Frameless Radiosurgery Using Stereoscopic X‐Ray Guidance: System Characteristics and 2 Year Clinical Experience33(2006); http://dx.doi.org/10.1118/1.2241627View Description Hide Description
Purpose: To evaluate targeting capabilities of a system for image guided “frameless” stereotactic irradiation. Method and Materials: System accuracy was investigated using an anthropomorphic head phantom into which a “hidden target” (radio‐opaque sphere) was inserted. The target was identified on planning CTimages and the phantom was fixed to the treatment couch. To align the target to isocenter, stereoscopic kV x‐rays were fused to planning DRRs. AP and lateral verification films were exposed using a 10 mm circular collimator, and the offset of the sphere within the radiation field was recorded. The entire process was repeated 50 times in order to achieve an accurate assessment of positioning capabilities. Retrospective data was evaluated from patients having undergone nearly 600 x‐ray guided single and multi‐fraction stereotactic irradiation procedures. From this data the following were investigated: the nature and magnitude of systematic and random positioning errors present in conventional procedures, confidence limits on the reproducibility of the mask immobilization device used for multi‐fraction treatments, and the potential for replacing rigid head fixation with image guided positioning. Results: In phantom studies, a vector displacement of σ = 0.39 mm relative to the “perfect” isocenter was observed. Based on a sample population of 50, this provides assurance of accuracy at the 95% confidence level.
Improvement in positioning accuracy was observed in multi‐fraction procedures; analysis of 565 fractions showed a mean vector deviation σ = 2.65 mm relative to traditional methods. The largest component of this discrepancy was in the superior/inferior direction. In comparison, a mean vector deviation σ = 1.62 mm was observed in patients for whom rigid fixation was used. Conclusions: Stereoscopic x‐ray imaging is an accurate positioning method for cranial stereotactic irradiation. The system is as accurate as rigid frame‐based methods and can improve the reproducibility of fractionated delivery.
TU‐E‐224A‐07: Evaluation of Dose Calculation of SRT/IMRT for Small Lung Lesions Using Monte Carlo Simulations33(2006); http://dx.doi.org/10.1118/1.2241628View Description Hide Description
Purpose: To assess the heterogeneity effect on stereotactic radiotherapy (SRT) of small lung lesions using Monte Carlo(MC) simulations and to evaluate the accuracy of dose calculation in a commercial treatment planning system (TPS) (Radionics, XKnifeRT) for SRT planning. Method and Materials: Five patients were randomly selected for this study. For these patients, the sizes of the planning treatment volume (PTV) ranged from 4.2 and 36 cc, and the average densities of the GTV and the ipsilateral lung ranged from 0.659 to 0.93 g/cm3 and from 0.244 to 0.358 g/cm3, respectively. The SRT treatment plans (9 photon beams) for these patients were first generated by the TPS and then recalculated by a MCdose calculation system with the same beam configuration and beam weights as in the TPS. Comparisons between the MC and the TPS calculations were made to assess the differences in isodose distributions, median dose (D50), maximum dose (defined as D1) and minimum dose (defined as D99). Results:Dose indices of D1, D50 and D99 calculated by the TPS for all patients are found to be significantly larger than those of the MC calculations for the PTV. The degree of dose overestimation by the TPS increases with decreasing target volume and target and ipsilateral lung densities. Specifically, for PTV volume sizes from 36 to 4.2 cc, the dose calculated from the TPS in D1, D50, and D99 are overestimated by up to 24.7 %, 32.3%, and 38.9% respectively. Conclusions: Although the TPS can produce accurate (3%/3mm) treatment plans for homogeneous geometry and for large target volumes in the lung, for small lung lesions the dose calculated by the TPS can be significantly overestimated due to inaccurate heterogeneity corrections.