K Snyder

Development and evaluation of a clinical model for lung cancer patients using stereotactic body radiotherapy (SBRT) within a knowledge‐based algorithm for treatment planning

The purpose of this study was to describe the development of a clinical model for lung cancer patients treated with stereotactic body radiotherapy (SBRT) within a knowledge-based algorithm for treatment planning, and to evaluate the model performance and applicability to different planning techniques, tumor locations, and beam arrangements. 105 SBRT plans for lung cancer patients previously treated at our institution were included in the development of the knowledge-based model (KBM). The KBM was trained with a combination of IMRT, VMAT, and 3D CRT techniques. Model performance was validated with 25 cases, for both IMRT and VMAT. The full KBM encompassed lesions located centrally vs. peripherally (43:62), upper vs. lower (62:43), and anterior vs. posterior (60:45). Four separate sub-KBMs were created based on tumor location. Results were compared with the full KBM to evaluate its robustness. Beam templates were used in conjunction with the optimizer to evaluate the models ability to handle suboptimal beam placements. Dose differences to organs-at-risk (OAR) were evaluated between the plans generated by each KBM. Knowledge-based plans (KBPs) were comparable to clinical plans with respect to target conformity and OAR doses. The KBPs resulted in a lower maximum spinal cord dose by 1.0±1.6Gy compared to clinical plans, p=0.007. Sub-KBMs split according to tumor location did not produce significantly better DVH estimates compared to the full KBM. For central lesions, compared to the full KBM, the peripheral sub-KBM resulted in lower dose to 0.035 cc and 5 cc of the esophagus, both by 0.4Gy±0.8Gy, p=0.025. For all lesions, compared to the full KBM, the posterior sub-KBM resulted in higher dose to 0.035 cc, 0.35 cc, and 1.2 cc of the spinal cord by 0.2±0.4Gy, p=0.01. Plans using template beam arrangements met target and OAR criteria, with an increase noted in maximum heart dose (1.2±2.2Gy, p=0.01) and GI (0.2±0.4, p=0.01) for the nine-field plans relative to KBPs planned with custom beam angles. A knowledge-based model for lung SBRT consisting of multiple treatment modalities and lesion locations produced comparable plan quality to clinical plans. With proper training and validation, a robust KBM can be created that encompasses both IMRT and VMAT techniques, as well as different lesion locations. PACS number(s): 87.55de, 87.55kh, 87.53Ly.The purpose of this study was to describe the development of a clinical model for lung cancer patients treated with stereotactic body radiotherapy (SBRT) within a knowledge‐based algorithm for treatment planning, and to evaluate the model performance and applicability to different planning techniques, tumor locations, and beam arrangements. 105 SBRT plans for lung cancer patients previously treated at our institution were included in the development of the knowledge‐based model (KBM). The KBM was trained with a combination of IMRT, VMAT, and 3D CRT techniques. Model performance was validated with 25 cases, for both IMRT and VMAT. The full KBM encompassed lesions located centrally vs. peripherally (43:62), upper vs. lower (62:43), and anterior vs. posterior (60:45). Four separate sub‐KBMs were created based on tumor location. Results were compared with the full KBM to evaluate its robustness. Beam templates were used in conjunction with the optimizer to evaluate the models ability to handle suboptimal beam placements. Dose differences to organs‐at‐risk (OAR) were evaluated between the plans generated by each KBM. Knowledge‐based plans (KBPs) were comparable to clinical plans with respect to target conformity and OAR doses. The KBPs resulted in a lower maximum spinal cord dose by 1.0±1.6Gy compared to clinical plans, p=0.007. Sub‐KBMs split according to tumor location did not produce significantly better DVH estimates compared to the full KBM. For central lesions, compared to the full KBM, the peripheral sub‐KBM resulted in lower dose to 0.035 cc and 5 cc of the esophagus, both by 0.4Gy±0.8Gy, p=0.025. For all lesions, compared to the full KBM, the posterior sub‐KBM resulted in higher dose to 0.035 cc, 0.35 cc, and 1.2 cc of the spinal cord by 0.2±0.4Gy, p=0.01. Plans using template beam arrangements met target and OAR criteria, with an increase noted in maximum heart dose (1.2±2.2Gy, p=0.01) and GI (0.2±0.4, p=0.01) for the nine‐field plans relative to KBPs planned with custom beam angles. A knowledge‐based model for lung SBRT consisting of multiple treatment modalities and lesion locations produced comparable plan quality to clinical plans. With proper training and validation, a robust KBM can be created that encompasses both IMRT and VMAT techniques, as well as different lesion locations. PACS number(s): 87.55de, 87.55kh, 87.53Ly

Optimization of Treatment Geometry to Reduce Normal Brain Dose in Radiosurgery of Multiple Brain Metastases with Single–Isocenter Volumetric Modulated Arc Therapy

Treatment of patients with multiple brain metastases using a single-isocenter volumetric modulated arc therapy (VMAT) has been shown to decrease treatment time with the tradeoff of larger low dose to the normal brain tissue. We have developed an efficient Projection Summing Optimization Algorithm to optimize the treatment geometry in order to reduce dose to normal brain tissue for radiosurgery of multiple metastases with single-isocenter VMAT. The algorithm: (a) measures coordinates of outer boundary points of each lesion to be treated using the Eclipse Scripting Application Programming Interface, (b) determines the rotations of couch, collimator, and gantry using three matrices about the cardinal axes, (c) projects the outer boundary points of the lesion on to Beam Eye View projection plane, (d) optimizes couch and collimator angles by selecting the least total unblocked area for each specific treatment arc, and (e) generates a treatment plan with the optimized angles. The results showed significant reduction in the mean dose and low dose volume to normal brain, while maintaining the similar treatment plan qualities on the thirteen patients treated previously. The algorithm has the flexibility with regard to the beam arrangements and can be integrated in the treatment planning system for clinical application directly.

Deriving detector‐specific correction factors for rectangular small fields using a scintillator detector

The goal of this study was to investigate small field output factors (OFs) for flattening filter-free (FFF) beams on a dedicated stereotactic linear accelerator-based system. From this data, the collimator exchange effect was quantified, and detector-specific correction factors were generated. Output factors for 16 jaw-collimated small fields (from 0.5 to 2 cm) were measured using five different detectors including an ion chamber (CC01), a stereotactic field diode (SFD), a diode detector (Edge), Gafchromic film (EBT3), and a plastic scintillator detector (PSD, W1). Chamber, diodes, and PSD measurements were performed in a Wellhofer water tank, while films were irradiated in solid water at 100 cm source-to-surface distance and 10 cm depth. The collimator exchange effect was quantified for rectangular fields. Monte Carlo (MC) simulations of the measured configurations were also performed using the EGSnrc/DOSXYZnrc code. Output factors measured by the PSD and verified against film and MC calculations were chosen as the benchmark measurements. Compared with plastic scintillator detector (PSD), the small volume ion chamber (CC01) underestimated output factors by an average of -1.0%±4.9%(max.=-11.7% for 0.5×0.5cm2 square field). The stereotactic diode (SFD) overestimated output factors by 2.5%±0.4%(max.=3.3% for 0.5×1cm2 rectangular field). The other diode detector (Edge) also overestimated the OFs by an average of 4.2%±0.9%(max.=6.0% for 1×1cm2 square field). Gafchromic film (EBT3) measurements and MC calculations agreed with the scintillator detector measurements within 0.6%±1.8% and 1.2%±1.5%, respectively. Across all the X and Y jaw combinations, the average collimator exchange effect was computed: 1.4%±1.1% (CC01), 5.8%±5.4% (SFD), 5.1%±4.8% (Edge diode), 3.5%±5.0% (Monte Carlo), 3.8%±4.7% (film), and 5.5%±5.1% (PSD). Small field detectors should be used with caution with a clear understanding of their behaviors, especially for FFF beams and small, elongated fields. The scintillator detector exhibited good agreement against Gafchromic film measurements and MC simulations over the range of field sizes studied. The collimator exchange effect was found to be important at these small field sizes. Detector-specific correction factors were computed using the scintillator measurements as the benchmark. PACS number(s): 87.56.Fc.The goal of this study was to investigate small field output factors (OFs) for flattening filter‐free (FFF) beams on a dedicated stereotactic linear accelerator‐based system. From this data, the collimator exchange effect was quantified, and detector‐specific correction factors were generated. Output factors for 16 jaw‐collimated small fields (from 0.5 to 2 cm) were measured using five different detectors including an ion chamber (CC01), a stereotactic field diode (SFD), a diode detector (Edge), Gafchromic film (EBT3), and a plastic scintillator detector (PSD, W1). Chamber, diodes, and PSD measurements were performed in a Wellhofer water tank, while films were irradiated in solid water at 100 cm source‐to‐surface distance and 10 cm depth. The collimator exchange effect was quantified for rectangular fields. Monte Carlo (MC) simulations of the measured configurations were also performed using the EGSnrc/DOSXYZnrc code. Output factors measured by the PSD and verified against film and MC calculations were chosen as the benchmark measurements. Compared with plastic scintillator detector (PSD), the small volume ion chamber (CC01) underestimated output factors by an average of ‐1.0%±4.9%(max.=‐11.7% for 0.5×0.5cm2 square field). The stereotactic diode (SFD) overestimated output factors by 2.5%±0.4%(max.=3.3% for 0.5×1cm2 rectangular field). The other diode detector (Edge) also overestimated the OFs by an average of 4.2%±0.9%(max.=6.0% for 1×1cm2 square field). Gafchromic film (EBT3) measurements and MC calculations agreed with the scintillator detector measurements within 0.6%±1.8% and 1.2%±1.5%, respectively. Across all the X and Y jaw combinations, the average collimator exchange effect was computed: 1.4%±1.1% (CC01), 5.8%±5.4% (SFD), 5.1%±4.8% (Edge diode), 3.5%±5.0% (Monte Carlo), 3.8%±4.7% (film), and 5.5%±5.1% (PSD). Small field detectors should be used with caution with a clear understanding of their behaviors, especially for FFF beams and small, elongated fields. The scintillator detector exhibited good agreement against Gafchromic film measurements and MC simulations over the range of field sizes studied. The collimator exchange effect was found to be important at these small field sizes. Detector‐specific correction factors were computed using the scintillator measurements as the benchmark. PACS number(s): 87.56.Fc

Use of jaw tracking in intensity modulated and volumetric modulated arc radiation therapy for spine stereotactic radiosurgery

PURPOSE This study was conducted to evaluate the advantages of jaw tracking for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) in spine radiosurgery. METHODS AND MATERIALS VMAT and IMRT plans were retrospectively generated for 10 RTOG 0631 spine radiosurgery protocol patients. A total of 8 plans for each patient were created for a Varian TrueBeam equipped with a Millennium 120 multileaf collimator. Plans were created to compare IMRT and VMAT plans with and without jaw tracking, as well as with different flattening-filter-free energies: 6 MV unflattened (6U) and 10 MV unflattened (10U). The plans were prescribed to the 90% isodose line to either 16 or 18 Gy in 1 fraction. Planning target volume coverage, conformity index, dose to the spinal cord, and distance to falloff from the 90% to 50% isodose line were evaluated. Ion chamber and film measurements were performed to verify calculated dose distributions. RESULTS Jaw tracking decreased spinal cord dose for both IMRT and VMAT plans, but a larger decrease was seen with the IMRT plans (P = .004 vs P = .04). The average D(10%) for the spinal cord (dose that covered 10% of the spinal cord) was least for the 6U IMRT plan with jaw tracking and was greatest for the 10U IMRT plan without jaw tracking. Measurements showed greater than 98.5% agreement for planar dose gamma analysis and less than 2.5% for point dose analysis. CONCLUSIONS The addition of jaw tracking to IMRT and VMAT can decrease spinal cord dose without a change in calculation accuracy. A lower dose to the spinal cord was achieved with 6U than with 10U, although in some cases, 10U may be justified.

A prediction model of radiation‐induced necrosis for intracranial radiosurgery based on target volume

Purpose: This study aims to extend the observation that the 12 Gy‐radiosurgical‐volume (V12Gy) correlates with the incidence of radiation necrosis in patients with intracranial tumors treated with radiosurgery by using target volume to predict V12Gy. V12Gy based on the target volume was used to predict the radiation necrosis probability (P) directly. Also investigated was the reduction in radiation necrosis rates (ΔP) as a result of optimizing the prescription isodose lines for linac‐based SRS. Methods: Twenty concentric spherical targets and 22 patients with brain tumors were retrospectively studied. For each case, a standard clinical plan and an optimized plan with prescription isodose lines based on gradient index were created. V12Gy were extracted from both plans to analyze the correlation between V12Gy and target volume. The necrosis probability P as a function of V12Gy was evaluated. To account for variation in prescription, the relation between V12Gy and prescription was also investigated. Results: A prediction model for radiation‐induced necrosis was presented based on the retrospective study. The model directly relates the typical prescribed dose and the target volume to the radionecrosis probability; V12Gy increased linearly with the target volume (R2 > 0.99). The linear correlation was then integrated into a logistic model to predict P directly from the target volume. The change in V12Gy as a function of prescription was modeled using a single parameter, s (=−1.15). Relatively large ΔP was observed for target volumes between 7 and 28 cm3 with the maximum reduction (8–9%) occurring at approximately 18 cm3. Conclusions: Based on the model results, optimizing the prescription isodose line for target volumes between 7 and 28 cm3 results in a significant reduction in necrosis probability. V12Gy based on the target volume could provide clinicians a predictor of radiation necrosis at the contouring stage thus facilitating treatment decisions.

Generation and verification of QFix kVue Calypso-compatible couch top model for a dedicated stereotactic linear accelerator with FFF beams

This study details the generation, verification, and implementation of a treatment planning system (TPS) couch top model for patient support system used in conjunction with a dedicated stereotactic linear accelerator. Couch top model was created within the TPS using CT simulation images of the kVue Calpyso‐compatible couchtop (with rails). Verification measurements were compared to TPS dose prediction for different energies (6 MV FFF and 10 MV FFF) and rail configurations (rails in and rails out) using: 1) central axis point‐dose measurements with pinpoint chamber in water‐equivalent phantom at 42 gantry angles for various field sizes (2×2 cm2,4×4 cm2,10×10 cm2); and 2) Gafchromic EBT3 film parallel to beam in acrylic slab to assess changes in surface and percent depth doses in PA geometry. To assess sensitivity of delivered dose to variations in patient lateral position, measurements at central axis using the pinpoint chamber geometry were taken at lateral couch displacements of 2, 5, and 10 mm for 6 MV FFF. The maximum percent difference for point‐dose measurements was 3.24% (6 MV FFF) and 2.30% (10 MV FFF). The average percent difference for point‐dose measurements was less than 1.10% for all beam energies and rail geometries. The maximum percent difference between calculated and measured dose can be as large as 13.0% if no couch model is used for dose calculation. The presence of the couch structures also impacts surface dose and PDD, which was evaluated with Gafchromic film measurements. The upstream shift in the depth of dose maximum (dmax) was found to be 10.5 mm for 6 MV FFF and 5.5 mm for 10 MV FFF for ‘Rails In’ configuration. Transmission of the treatment beam through the couch results in an increase in surface dose (absolute percentage) of approximately 50% for both photon energies (6 MV FFF and 10 MV FFF). The largest sensitivity to lateral shifts occurred at the lateral boundary of the rail structures. The mean magnitude (standard deviation) of the deviation between shifted and centered measurements over all field sizes tested was 0.61% (0.61%) for 2 mm shifts, 0.46% (0.67%) for 5 mm shifts, and 0.86% (1.46%) for 10 mm shifts. PACS numbers: 87.56.‐v, 87.56.Da, 87.56.Fc

Characterization and evaluation of 2.5 MV electronic portal imaging for accurate localization of intra- and extracranial stereotactic radiosurgery

2.5 MV electronic portal imaging, available on Varian TrueBeam machines, was characterized using various phantoms in this study. Its low-contrast detectability, spatial resolution, and contrast-to-noise ratio (CNR) were compared with those of conventional 6 MV and kV planar imaging. Scatter effect in large patient body was simulated by adding solid water slabs along the beam path. The 2.5 MV imaging mode was also evaluated using clinically acquired images from 24 patients for the sites of brain, head and neck, lung, and abdomen. With respect to 6 MV, the 2.5 MV achieved higher contrast and preserved sharpness on bony structures with only half of the imaging dose. The quality of 2.5 MV imaging was comparable to that of kV imaging when the lateral separation of patient was greater than 38 cm, while the kV image quality degraded rapidly as patient separation increased. Based on the results of patient images, 2.5 MV imaging was better for cranial and extracranial SRS than the 6 MV imaging. PACS number(s): 87.57.C.2.5 MV electronic portal imaging, available on Varian TrueBeam machines, was characterized using various phantoms in this study. Its low‐contrast detectability, spatial resolution, and contrast‐to‐noise ratio (CNR) were compared with those of conventional 6 MV and kV planar imaging. Scatter effect in large patient body was simulated by adding solid water slabs along the beam path. The 2.5 MV imaging mode was also evaluated using clinically acquired images from 24 patients for the sites of brain, head and neck, lung, and abdomen. With respect to 6 MV, the 2.5 MV achieved higher contrast and preserved sharpness on bony structures with only half of the imaging dose. The quality of 2.5 MV imaging was comparable to that of kV imaging when the lateral separation of patient was greater than 38 cm, while the kV image quality degraded rapidly as patient separation increased. Based on the results of patient images, 2.5 MV imaging was better for cranial and extracranial SRS than the 6 MV imaging. PACS number(s): 87.57.C

MO-F-CAMPUS-T-01: Radiosurgery of Multiple Brain Metastases with Single-Isocenter VMAT: Optimizing Treatment Geometry to Reduce Normal Brain Dose

Purpose: To develop an optimization algorithm to reduce normal brain dose by optimizing couch and collimator angles for single isocenter multiple targets treatment of stereotactic radiosurgery. Methods: Three metastatic brain lesions were retrospectively planned using single-isocenter volumetric modulated arc therapy (VMAT). Three matrices were developed to calculate the projection of each lesion on Beam’s Eye View (BEV) by the rotating couch, collimator and gantry respectively. The island blocking problem was addressed by computing the total area of open space between any two lesions with shared MLC leaf pairs. The couch and collimator angles resulting in the smallest open areas were the optimized angles for each treatment arc. Two treatment plans with and without couch and collimator angle optimization were developed using the same objective functions and to achieve 99% of each target volume receiving full prescription dose of 18Gy. Plan quality was evaluated by calculating each target’s Conformity Index (CI), Gradient Index (GI), and Homogeneity index (HI), and absolute volume of normal brain V8Gy, V10Gy, V12Gy, and V14Gy. Results: Using the new couch/collimator optimization strategy, dose to normal brain tissue was reduced substantially. V8, V10, V12, and V14 decreased by 2.3%, 3.6%, 3.5%, and 6%, respectively. There were no significant differences in the conformity index, gradient index, and homogeneity index between two treatment plans with and without the new optimization algorithm. Conclusion: We have developed a solution to the island blocking problem in delivering radiation to multiple brain metastases with shared isocenter. Significant reduction in dose to normal brain was achieved by using optimal couch and collimator angles that minimize total area of open space between any of the two lesions with shared MLC leaf pairs. This technique has been integrated into Eclipse treatment system using scripting API.

MO-F-CAMPUS-T-04: Development and Evaluation of a Knowledge-Based Model for Treatment Planning of Lung Cancer Patients Using Stereotactic Body Radiotherapy (SBRT)

Purpose: To describe the development of a knowledge-based treatment planning model for lung cancer patients treated with SBRT, and to evaluate the model performance and applicability to different planning techniques and tumor locations. Methods: 105 lung SBRT plans previously treated at our institution were included in the development of the model using Varian’s RapidPlan DVH estimation algorithm. The model was trained with a combination of IMRT, VMAT, and 3D–CRT techniques. Tumor locations encompassed lesions located centrally vs peripherally (43:62), upper vs lower (62:43), and anterior vs posterior lobes (60:45). The model performance was validated with 25 cases independent of the training set, for both IMRT and VMAT. Model generated plans were created with only one optimization and no planner intervention. The original, general model was also divided into four separate models according to tumor location. The model was also applied using different beam templates to further improve workflow. Dose differences to targets and organs-at-risk were evaluated. Results: IMRT and VMAT RapidPlan generated plans were comparable to clinical plans with respect to target coverage and several OARs. Spinal cord dose was lowered in the model-based plans by 1Gy compared to the clinical plans, p=0.008. Splitting the model according to tumor location resulted in insignificant differences in DVH estimation. The peripheral model decreased esophagus dose to the central lesions by 0.5Gy compared to the original model, p=0.025, and the posterior model increased dose to the spinal cord by 1Gy compared to the anterior model, p=0.001. All template beam plans met OAR criteria, with 1Gy increases noted in maximum heart dose for the 9-field plans, p=0.04. Conclusion: A RapidPlan knowledge-based model for lung SBRT produces comparable results to clinical plans, with increased consistency and greater efficiency. The model encompasses both IMRT and VMAT techniques, differing tumor locations, and beam arrangements. Research supported in part by a grant from Varian Medical Systems, Palo Alto CA.

Evaluation and verification of the QFix EncompassTM couch insert for intracranial stereotactic radiosurgery

Abstract The QFix EncompassTM stereotactic radiosurgery (SRS) immobilization system consists of a thermoplastic mask that attaches to the couch insert to immobilize patients treated with intracranial SRS. This study evaluates the dosimetric impact and verifies a vendor provided treatment planning system (TPS) model in the Eclipse TPS. A thermoplastic mask was constructed for a Lucy 3D phantom, and was scanned with and without the EncompassTM system. Attenuation measurements were performed in the Lucy phantom with and without the insert using a pinpoint ion chamber for energies of 6xFFF, 10xFFF and 6X, with three field sizes (2 × 2, 4 × 4, and 6 × 6 cm2). The measurements were compared to two sets of calculations. The first set utilized the vendor provided Encompass TPS model (EncompassTPS), which consists of two structures: the Encompass and Encompass base structure. Three HU values for the Encompass (200, 300, 400) and Encompass Base (−600, −500, −400) structures were evaluated. The second set of calculations consists of the Encompass insert included in the external body contour (EncompassEXT) for dose calculation. The average measured percent attenuation in the posterior region of the insert ranged from 3.4%–3.8% for the 6xFFF beam, 2.9%–3.4% for the 10xFFF, and 3.3%–3.6% for the 6X beam. The maximum attenuation occurred at the region where the mask attaches to the insert, where attenuation up to 17% was measured for a 6xFFF beam. The difference between measured and calculated attenuation with either the EncompassEXT or EncompassTPS approach was within 0.5%. HU values in the EncompassTPS model that provided the best agreement with measurement was 400 for the Encompass structure and −400 for the Encompass base structure. Significant attenuation was observed at the area where the mask attaches to the insert. Larger differences can be observed when using few static beams compared to rotational treatment techniques.