C. Fraser

SU-D-204-07: Retrospective Correlation of Dose Accuracy with Regions of Local Failure for Early Stage Lung Cancer Patients Treated with Stereotactic Body Radiotherapy

PURPOSE To correlate dose distributions computed using six algorithms for recurrent early stage non-small cell lung cancer (NSCLC) patients treated with stereotactic body radiotherapy (SBRT), with outcome (local failure). METHODS Of 270 NSCLC patients treated with 12Gyx4, 20 were found to have local recurrence prior to the 2-year time point. These patients were originally planned with 1-D pencil beam (1-D PB) algorithm. 4D imaging was performed to manage tumor motion. Regions of local failures were determined from follow-up PET-CT scans. Follow-up CT images were rigidly fused to the planning CT (pCT), and recurrent tumor volumes (Vrecur) were mapped to the pCT. Dose was recomputed, retrospectively, using five algorithms: 3-D PB, collapsed cone convolution (CCC), anisotropic analytical algorithm (AAA), AcurosXB, and Monte Carlo (MC). Tumor control probability (TCP) was computed using the Marsden model (1,2). Patterns of failure were classified as central, in-field, marginal, and distant for Vrecur ≥95% of prescribed dose, 95-80%, 80-20%, and ≤20%, respectively (3). RESULTS Average PTV D95 (dose covering 95% of the PTV) for 3-D PB, CCC, AAA, AcurosXB, and MC relative to 1-D PB were 95.3±2.1%, 84.1±7.5%, 84.9±5.7%, 86.3±6.0%, and 85.1±7.0%, respectively. TCP values for 1-D PB, 3-D PB, CCC, AAA, AcurosXB, and MC were 98.5±1.2%, 95.7±3.0, 79.6±16.1%, 79.7±16.5%, 81.1±17.5%, and 78.1±20%, respectively. Patterns of local failures were similar for 1-D and 3D PB plans, which predicted that the majority of failures occur in centraldistal regions, with only ∼15% occurring distantly. However, with convolution/superposition and MC type algorithms, the majority of failures (65%) were predicted to be distant, consistent with the literature. CONCLUSION Based on MC and convolution/superposition type algorithms, average PTV D95 and TCP were ∼15% lower than the planned 1-D PB dose calculation. Patterns of failure results suggest that MC and convolution/superposition type algorithms predict different outcomes for patterns of failure relative to PB algorithms. Work supported in part by Varian Medical Systems, Palo Alto, CA.

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.

Changes in pharyngeal constrictor volumes during head and neck radiation therapy: Implications for dose delivery

Objective: The objective of this study was to evaluate the anatomical changes and associated dosimetric consequences to pharyngeal constrictor muscles (PCMs) that occur during head and neck (H and N) radiotherapy (RT). Materials and Methods: A cohort of 13 oropharyngeal cancer patients with daily cone beam computed tomography (CBCT) was retrospectively studied. On every 5th CBCT image, PCM was manually delineated by a radiation oncologist. The anterior-posterior PCM thickness was measured at the midline level of C3 vertebral body. Delivered dose to PCM was estimated by calculating dose on daily images and performing dose accumulation on corresponding planning CT images using a parameter-optimized B-spline-based deformable image registration algorithm. The mean and maximum delivered dose (Dmean, Dmax) to PCM were determined and compared with the corresponding planned quantities. Results: The average (±standard deviation) volume increase (ΔV) and thickness increase (Δt) over the course of 35 total fractions were 54 ± 33% (11.9 ± 7.6 cc) and 63 ± 39% (2.9 ± 1.9 mm), respectively. The resultant cumulative mean dose increase from planned dose to PCM (ΔDmean) was 1.4 ± 1.3% (0.9 ± 0.8 Gy), while the maximum dose increase (ΔDmax) was 0.0 ± 1.6% (0.0 ± 1.1 Gy). Patients who underwent adaptive replanning (n = 6) showed a smaller mean dose increase than those without (n = 7); 0.5 ± 0.2% (0.3 ± 0.1 Gy) versus 2.2 ± 1.4% (1.4 ± 0.9 Gy). There were statistically significant (P = 0.001) strong correlations between ΔDmean and Δt (Pearson coefficient r = 0.78), as well as between ΔDmean and ΔV (r = 0.52). Conclusion: The patients underwent considerable anatomical changes to PCM during H and N RT. However, the resultant increase in dose to PCM was minor to moderate. PCM thickness measured at C3 level is a good predictor for the mean dose increase to PCM.

SU‐E‐T‐651: Radiobiological Effect of Target Volume in SBRT of Lung Tumor: Comparison of Treatment Planning Algorithms Between Pencil Beam Algorithm and Monte Carlo Method

Purpose: To use equivalent uniform dose (EUD) and tumorcontrol probability (TCP) to retrospectively analyze the radiobiological effect of target volumes in patients with NSCLC planned and treated with Stereotactic Body Radiotherapy(SBRT). Methods: Eighty‐three stage I–IIlungcancer patients with 86 lesions treated with SBRT were retrospectively analyzed. For each patient, a Pencil Beam (PB) algorithm‐based treatment plan with a dose regimen of 12 Gy/fraction in 4 fractions was generated. To overcome the known uncertainties of conventional PB algorithm in lungtissue,Monte Carlo(MC) treatment plans were also created in the iPlan (BrainLab) system using the same monitor units derived from the PB‐based plan. Niemierkos EUD and TCP (Poisson model) were computed using different surviving fraction (SF) parameters for each dose calculation algorithm. The radiobiological effects of target volume were analyzed by correlating EUD and TCP with PTV volumes. Results: Mean PTV volume was 39.31 +/− 28.96 cc. The mean PB EUDs were 50.61, 50.60, 50.57 and 50.55 Gy for SF parameter values of 0.36, 0.34, 0.3 and 0.28, compared with MC EUDs 43.97, 43.84, 43.56 and 43.41 Gy. The mean PB TCP values were up to 18% higher than the mean MC TCP. EUDs calculated using both PB and MC were not sensitive to SF parameters, whereas they were for TCP calculation. Overall, MC EUDs were more sensitive to PTV volumes than PB EUDs, measured by Pearson correlation 0.46 vs. 0.07. Larger PTV volume decreased PB TCP values while this was not the case for MC TCP Conclusions: This work demonstrates encouraging evidence that radiobiological effect of target volume and dose calculation algorithm selection is significant in EUD and TCP estimations. Further studies confirming this relationship and relating to treatment outcomes are warranted. Work supported in part by NIH R01 CA106770

SU-FF-J-112: Accurate Targeting Breast Cancer in Real-Time Stereovision-Guided Radiotherapy

Purpose: A recent study by Korreman et al (IJROBP, vol 65, 1375–1380, 2006) demonstrates possible >85% relative reduction of cardiac‐pulmonary complications for tangential breast irradiation by using deep inspiration breath‐hold or gating techniques. The aim of this work is to minimize the risks by correcting the breast displacements using real‐time stereovision guidance. Materials/Methods: Twenty breast‐cancer patients were accrued over the last seventeen months on an IRB‐approved study. Planning target volume (PTV) was encompassed by the prescribed isodose in tangential beams with intensity modulation. The surgical bed was boosted using an electron field or conformal photon beams. The planning information (include CTimages) were transferred to an in‐house stereovision‐guided program which automatically created the reference surfaceimages from the CT‐based plans. The reference surfaces were matched with the real‐time surfaceimages, captured with 3D cameras mounted in the CT simulation room and treatment vault, to determine the displacements of CT setup markers, daily initial setup isocenter, final target position, and target motion during treatment. Large and significant position error were corrected based the image‐guidance.Images and on‐line adjustments were automatically stored for the analysis. Portal images were taken weekly for the position verification. Results: The 3D isocenter displacements improved from ∼10 mm to ∼ 4 mm after the application of IGRT. Isocenter setup error and simulation‐marking error were detected and corrected. All major changes were confirmed with portal images. The intrafractional motion caused only ∼2.0 mm target displacement. The entire procedure (including the table shifts) took < 5 minutes per day. Conclusions: The clinical results demonstrate improvement for breast target positioning using stereovision guidance. This in‐house IGRT for BC is clinically feasible, efficient, and accurate. Research initially sponsored by NIH SBIR‐1R43CA91690‐01 and CA‐88843 grants.