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AIM: The primary objective was to assess set-up errors (SE) and secondary objective was to determine optimal safety margin (SM). BACKGROUND: To evaluate the SE and its impact on the SM utilizing electronic portal imaging (EPI) for pelvic conformal radiotherapy. MATERIAL AND METHODS: 20 cervical cancer patients were enrolled in this prospective study. Supine position with ankle and knee rest was used during CT simulation. The contouring was done using consensus guideline for intact uterus. 50â¯Gy in 25 fractions were delivered at the isocenter with ≥95% PTV coverage. Two orthogonal (Anterior and Lateral) digitally reconstructed radiograph (DRR) was constructed as a reference image. The pair of orthogonal [Anterior-Posterior and Right Lateral] single exposure EPIs during radiation was taken. The reference DRR and EPIs were compared for shifts, and SE was calculated in the X-axis, Y-axis, and Z-axis directions. RESULTS: 320 images (40 DRRs and 280 EPIs) were assessed. The systematic error in the Z-axis (AP EPI), X-axis (AP EPI), and Y-axis (Lat EPI) ranged from -12.0 to 11.8â¯mm, -10.3 to 7.5â¯mm, and -8.50 to 9.70â¯mm, while the random error ranged from 1.60 to 6.15â¯mm, 0.59 to 4.93â¯mm, and 1.02 to -4.35â¯mm. The SM computed were 7.07, 6.36, and 7.79â¯mm in the Y-axis, X-axis, and Z-axis by Van Herk's equation, and 6.0, 5.51, and 6.74â¯mm by Stroom's equation. CONCLUSION: The computed SE helps defining SM, and it may differ between institutions. In our study, the calculated SM was approximately 8â¯mm in the Z-axis, 7â¯mm in X and Y axis for pelvic conformal radiotherapy.
RESUMO
Background: Radiobiological model-based studies of photon-modulated radiotherapy for pancreatic cancer have reported reduced gastrointestinal (GI) toxicity, although the risk is still high. The purpose of this study was to investigate the potential of 3D-passive scattering proton beam therapy (3D-PSPBT) in limiting GI organ at risk (OAR) toxicity in localized pancreatic cancer based on dosimetric data and the normal tissue complication probability (NTCP) model. Methods: The data of 24 pancreatic cancer patients were retrospectively analyzed, and these patients were planned with intensity-modulated radiotherapy (IMRT), volume-modulated arc therapy (VMAT), and 3D-PSPBT. The tumor was targeted without elective nodal coverage. All generated plans consisted of a 50.4-GyE (Gray equivalent) dose in 28 fractions with equivalent OAR constraints, and they were normalized to cover 50% of the planning treatment volume (PTV) with 100% of the prescription dose. Physical dose distributions were evaluated. GI-OAR toxicity risk for different endpoints was estimated by using published NTCP Lyman-Kutcher-Burman (LKB) models. Analysis of variance (ANOVA) was performed to compare the dosimetric data, and ΔNTCPIMRT-PSPBT and ΔNTCPVMAT-PSPBT were also computed. Results: Similar homogeneity and conformity for the clinical target volume (CTV) and PTV were exhibited by all three planning techniques (P > 0.05). 3D-PSPBT resulted in a significant dose reduction for GI-OARs in both the low-intermediate dose range (below 30 GyE) and the highest dose region (D max and V 50 GyE) in comparison with IMRT and VMAT (P < 0.05). Based on the NTCP evaluation, the NTCP reduction for GI-OARs by 3D-PSPBT was minimal in comparison with IMRT and VMAT. Conclusion: 3D-PSPBT results in minimal NTCP reduction and has less potential to substantially reduce the toxicity risk of upper GI bleeding, ulceration, obstruction, and perforation endpoints compared to IMRT and VMAT. 3D-PSPBT may have the potential to reduce acute dose-limiting toxicity in the form of nausea, vomiting, and diarrhea by reducing the GI-OAR treated volume in the low-to-intermediate dose range. However, this result needs to be further evaluated in future clinical studies.