A comparison of 6 MV and 10 MV photon beams for volumetric modulated arc therapy
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Purpose: To study the dosimetric differences of using 10 MV photons instead of 6 MV photons used for treatment plans incorporating VMAT. Methods and Materials: Ten distal esophageal cancer cases and ten prostate bed cancer cases that had been previously treated with 6 MV photons using VMAT at Roswell Park Cancer Institute were collected for a retrospective study. These cases were re-planned with 10 MV photons. Both sets of plans were created using the Varian Eclipse 11(TM) treatment planning system. Minimum dose to PTV was kept the same for both 6 MV and 10 MV cases. Number of arcs, collimator rotation angle, and gantry rotation angle were kept the same for both sets of plans. The same structure set was used for both the 6 MV and 10 MV plans. The 10 MV treatment plans incorporated additional structures that were used only for plan optimization. Esophageal cancer cases used a dose constraint list primarily adapted from TD 5/5 values, and prostate bed cancer cases used a dose constraint list adapted from RTOG report 0534. Results were collected for the OAR dose constraints, the dose to the target structures, the number of MUs for each arc, the Body V 5 , and the Conformity Index for each cancer site. The Body V 5 was recorded to evaluate the volume of healthy tissue that is irradiated by a low dose. The mean values were used to compare the 6 MV and 10 MV results. To determine the statistical significance of the 10 MV plans, a paired Student t test was performed. Results and Analysis: For the 10 esophageal cancer cases, using 10 MV resulted in a statistically significant lower mean for the OAR dose constraints for the liver, heart, stomach, esophagus, right kidney, and left kidney. Using 10 MV resulted in statistically significant lower mean MUs for each arc and a statistically significant lower mean hotspot. For the 10 prostate bed cancer cases, using 10 MV resulted in a statistically significant lower mean for the OAR dose constraints for the rectum, bladder-CTV, left femoral head, and right femoral head. Using 10 MV resulted in a statistically significant lower mean Body V 5 and a statistically significant lower mean Conformity Index. Conclusion: Based on the results obtained from the esophageal cancer cases and prostate bed cancer cases, there are dosimetric benefits to using 10 MV instead of 6 MV for VMAT based treatment plans. These benefits involve sparing more dose from the OAR located at each cancer site. Additionally, it was observed that for prostate bed cancer, 10 MV resulted in a more conformal treatment plan.