Purpose: To compare prostate volumetrically modulated arc therapy (VMAT) treatments planned with 10MV vs 18MV photon beams to determine the practicality of 18 MV VMAT treatments. Recent studies indicate significantly lower than previously reported neutron doses associated with high-energy photon beams.
Methods: Clinically delivered prostate patient treatments planned using a 10MV photon VMAT technique were selected and re-planned with 18MV photon beams. Both sets of plans were made to meet the same clinical goals and follow the same optimization constraints. The resultant plans were then compared across various criteria such as modulation factor, 50% isodose volume, and average dose to OARs. All treatments were planned in RayStation v8 utilizing clinically accepted beam model data.
Results: All selected plans had the same prescription, and were treated on the same treatment machine. On average, 18MV photon plans were delivered with a ~17% lower modulation factor than the corresponding 10MV plans. The average 50% isodose volume of 18MV plans was found to be ~10 % larger than that of the 10MV treatments and average doses to the bladder and rectum were also larger on average. It was consistently observed however that in plans designed to treat larger patients 18MV photon plans were more likely to outperform their 10MV counterparts.
Conclusion: On average treatments planned with 10 MV photons proved to be superior in 50% isodose volume and OAR averages doses. For larger patients 18MV prostate treatments are likely to produce dosimetrically preferable plans. Criteria used to distinguish that patient population will be presented. Since high-energy intensity-modulated treatments for prostate therapy may not be as detrimental as previously estimated, their place in clinical radiation therapy has to be firmly established.