Purpose: External beam radiation therapy has gained popularity in treating prostate tumors. This study aims to re-evaluate the rectum dose constraints from conventional prostate treatment plans.
Methods: RTOG 0924 protocol has been used as the guideline for the treatment plan with dose prescription, rectum contour and dose constraints. There were 41 patients treated during the period from May 2016 to Feb 2018 following RTGO 0924 protocol. Assume the proctitis is related to the relative high dose area in the rectum wall during the prostate treatment. Three volumes were used for the dose constraints re-evaluation: PTV prostate, Rectum and Rectum wall (R-wall-5mm-3cm) defined as 5mm outer wall of rectum structure within 3cm from the PTV prostate. RTOG 0924 recommended the rectum dose constraints as relative volume dose constraints such as V70<15%. As individual prostate patient has different prostate volume (also different overlap length with rectum in SI direction) and different total rectum volume, patient-specific rectum dose constraints were made here and used for re-evaluation.For all 41 prostate patients, the volumes of PTV prostate, R-wall-5mm-3cm and Rectum are 146.10cc, 37.75cc and 99.41cc respectively. The ratio of R-wall-5mm-3cm and PTV prostate volume and ratio of rectum and PTV prostate volume were used for patient specific rectum dose constraints re-evaluation
Results: The ratio of R-wall-5mm-3cm and PTV prostate volume is 0.29±0.09. And the ratio of rectum and PTV prostate volume is 0.80±0.50. The second ratio is in a very large random range. Therefore, it’s hard to predict the side effect based on the relative volume dose constraints.
Conclusion: This study showed that, for prostate radiation therapy, 1) patient-specific dose constraints are vital for individual patients and 2) the organ at risk for the dose constraints need re-evaluation.