SU-E-T-602: Comparison of Treatment Techniques for Pelvic Node Irradiation: Intensity-Modulated Photons versus...

Radiotherapy planning for iliac pelvic nodes can be challenging due to the close proximity of sensitive healthy tissues such as the bowel and rectum. Modern treatment techniques like photon intensity-modulated radiotherapy (IMRT) and intensity-modulated proton therapy (IMPT) offer improved healthy tissue sparing for similar target coverage. In this study we compare IMRT and IMPT plans for six post-cystectomy patients.

A dose of 50.4 Gy was prescribed to the planning target volume (PTV), which for IMRT is the clinical target volume (CTV) plus a 5 mm expansion for geometric uncertainties due to CTV and patient positioning errors, and for proton beams is the CTV plus the lateral 5 mm margin plus an additional longitudinal margin to allow for the proton range uncertainty. The optimization objectives are: 98% of the PTV receive at least 95% of the prescription, target maximum dose = 107% of prescription, rectum V[40Gy] < 30% and max = 105%, and bowel V[45Gy] < 125 cc and max = 107%. All IMRT and IMPT plans are made to achieve the target coverage objective.

Using IMPT, the rectum would receive a mean dose of 9.0 Gy with an average (over the six patients) maximum dose of 38.1 Gy. Using IMRT, the rectum would receive a mean dose of 13.0 Gy and an average maximum dose of 37.6 Gy. The IMPT plans give a mean dose of 17.9 Gy and a maximum dose of 53.4 Gy for the bowel, whereas the IMRT plans give a mean dose of 23.8 Gy and a maximum dose of 53.2 Gy. Both the rectum and bowel show slightly lower mean doses for IMPT.

Our results indicate that IMPT plans improve normal tissue sparing as compared to IMRT plans and provide adequate dose coverage of the target volume.