Pelvic Lymph Node Irradiation Including Pararectal Sentinel Nodes for Prostate Cancer Patients: Treatment Optimization Comparing Intensity Modulated X-rays, Volumetric Modulated Arc Therapy, and Intensity Modulated Proton Therapy

We aimed to assess the dosimetric impact of advanced delivery radiotherapy techniques using either intensity modulated x-ray beams (IMXT), volumetric modulated arc therapy (VMAT), or intensity modulated proton therapy (IMPT), for high-risk prostate cancer patients with sentinel nodes in the pararectal region. Twenty high-risk prostate cancer patients were included in a prospective trial evaluating sentinel nodes on pelvic SPECT acquisition. To be eligible for the dosimetric study, patients had to present with pararectal sentinel nodes usually not included in the clinical target volume encompassing the pelvic lymph nodes. Radiotherapy-plans including the prostate, the seminal vesicles, and the pelvic lymph nodes with the pararectal sentinel nodes were optimized for 6 eligible patients. IMXT and IMPT were delivered with 7 and 3 beams respectively and VMAT with 2 arcs. Results were assessed with Dose-Volume Histograms and predictive normal tissue complication probabilities (NTCPs) models between the three competing treatment modalities aiming to deliver a total dose of 50.4 Gy in 1.8 Gy daily fractions. Target coverage was optimized with IMPT when compared to IMXT and VMAT. Coverage of the sentinel node was slightly better with IMXT (D-98% = 57.3 +/- 5.1 Gy) when compared with VMAT (D-98% = 56.2 +/- 4.1 Gy). The irradiation of rectal, bladder, small bowel, and femoral heads volumes was significantly reduced with IMPT when compared to IMXT and VMAT. NTCPs rates for rectal and bladder >= grade-3 late toxicity were better with IMPT (0.4 +/- 0.0% and 0.0 +/- 0.0%) compared with IMXT (4.6 +/- 3.3% and 1.4 +/- 1.1%), and VMAT (4.5 +/- 4.0% and 1.6 +/- 1.6%), respectively. Acceptable dose-volume distributions and low rectal and urinary NTCPs were estimated to geometrically complex pelvic volumes such as the ones proposed in this study using IMXT, VMAT and IMPT. IMPT succeeded, however, to propose the best physical and biological treatment plans compared to both X-ray derived plans.