Comparison Study of Intensity Modulated Arc Therapy (IMAT) using Single or Multiple Arcs to Intensity Modulated Radiation Therapy (IMRT) for High-risk Prostate Cancer

Abstract

IMAT is a complex form of IMRT that allows dose delivery in a single or multiple arcs. The treatment of nodal stations in high-risk prostate cancer is controversial, however, there is emerging data to support enhanced biochemical disease free survival with nodal irradiation. We compared the plan quality and plan efficiency of IMRT versus one arc (1Arc) versus three arcs (3Arc) for treatment of high-risk prostate cancer. Fifteen patients with high-risk prostate cancer as defined by NCCN, were studied. Prostate (PTVp), right pelvic lymph nodes (PTVrln), left pelvic lymph nodes (PTVlln), and organs at risk were contoured. The total planned dose was 79.20Gy/1.8Gy/fraction. PTVp, PTVrln, PTVlln received 50.40Gy/1.8Gy/fraction followed by a boost to PTVb of 28.80Gy. Three plans were generated for each patient: 7 beams IMRT, 1Arc (1full rotation, 1 isocenter), 3Arc (1 full, 2 partial rotations for the lymph nodes, 3 isocenters) using Rapid Arc technology. All 3 plans were calculated to maintain Dmax ≤110% with 95% coverage to PTVt (PTVp+PTVrln+PTVlln). All plans were optimized using the same dose constraints. Nine patients were treated using 3Arc technique, 4 with IMRT and 3 using 1Arc. All plans had excellent coverage, i.e., 95% of dose to 95% of PTVt. The average RTOG conformity index (prescription volume/target volume), was 1.17, 1.20, 1.15 for IMRT, 1Arc, 3Arc, respectively, with 3Arc exhibiting the highest conformity. The 3 plans' homogeneity index was within 1% of each other. Similarly, Dmean of bladder was within 2% of each other. The lowest mean dose for the rectum was achieved with the IMRT plan, with 10% lower dose than the arc plans. The Paddick gradient index (50% isodose volume/prescription volume) of the 3Arc was superior to IMRT by 27.4% (p = 0.006). The average monitor unit (MU) was highest in the IMRT plan (1685) versus 1Arc (575) versus 3Arc (1079), (p =0.0001 for 1arc). The average beam on time was 6 minutes for IMRT compared to IMAT, p = 0.001. The beam on time of the arc therapies was 1.3 and 2.9 minutes for 1Arc and 3Arc, respectively. Intensity Modulated Arc Therapy delivers acceptable coverage compared to IMRT for high-risk prostate cancer, it also has the potential to achieve a more favorable dose gradient profile, conformity, lower MU and beam on time compared to IMRT. The 3Arc plan proved to have a superior gradient and conformity indices in contrast to the 1Arc plan. In regards to dose to organs at risk, there was no significant difference except that a lower rectal dose was seen in the IMRT plans.