Purpose: To compare intensity-modulated photon radiotherapy (IMRT) with three-dimensional conformal proton therapy (3D-CPT) for early-stage prostate cancer, and explore the potential utility of intensity-modulated proton therapy (IMPT).
Methods and materials: Ten patients were planned with both 3D-CPT (two parallel-opposed lateral fields) and IMRT (seven equally spaced coplanar fields). Prescribed dose was 79.2 Gy (or cobalt Gray-equivalent, [CGE] for protons) to the prostate gland. Dose-volume histograms, dose conformity, and equivalent uniform dose (EUD) were compared. Additionally, plans were optimized for 3D-CPT with nonstandard beam configuration, and for IMPT assuming delivery with beam scanning.
Results: At least 98% of the planning target volume received the prescription dose. IMRT plans yielded better dose conformity to the target, whereas proton plans achieved higher dose homogeneity and better sparing of rectum and bladder in the range below 30 Gy/CGE. Bladder volumes receiving more than 70 Gy/CGE (V70) were reduced, on average, by 34% with IMRT vs. 3D-CPT, whereas rectal V70 were equivalent. EUD from 3D-CPT and IMRT plans were indistinguishable within uncertainties for both bladder and rectum. With the use of small-angle lateral-oblique fields in 3D-CPT and IMPT, the rectal V70 was reduced by up to 35% compared with the standard lateral configuration, whereas the bladder V70 increased by less than 10%.
Conclusions: In the range higher than 60 Gy/CGE, IMRT achieved significantly better sparing of the bladder, whereas rectal sparing was similar with 3D-CPT and IMRT. Dose to healthy tissues in the range lower than 50% of the target prescription was substantially lower with proton therapy.