Dosimetric advantages of IMRT simultaneous integrated boost for high-risk prostate cancer

Purpose A sequential two-phase process, initial and boost irradiation, is the common practice for the radiotherapy management of high-risk prostate cancer. In this work, we explore the feasibility of using intensity modulated radiation therapy (IMRT) simultaneous integrated boost (SIB), a single-phase process, to simultaneously deliver high dose to the prostate and lower dose to the pelvic nodes. In addition, we introduce the concept of voxel-equivalent dose for the comparison of treatment plans. Methods and materials The SIB is designed to deliver the same dose (e.g., 45 Gy, 25 × 1.8 Gy) as the conventional method to the pelvic nodes and to deliver higher doses to prostate in the same 25 fractions (i.e., hypofractionation). The equivalent uniform dose (EUD) was used to determine suitable SIB fractionations that deliver the biologically equivalent doses to prostate. For tumor, the EUD was estimated based on the linear quadratic (LQ) model. The most recent LQ parameters derived from clinical data for prostate cancer were used. The sensitivity of LQ parameters was evaluated. The EUD for normal tissue was computed based on the widely used Lyman model. To be able to consider biologic effectiveness spatially (e.g., voxel by voxel), we propose a new concept, termed the voxel-equivalent dose (VED). The calculation of VED was similar to that for EUD, except that it was done within a voxel. To demonstrate dosimetric feasibility and advantages of the proposed IMRT SIB, we have performed a retrospective planning study on selected patient cases using commercial IMRT and three-dimensional (3D) planning systems. Four treatment scenarios were considered: ( 1 ) the conventional 3D plan for initial whole-pelvic irradiation and subsequent conventional 3D boost plan for prostate gland, ( 2 ) the conventional 3D plan for initial whole-pelvic irradiation and subsequent IMRT boost plan for prostate, ( 3 ) IMRT plan for initial whole-pelvic irradiation and subsequent IMRT boost plan for prostate, and ( 4 ) IMRT SIB. EUDs and VED-based dose–volume histograms for prostate, pelvic nodes, small bowel, rectum, bladder, and other tissue for all 4 scenarios were compared. Results A series of equivalent hypofractionation regimens suitable for the IMRT SIB were obtained for high-risk prostate cancer. For example, the conventional treatment regimen of 42 × 1.8 Gy (EUD = 75.4 Gy) would be equivalent to a SIB regimen of 25 × 2.54 Gy. From the comparison of 3D VED dose distributions and dose–volume histograms between the SIB and the conventional two-phase irradiation, we found that the SIB offers better or equivalent dose conformity to prostate and pelvic nodes and better sparing to the critical structures. For example, for the 4 treatment scenarios with a prostate EUD of 75.4 Gy, the corresponding rectal EUDs are 67.1 (3D + 3D), 65.6 Gy (3D + IMRT), 63.7 Gy (IMRT + IMRT), and 62.0 Gy (SIB). Conclusions A new IMRT simultaneous integrated boost strategy that irradiates prostate via hypofractionation while irradiating pelvic nodes with the conventional fractionation is proposed for high-risk prostate cancer. Compared to the conventional two-phase treatment, the proposed SIB technique offers potential advantages, including better sparing of critical structures, more efficient delivery, shorter treatment duration, and better biologic effectiveness.