Planning With Patient-Specific Rectal Sub-Region Constraints Decreases Probability of Toxicity in Prostate Cancer Radiotherapy

Background: A rectal sub-region (SRR) has been previously identified by voxel-wise analysis in the inferior-anterior part of the rectum as highly predictive of rectal bleeding (RB) in prostate cancer radiotherapy. Translating the SRR to patient-specific radiotherapy planning is challenging as new constraints have to be defined. A recent geometry-based model proposed to optimize the planning by determining the achievable mean doses (AMDs) to the organs at risk (OARs), taking into account the overlap between the planning target volume (PTV) and OAR. The aim of this study was to quantify the SRR dose sparing by using the AMD model in the planning, while preserving the dose to the prostate. Material and Methods: Three-dimensional volumetric modulated arc therapy (VMAT) planning dose distributions for 60 patients were computed following four different strategies, delivering 78 Gy to the prostate, while meeting the genitourinary group dose constraints to the OAR: (i) a standard plan corresponding to the standard practice for rectum sparing (STD(pl)), (ii) a plan adding constraints to SRR (SRR(pl)), (iii) a plan using the AMD model applied to the rectum only (AMD_RECT(pl)), and (iv) a final plan using the AMD model applied to both the rectum and the SRR (AMD_RECT_SRR(pl)). After PTV dose normalization, plans were compared with regard to dose distributions, quality, and estimated risk of RB using a normal tissue complication probability model. Results: AMD_RECT_SRR(pl) showed the largest SRR dose sparing, with significant mean dose reductions of 7.7, 3, and 2.3 Gy, with respect to the STD(pl), SRR(pl), and AMD_RECT(pl), respectively. AMD_RECT_SRR(pl) also decreased the mean rectal dose by 3.6 Gy relative to STD(pl) and by 3.3 Gy relative to SRR(pl). The absolute risk of grade ≥1 RB decreased from 22.8% using STD(pl) planning to 17.6% using AMD_RECT_SRR(pl) considering SRR volume. AMD_RECT_SRR(pl) plans, however, showed slightly less dose homogeneity and significant increase of the number of monitor units, compared to the three other strategies. Conclusion: Compared to a standard prostate planning, applying dose constraints to a patient-specific SRR by using the achievable mean dose model decreased the mean dose by 7.7 Gy to the SRR and may decrease the relative risk of RB by 22%.