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

Abstract

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%.

Selective sparing of bladder and rectum sub-regions in radiotherapy of prostate cancer combining knowledge-based automatic planning and multicriteria optimization

“…Selective sparing of the SRR was also recently discussed by Lafond et al [26] , reporting a significant reduction in mean dose for the rectum and SRR compared to clinical plans (3.6 and 7.7 Gy respectively). In our study, four sub-structures were simultaneously spared, resulting in a mean dose reduction between 1.3 and 2.2 Gy, with a maximum reduction up to 3–5 Gy.…”

“…Previous studies have employed these techniques, including both 2D analyses based on surface maps and 3D analyses [8] , [9] , [12] , [13] , [14] , [15] , [16] , [17] , [18] , [19] , [20] , [21] , [22] , [23] , [24] , [25] . To the best of our knowledge, there have been limited studies focusing on adapting treatment plans to spare SRSs volumes [26] , [27] . To address this issue, advanced techniques such as Multi-Criteria Optimization (MCO) could be employed for the fine tuning of the dose distributions.…”

Spatial descriptions of radiotherapy dose: normal tissue complication models and statistical associations

“…The propagation of identified sensitive sub-regions to an individual would facilitate toxicity-minimised planning, without the need to modify current optimisation methods (Acosta and De Crevoisier 2019). This has been demonstrated by Lafond et al (2020). A subsequent natural extension would be the possibility to implement these models directly into the optimisation engine.…”

Assessment of rectal toxicities after radiation therapy for localized prostate cancer: experience of the Akanda Cancer Institute in Gabon

“…Rectal toxicity is one of the main side effects arising when treating prostate cancer with radiotherapy [ 7 ]. Odrazka et al [ 8 ] underlined that the rectum and bladder are the crucial organs at risk for curative radiation therapy of localized prostate cancer.…”