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 (STDpl), (ii) a plan adding constraints to SRR (SRRpl), (iii) a plan using the AMD model applied to the rectum only (AMD_RECTpl), and (iv) a final plan using the AMD model applied to both the rectum and the SRR (AMD_RECT_SRRpl). 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_SRRpl showed the largest SRR dose sparing, with significant mean dose reductions of 7.7, 3, and 2.3 Gy, with respect to the STDpl, SRRpl, and AMD_RECTpl, respectively. AMD_RECT_SRRpl also decreased the mean rectal dose by 3.6 Gy relative to STDpl and by 3.3 Gy relative to SRRpl. The absolute risk of grade ≥1 RB decreased from 22.8% using STDpl planning to 17.6% using AMD_RECT_SRRpl considering SRR volume. AMD_RECT_SRRpl 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%.
Highlights
Rectal toxicity is one of the main side effects arising when treating prostate cancer with radiotherapy
The clinical target volume (CTV) and the organs at risk (OARs) were manually delineated on computed tomography (CT) slices according to the French Genitourinary Group (GETUG) recommendations [14, 15]
This paper proposed a methodology for decreasing rectal toxicity by adding a patient-specific sub-region in the prostate inverse radiotherapy planning and by using a specific achievable mean dose model
Summary
Rectal toxicity is one of the main side effects arising when treating prostate cancer with radiotherapy. By increasing mechanically the anterior perirectal space, using hydrogel spacer, has been shown to significantly reduce rectal irradiation [3,4,5] Such an approach is, invasive and expensive. A more appealing approach, in the context of dose escalation, would be to intervene at the planning step by adding dosimetric constraints to particular portions of the rectum which may be highly radiosensitive. This requires, a robust technique for identifying patient-specific rectal subregions (SRR) that should be spared in the treatment planning. 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
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