Defining an 'ideal' continuous dose-volume constraint curve using literature recommended individual dose-volume thresholds for reducing rectal toxicity from external beam prostate radiation therapy

We postulated that a contiguous rectal dose-volume threshold curve obtained via a best mathematical fit of the toughest literature recommended dose-volume thresholds could be deemed as “safe” related to the risk of rectal toxicity. The practicality of restricting rectal doses to such ideal volume thresholds in clinical practice was tested by comparing data collected from a pool of prostate patients treated with external beam radiation therapy (EBRT) in our institution. Available rectal dose-volume constraint recommendation in prostate EBRT was collected. At each dose level, the toughest recommended volume limit was identified and an ideal dose-volume threshold curve was generated using a best mathematical fit. Rectal dose-volume data on 105 prostate patients treated to 75.6 Gy dose with intensity modulated radiation therapy (IMRT) in our institution were retrospectively reviewed. The “ideal” rectal dose-volume threshold curve is defined by V 75 ≤ 7%, V 70 ≤ 11%, V 65 ≤ 15%, V 60 ≤ 20%, V 50 ≤ 30%, V 40 ≤ 43%, and V 30 ≤ 60%. At doses larger or equal to 65 Gy, our patient averaged dose-volume histograms (DVHs) for all groups converged with the ideal curve. At intermediate and low doses (less than 60 Gy), higher volumes of rectum were irradiated than described by the hypothetical curve, with pelvic group receiving more rectal dose than prostate only group respectively. Rectal mean dose correlated strongly with V 40 values (R 2 = 0.84). Our “ideal” rectal dose-volume curve generated from literature dose-volume thresholds in the most stringent manner is realistic and achievable at high doses (≥60 Gy) and not realistic at intermediate and low doses (≤50 Gy). Standardizing rectal definition and dose-volume metrics will make published analyses and dose-volume constraint recommendations easier to follow.