Prediction of patient-reported bowel quality of life (QOL) after dose-escalated radiation therapy (RT) for prostate cancer by rectal dosimetry.

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Abstract
84 Background: Rectal dose-volume histogram (DVH) parameters and a consensus normal tissue complication probability (NTCP) model predict physician scored toxicity but have not been well studied for patient reported outcomes (PRO) following external beam RT for prostate cancer. Methods: PRO were prospectively collected at baseline and 12 month intervals by expanded prostate cancer index composite (EPIC) for 98 patients from 2004-2009. Linear regression modeling for bowel score at 1 and 2 years was performed using pre-treatment bowel score and DVH parameters [V80 (% of rectal volume >80 Gy), V75, V70, V50, V25, and NTCP]. Results: Median RT dose was 78 Gy with image-guidance in 81%, intensity-modulation in 72%, and pelvic RT in 11%. Rectal DVH parameters are presented in the Table. Pretreatment and 2 year PRO were available for 92% of patients. The mean pretreatment bowel summary score declined from 96.0 (sd 8.0) to 91.8 (13.5) at 2 years. Acute proctitis (grade 1-2) during RT was associated with a decline in 2 year bowel QOL, while acute diarrhea (grade 1-2) was not. At 2 years 63% of patients had a decline dose, NTCP and bowel QOL (Table). Akaike information criterion analysis followed by leave-one-out cross validation, indicated that V75 provided the best predictive model for 2 year bowel QOL followed by NTCP and V70. Conclusions: Most previous studies have highlighted V70 for toxicity (such as rectal bleeding). The association with V75 and rectal QOL has not been previously reported and supports potential changes in how prostate plans are designed to maintain QOL. This is also the first validation of a consensus NTCP model for bowel QOL. [Table: see text]