Dosimetric Parameters Predicting Late Small Bowel Toxicity In Rectal Cancer Patients Receiving Neo-Adjuvant Chemo-Radiation

Abstract

Neo-adjuvant long-course chemo-radiation (CRT) followed by surgery and adjuvant treatment is the current standard of care for locally advanced rectal cancer patients. With improved survival, there is concern for late treatment related toxicity, especially for the small bowel, which is a dose limiting structure. The aim of this study was to identify the late small bowel (SB) toxicity associated with neo-adjuvant long-course CRT and to identify correlations of these events with radiation dosimetry. 501 consecutive patients with locally advanced rectal cancers (clinical T3/T4 or N1/N2) who received CRT followed by surgery and had dosimetric data available for analysis were included in this study. The correlation of late SB toxicity with radiation dosimetry was evaluated and reported. Among the 501 patients with a median follow-up of 58 months that received CRT, there were 26 (5.2%) events of late SB toxicity ≥ grade 2, and 21 (4.2%) events ≥ grade 3. Univariate analysis identified SB volumes exposed to 30 Gy as being significantly associated with grade ≥3 toxicity (p = 0.019), while volumes of 25 Gy and 30 Gy were most predictive of grade ≥ 2 toxicity (p = 0.023, 0.003). Logistic regression analysis identified volumes of 458 cc receiving 5 Gy (p = 0.007), 330 cc for 15 Gy (p = 0.02), 274 cc for 25 Gy (p = 0.02), and 190 cc for 30 Gy (p = 0.02) as predictive for grade ≥2 SB toxicity. This study reports on the late SB toxicity associated with CRT in a large cohort of rectal cancer patients. Dosimetric analysis identified SB volumes that are significantly associated with late SB toxicity of ≥ grade 2. When planning CRT for rectal cancer patients, restricting V30 to <190cc will be a useful guideline to keep grade ≥2 late SB toxicity to <5%.