Dosimetric Parameters Predicting Late Small Bowel Toxicity in Patients With Rectal Cancer Receiving Neoadjuvant Chemoradiation.

Purpose: The aim of this study was to identify dosimetric parameters that predict late small bowel (SB) toxicity after neoadjuvant long course chemoradiation (CRT) for rectal cancer.

Methods And Materials: Four hundred eighty-six 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 dose-volume relationship between small bowel irradiation and late small bowel toxicity was evaluated and a mathematical model to predict for late SB toxicity was derived.

Results: Among the 486 patients with a median follow-up of 60 months from completion of radiation, 36 (7.4%) patients experienced ≥ grade 2 and 21 (4.3%) developed ≥ grade 3 late SB toxicity. A statistically significant association between the development of grade ≥3 late small bowel toxicity and the volume of small bowel irradiated was found at each dose level from 5 to 40 Gy (P < .001 for all dose volumes) in 5 Gy intervals. The average SB volume for patients who experienced grade ≥2 SB toxicity was 2149.9 cm and the average SB volume for patients who experienced grade ≥3 SB toxicity was 2179.9 cm. The predicted V30 for a 5% risk for grade ≥2 SB toxicity was 101.5 cm and for grade ≥3 SB toxicity was 201.5 cm. The volume of small bowel receiving at least 30 Gy (V30) was most strongly associated with grade ≥3 SB toxicity.

Conclusions: This study demonstrates the significant dose-volume relationship between volume of small bowel receiving 30 Gy (V30 Gy) and late grade ≥3 SB toxicity. When planning CRT for patients with rectal cancer, restricting V30 to <200 cm will be a useful guideline to minimize the 5 year grade ≥3 late SB toxicity to <5%.