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

ABSTRACT

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 cm3 and the average SB volume for patients who experienced grade ≥3 SB toxicity was 2179.9 cm3. The predicted V30 for a 5% risk for grade ≥2 SB toxicity was 101.5 cm3 and for grade ≥3 SB toxicity was 201.5 cm3. 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 cm3 will be a useful guideline to minimize the 5 year grade ≥3 late SB toxicity to <5%.