Purpose: To investigate the reduction of irradiated small bowel volume with intensity modulated radiation therapy (IMRT) planning in rectal cancer and to asses the variability of the irradiated small bowel in the conventional planning as in the IMRT planning (IMPlan) by obtaining weekly CT scans.
Patients and materials: Twelve patients with rectal cancer had treatment planning CT scans of the pelvis with small bowel contrast obtained for planning and once a week during treatment (65 CT scans total). The scans were registered using the bony structures. The clinical target volume (CTV), small bowel, large bowel and bladder were outlined on each slice. The first CT scan was used for IMPlan and conventional three-field planning (ConPlan), which were then applied to the CT scans obtained during therapy.
Results: The median value among patients of the mean volume over a patient's scan of small bowel irradiated > or =95% was 112 cm3 (standard deviation (SD): 31 cm3) for the ConPlan and 42 cm3 (SD: 17 cm3) for the IMPlan. The median total bladder volume was 148 cm3 (SD: 130 cm3). There was a good correlation between the volume of irradiated small bowel and the bladder volume for IMPlan with <50 cm3 irradiated small bowel and ConPlan with <150 cm3 (p = 0.002).
Conclusion: The use of IMRT led to a potentially clinically meaningful reduction in the volume of small bowel irradiated, even when accounting for small bowel motion. A full bladder was of greatest benefit in individuals with the smallest volume of small bowel in the treatment field.