The variability of the clinical target volume for rectal cancer due to internal organ motion during adjuvant treatment

Purpose: This study defined the clinical target volume (CTV) for the adjuvant treatment of rectal cancer and applied this definition to multiple CT scans obtained during the typical 5-week course of treatment to measure the modification to the CTV due to internal organ motion that would be needed to define the planning target volume (PTV). Methods and Materials: Ten patients with rectal cancer had weekly treatment planning CT scans during adjuvant radiation therapy. All patients were given oral contrast, placed prone on a rigid foam cradle with a depressed area for small bowel exclusion, and instructed to have a full bladder. The CT scans were registered according to the bones of the pelvis, and the CTV was outlined on each CT slice. Movement of the CTV in all dimensions was measured. The CT scan with the lowest and highest bladder volume for each patient was used to calculate the CTV movement due to bladder filling. Results: The largest difference in the CTV occurred 10 cm caudal to the anus, with a standard deviation of 1 cm. Bladder filling displaced the anterior border of the CTV an average of 7 mm over a cranial to caudal length of 2.5 cm. Other borders of the CTV were based on muscle, bone, or major blood vessels and were stable. Conclusion: Modification of the CTV to design a PTV can be unequal, with the largest change at the anterior border of the inferior pelvis