Effect of different patient positions on target dose coverage in rectal cancer IMRT

Abstract

Objective To compare the effect between the supine and prone patient positions upon target dose coverage during intensity-modulated radiotherapy (IMRT) for rectal cancer, aiming to provide clinical reference for the selection of position for rectal cancer patients. Methods Twenty-four patients diagnosed with rectal cancer receiving postoperative adjuvant radiotherapy were selected and divided into the supine (n=12) and prone position groups (n=12). Before and during the IMRT (1-4 weeks), all patients received CT scans, which were defined as: Plan, 1W, 2W, 3W and 4W, respectively. The organs at risk were delineated based on CT scan images. Plan, 1W, 2W, 3W and 4W CT scan images were fused. The CTV and PTV from Plan CT scan were copied to the 1-4W CT scan images, and the therapeutic plans from Plan CT scan were copied as well. The target dose coverage was assessed and the failure rate of target dose coverage was calculated. The couch-position data for each patient during each cycle of IMRT were recorded by using the MOSAIQ network and the overall deviation (S) of couch position was calculated. Results The failure rates of CTV and PTV target dose coverage in the prone position group were higher than those in the supine position group (18.60% VS 0%, 69.76% VS 53.65%). The S value was significantly correlated with the target dose coverage (r=-0.683, P=0.000). The S value in the prone position group was (1.23±0.76) cm, significantly greater than (0.28±0.18) cm in the supine position (P=0.001), and the most significant deviation was noted in the y (head and foot) and z (frontal and dorsal) directions (P=0.003 and 0.003). Compared with the supine group, the V5 and V10 of the small intestine were significantly less (P=0.003 and 0.004) and the chronic toxicity (NTCPC) was considerably reduced (P=0.041) in the prone position group. Conclusions A better target dose coverage can be maintained during IMRT with a supine position during rectal cancer IMRT, whereas the positioning repeatability is worsened with a prone position due to use of the belly board, thereby affecting the target dose coverage. Although the prone position combined with belly board can reduce the tolerated dosage of the small intestine, effective measures should be taken to guarantee the patient positioning repeatability. Key words: Rectal neoplasms/intensity-modulated radiotherapy; Radiotherapy dosage; Belly board