Factors Influencing Bowel Sparing in Intensity Modulated Whole Pelvic Radiotherapy for Gynaecological Malignancies

Background And Purpose: To evaluate the influence of uterus and bladder size on large and small bowel sparing with intensity modulated whole pelvic radiotherapy (IM-WPRT) in gynecologic patients.

Patients And Methods: Twenty patients were selected; 10 women with cervical cancer treated with definitive radiotherapy (group 'DEF') and 10 endometrial cancer patients treated postoperatively (group 'POST'). Bladder, rectal wall, small (SB) and large bowel (LB) were delineated as organs at risk. A conformal four field technique and a seven field IMRT plan (prescription dose 50.4 Gy) were compared in terms of DVH and various target parameters.

Results: At doses between 40 and 50.4 Gy statistically significant improvements (P<0.05) were observed for IM-WPRT for irradiated volume of rectal wall and bladder. In both patient groups, with IMRT the average irradiated volume of SB was reduced by a factor of 6 at 50.4Gy. This ratio was 2 for LB. In the DEF group the effect of SB-sparing with IMRT correlated with bladder size (correlation coefficient 0.70) while it did not correlate in the postoperative group. The effect of LB-sparing decreased with increasing bladder size in both groups but the impact of IMRT was larger for postoperative patients.

Conclusions: IMRT significantly reduced the absolute volume of rectal wall, bladder and bowel irradiated at the prescribed dose level in gynaecologic patients. Main differences between POST and DEF patients receiving IM-WPRT were absolute volumes of LB irradiated to doses between 35 and 50Gy, suggesting an impact of intact uterus on LB volume in the pelvis. POST patients seem to benefit most from elective nodal IMRT. Bladder filling is an important co-factor influencing the benefit of IMRT with respect to OAR sparing.