Determination of optimal planning target volume margins in patients with gynecological cancer.

Purpose: To define optimal planning target volume (PTV) margins for intensity modulated radiotherapy (IMRT) ± knee-heel support (KHS) in patients treated with adjuvant radiotherapy.

Methods: Computed tomography (CT) scans ± KHS of 10 patients were taken before and at 3rd and 5th week of treatment, fused and compared with initial IMRT plans.

Results: A PTV margin of 15 mm in anteroposterior (AP) and superoinferior (SI) directions and 5 mm in lateral directions were found to be adequate without any difference between ± KHS except for the SI shifts in CTV-primary at the 3rd week. Five mm margin for iliac CTV was found to be inadequate in 10-20% of patients in SI directions however when 7 mm margin was given for iliac PTV, it was found to be adequate. For presacral CTV, it was found that the most striking shift of the target volume was in the direction of AP. KHS caused significantly less volume of rectum and bladder in the treated volume.

Conclusions: PTV margin of 15 mm in SI and AP, and 5 mm in lateral directions for CTV-primary were found to be adequate. A minimum of 7 mm PTV margin should be given to iliac CTV. The remarkable shifting in presacral CTV was believed to be due to the unforeseen hip malposition of obese patients. The KHS seems not to provide additional beneficial effect in decreasing the shifts both in CTV-primary and lymphatic, however it may have a beneficial effect of decreasing the OAR volume in PTV margins.