Nuclear Medicine & Radiation Therapy

The Impact of Planning Target Volume Margins on Four-Field Pelvic Radiotherapy, Intensity Modulated Radiotherapy and Volumetric Modulated Arc Therapy in Cervical Cancer: A Dosimetric Comparison

Abstract

Carminia Lapuz, Claire Dempsey, Michelle Evans, Leah Ponman, Jackie Patterson, Christopher Oldmeadow, Geetha Govindarajulu and Swetha Sridharan

Objective: To compare conventional Four-Field Pelvic Radiotherapy (4FRT), Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) in cervical cancer using increasing Clinical Target Volume (CTV) to Planning Target Volume (PTV) margins to account for daily variability in uterine position and to assess whether dosimetric advantages with advanced planning techniques continue with larger PTV margins.

Methods: Twenty patients with locally advanced cervical cancer previously treated with definitive radiation therapy were selected for the study. For each patient, computed tomography planning scans were obtained and PTVs were created with CTV to PTV uterine margins of 1.5 cm, 2 cm, 2.5 cm and 3 cm if anatomically feasible. 4FRT, IMRT and VMAT plans were generated and evaluated for target coverage, conformity, and homogeneity, dose to Organs at Risk (OAR), Total Monitor Units (MU) and delivery time.

Results: A total of 136 plans were generated. Target coverage was excellent for all plans generated regardless of technique. VMAT and IMRT were associated with significantly reduced dose to OAR compared with 4FRT for all CTV to PTV uterine margins (p ≤ 0.05). VMAT plans were associated with lower rectum V40 (p ≤ 0.02) and bowel V40 (p ≤ 0.04) for the smaller uterine margins and lower MU and delivery time (p ≤ 0.01).

Conclusion: VMAT and IMRT result in significantly lower doses to OAR compared with 4FRT in cervical cancer, regardless of CTV to PTV margins used to account for uterine motion. VMAT was associated with shorter delivery times compared with IMRT, which is useful in cervical cancer radiation therapy where intrafraction uterine motion may result in target under coverage if inadequate uterine margins are used. Accurate localization of the uterus with daily image guidance is critical when considering these advanced planning techniques for definitive radiation therapy in cervical cancer.