Reducing CTV to PTV Margins with Daily Adaptive Radiotherapy in the Postoperative Treatment of Endometrial and Cervical Cancer.

In the postoperative treatment of endometrial and cervical cancer, a PTV expansion of 1.5cm or greater would be generally recommended to ensure adequate coverage of CTV with no or minimal image guidance. Online adaptive radiotherapy (oART) has demonstrated to be feasible to reduce inter-fractional radiotherapy errors as it re-optimizes treatment plan every fraction. In this study, we hypothesized using daily cone-beam computed tomography (CBCT)-guided oART would reduce CTV to PTV margins in the postoperative treatment of endometrial and cervical cancer. Seven patients from a single-center with postoperative endometrial and cervical cancer treated with daily CBCT-guided oART were retrospectively reviewed. A total dose of 45Gy is prescribed in CTV-vaginal cuff (CTV-V) and CTV- regional lymph nodes (CTV-N) with daily fractions of 1.8Gy for five fractions per week. A total of 175 radiotherapy fractions of daily pre-treatment CBCTs and post-treatment CBCTs scans were uploaded to oART emulator for CTV-V and CTV-N contouring by a single observer. CTVpre to PTVpre margins of 5, 7, 10, 12, 15mm was used in all directions. Each post-treatment CBCT was rigidly registered to the pre-treatment CBCT with respect to bony anatomy. The CTVpost was projected onto the PTVpre and assess required planning margins to encompass the CTVpost. The average total treatment time (post-treatment CBCTs - pre-treatment CBCTs) was 23 min 14s (range: 18min 53s ∼ 28min 53s). For all fractions, the adapted plan was selected, and the volume of the PTV-V and PTV-N receiving 100% of the prescribed dose or more (V100%) was more than required 95%. A uniform three-dimensional CTV-Npre to PTV-Npre margin of 5mm could encompass CTV-Npost in all fractions (100%; 175/175f). A uniform three-dimensional CTV-Vpre to PTV-Vpre margin of 10mm could encompass CTV-Vpost in all fractions (100%; 175/175f) and 5mm could encompass 98% fractions (172/175f). In the fractions of 5mm margins that failed to encompass CTV-Vpost, the volume of CTV missed three times in anterior-posterior directions and once in lateral direction. Compared with scheduled plan, adapted plan with 5mm margins significantly reduced the mean dose to bladder (Dmean: 103.97 cGy vs 107.20 cGy, P<0.05) and rectum (Dmean: 117.82 cGy vs 123.67 cGy, P<0.05), and achieved better PTV-N (V100%: 97.8% vs 90.0%, P<0.05) and PTV-V (V100%:96.8% vs 85.4%, P<0.05) dose coverage in 175 fractions. In this study, 5mm CTV to PTV margins was adequate to encompass 100% fractions of CTV-N and 98% fractions of the CTV-V. Adapted plan with 5mm margins significantly reduced the dose to critical organ at risks while improving target coverage. [ABSTRACT FROM AUTHOR]