Dosimetric evolution of the breast electron boost target using 3D ultrasound imaging

Abstract

Purpose: To investigate the effect of treatment planning, patient setup, and interfraction motion errors on the delivered dose for external beam electron boosts for postoperative early stage breast cancer patients.

Methods and materials: For 5 patients, 10-15 Gy was prescribed and administered via a conventionally defined electron boost treatment field - no dose distribution was calculated. Two computed tomography (CT) data sets were acquired on an average of 47 days apart. Using Monte Carlo techniques the clinically defined electron beams were reconstructed on CT1 and CT2, and a dosimetric comparison between the two data sets was made. Additionally, 3D ultrasound (US) imaging was performed to monitor interfraction motion. 3D US images were acquired concurrently with the CT images, as well as prior to each boost fraction in the treatment room. Taking into account interfraction motion, the dose to the clinical target volume (CTV) was calculated.

Results: Based on conventionally determined treatment fields the CT1-based CTV D95 averaged 49% (range 12-89%) of the prescribed dose. Representing setup errors, the CT2-based CTV D95 averaged 47% (range 16-91%) of the prescribed dose. Considering interfraction motion, the average radial displacement was 11 mm, and the resulting CTV D95 was further reduced in 2/5 patients.

Conclusions: Poor initial coverage at the time of planning is exacerbated by breast mobility and interfraction tumour bed motion, increasing the uncertainty in the delivered dose.