Automating RTOG-defined target volumes for postmastectomy radiation therapy

Consistency in defining and contouring target structures in radiation therapy (RT) is critical for highly conformal RT, for evaluating treatment plans, and for quality assurance in multi-institutional RT trials. The Radiation Therapy Oncology Group (RTOG) has published consensus guidelines for contouring targets for postmastectomy RT. To aid in contouring such structures, we evaluated the potential use of an automated contouring technique, known as deformable image registration-based breast segmentation (DEF-SEG).The RTOG definitions were used to contour the chest wall (CW); levels I, II, and III axillary nodes (Ax1, Ax2, Ax3); supraclavicular (SCV) nodes; internal mammary (IM) nodes; and the heart. Left-sided and right-sided templates were created. The DEF-SEG was then used to generate auto-segmented contours from the appropriate template to computed tomographic scans of 20 test cases (10 left, 10 right). To assess the accuracy of this method, those contours were manually modified as necessary to match the RTOG definitions, and the extent of the overlap was compared. The dosimetric impact of the difference in contours was then evaluated by comparing dose-volume histograms for modified and unmodified contours.Mean volume-overlap ratios between the unmodified DEF-SEG-generated contours and modified contours were as follows: CW, 0.91; Ax1, 0.68; Ax2, 0.64; Ax3, 0.68; SCV node, 0.66; IM node, 0.32, and the heart, 0.93. Mean differences in volume receiving 45 Gy (V45) for the modified versus unmodified contours were as follows: CW, 2.1%; SCV node, 4.8%; Ax1, 5.1%; Ax2, 5.6%; Ax3, 3.0%; and IM node, 10.1%. Mean differences in V10 between the modified heart and the unmodified heart were 0.4% for right-sided treatment and 0.5% for left-sided treatment.The DEF-SEG can be helpful for delineating structures according to the RTOG consensus guidelines, particularly for the CW and the heart. No clinically significant dosimetric differences were found between the modified and unmodified contours. The DEF-SEG may be useful for evaluating treatment plans for postmastectomy RT in multi-institutional trials.