Optimal Target Delineation for Highly Conformal Photon and Proton Radiation Therapy Techniques in Breast Cancer with Regional Nodal Irradiation

Abstract

Regional nodal irradiation (RNI) improves disease-free and distant disease-free survival in select women with breast cancer. For the most part, trials demonstrating this utilized 3-dimensional conformal radiotherapy (3D-CRT). Multiple atlases guide targeting using conformal techniques. We hypothesize that nodal failure patterns adequately covered by 3D-CRT may be under-covered with more conformal techniques. The Radiotherapy Comparative Effectiveness Consortium (RADCOMP), Radiation Therapy Oncology Group (RTOG), and the European Society for Radiotherapy and Oncology (ESTRO) breast cancer nodal atlases were utilized for contouring on a representative patient CT scan. 5mm PTV margins were applied. 3D-CRT, volumetric modulated arc therapy (VMAT), and pencil beam scanning proton (PBSPT) plans were prepared for each set of target volumes by two blinded dosimetrists. PET-positive lymph nodes of patients treated for breast cancer (2009-2017) were mapped (5 mm diameter brush at the center of the node) onto the representative patient utilizing anatomical landmarks. Lymph nodes were designated as adequately covered if they fell within the 95% isodose line. FDG-avid lymph nodes (n=389) were mapped from 102 patients with locally advanced (n=51; median 2, range 1-8 nodes) and metastatic (n=51; median 4, range 1-19 nodes) breast cancer: Axilla (AX) n=284, supraclavicular (SCV) n=60, internal mammary (IMN) n=45. With 3D-CRT (partially wide tangents, AP/PA SCV), coverage was excellent across all nodal levels (Table 1). With VMAT/PBSPT, overall coverage by 95% isodose was encouraging regardless of atlas: ESTRO (89%/88%), RTOG (93%/91%), and RADCOMP (98%/96%). At least 99% of level I and 94% level II lymph nodes were covered in all plans. Areas of under-coverage in level III, SCV, and IMN-SCV junction were identified in the ESTRO and RTOG atlases (Table 1). A greater than expected number of IMN nodes (n=45) were identified. The RADCOMP atlas improved coverage at the expense of slightly increased heart and lung doses (Table 1). Comprehensive target delineation, as detailed in the RADCOMP atlas is strongly recommended when utilizing conformal techniques for RNI to prevent under-coverage of areas of potential nodal failure.Abstract 111; Table 1Locoregional nodal coverage (%) by the 95% isodose line, lung V20 and heart dose.PlanTechniqueAll nodes (%)Level I Ax n (%)Level II Ax n (%)Level III Ax n (%)SCV n (%)IMN n (%)Left lung V20 (%)Heart Mean (cGy)3DPWT, AP/PA SCV379 (97%)161 (99%)86 (100%)36 (100%)56 (93%)40 (89%)44%987ESTROVMAT347 (89%)162 (100%)86 (100%)28 (78%)31 (52%)40 (89%)15.4%831ESTROPBSPT344 (88%)162 (100%)81 (94%)28 (78%)33 (55%)40 (89%)10.5%206RTOGVMAT362 (93%)160 (99%)86 (100%)33 (92%)45 (75%)38 (84%)16.7%853RTOGPBSPT354 (91%)160 (99%)81 (94%)31 (86%)43 (72%)39 (87%)10%204RADCOMPVMAT381 (98%)162 (100%)86 (100%)36 (100%)57 (95%)40 (89%)16.6%950RADCOMPPBSPT381 (98%)162 (100%)86 (100%)36 (100%)57 (95%)40 (89%)12.1%259 Open table in a new tab