June 2020: Central review of INSEMA radiotherapy planning

The INSEMA trial (NCT02466737; GBG 75) was a prospective, randomized trial comparing sentinel lymph node biopsy (SLNB) versus no axillary surgery in patients with early invasive breast cancer (clinically/imaging ≤5cm, c/iN0) who were candidates for breast-conserving surgery (BCS) including postoperative whole-breast irradiation (WBI). In a second phase, patients with involved sentinel lymph nodes (one to three macrometastases) were randomized to either SLNB alone or completion axillary lymph node dissection (ALND). The rationale of the trial was based on available data at the time of protocol design (2011-2014). After publication of the radiation field design in the American College of Surgeons Oncology Group Z0011 trial, a radiation therapy quality assurance review was integrated into the INSEMA trial. Although the INSEMA protocol required that patients receive WBI using standard tangential fields, partial involvement of ipsilateral axillary levels within the finalized irradiated volume cannot be avoided in the majority of patients owing to individual anatomic conditions and was not considered as a protocol violation. The aim of the study was to investigate the role of patient characteristics, extent of axillary surgery, and radiation techniques for dose distribution in ipsilateral axillary levels.

The INSEMA trial randomized a total of 5,542 patients who underwent BCS between September 2015 and April 2019. Of these, 276 patients from 108 radiation therapy facilities were included in the central review, using the planning records of the first 3 patients treated at each site. Of the 276 patients, 41 had major deviations (no axillary contouring or submission of insufficient records) leading to exclusion. A total of 235 (85.1%) radiation therapy planning records were delineated according to the INSEMA protocol, including 9 (3.8%) cases with minor deviations. At least 25% of INSEMA patients were unintentionally treated with ≥95% of the prescribed breast radiation dose in axillary level I. Approximately 50% of patients were irradiated with a median radiation dose of more than 85% of prescription dose in level I. Irradiated volumes and applied doses were significantly lower in levels II and III compared with level I. However, 25% of patients still received a median radiation dose of ≥75% of prescription dose to level II. Subgroup analysis revealed a significant association between incidental radiation dose in the axilla and obesity. Younger age, boost application, and fractionation schedule showed no impact on axillary dose distribution. Assuming ≥80% of prescribed breast dose as the optimal dose for curative radiation of low-volume disease in axillary lymph nodes, at least 50% of reviewed INSEMA patients received an adequate dose in level I, even with contemporary 3-dimensional techniques. Dose coverage was much less in axillary levels II and III, and far below therapeutically relevant doses.

In conclusion, this central review yields first insights into contemporary radiation therapy techniques and schedules used and the extent of unintentional axillary radiation among selected INSEMA patients with BCS. Collection and analysis of these data will also be available for the complete INSEMA population in the near future.

Hildebrandt G, Stachs A, Gerber B, et al. Central Review of Radiation Therapy Planning Among Patients with Breast-Conserving Surgery: Results from a Quality Assurance Process Integrated into the INSEMA Trial. Int J Radiat Oncol Biol Phys. 2020; doi:10.1016/j.ijrobp.2020.04.042

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