Assessment of Pectoralis muscle invasion using combined DBT and contrast-enhanced mammography: Retrospective multi-reader study.

Objectives: To evaluate combined digital breast tomosynthesis and contrast-enhanced mammography (DBT/CEM) for predicting pectoralis muscle invasion.

Methods: This retrospective multi-reader cohort study included research patients who underwent combined DBT/CEM for breast cancer staging and had prepectoral masses. Images were independently reviewed by six fellowship-trained breast radiologists. Diagnostic performance, reader confidence, and inter-reader agreement were calculated for each image type/modality.

Results: Among 10 patients with prepectoral masses on DBT/CEM, muscle invasion was present in 3 and absent in 7. The overall diagnostic accuracy of DBT/CEM for PMI was 0.6 (range 0.4-0.9); for predefined radiologic signs it was 0.5-0.7 for low energy (LE) CEM, 0.4-0.7 for DBT, and 0.4-0.8 for recombined (RC) CEM. Muscle deformity on MLO views had the highest accuracy (0.7-0.8). On a scale of 1-3, mean radiologist confidence for combined DBT/CEM was 1.9 (1.5-2.3; SD=0.65). Median confidence ranged from 1.9 for RC to 2.2 for DBT. Per-case reader agreement was poor (K=-0.01) for DBT/CEM; poor to slight (K= -0.13-0.40, median 0.28) for RC; slight to fair (K = 0.04-0.43, median 0.27 and K = 0.02-0.42, median 0.19, respectively) for DBT and LE. In two patients with subpectoral breast implants CEM was accurate in PMI detection, while MRI had one false-positive result.

Conclusion: Combined DBT/CEM accuracy and inter-reader agreement are suboptimal for PMI evaluation, except in patients with breast implants. RC images marginally improve accuracy compared to LE images but have lowest radiologist confidence. DBT has lowest accuracy but highest confidence. Muscle deformity on MLO view was the most accurate sign.

Critical relevance statement: Combined DBT/CEM demonstrated suboptimal diagnostic accuracy, reader confidence, and inter-reader agreement for detecting pectoralis muscle invasion (PMI) in prepectoral breast cancer (BC) except for patients with subpectoral breast implants, where recombined images on implant-displaced CEM views performed better than MRI.