Transfer learning for automatic sarcopenia evaluation at T12 vertebral level

Abstract

Purpose or Objective Sarcopenia is a progressive loss of muscle mass and is emerging as a potential important prognostic factor for RT patients. It is typically assessed by segmenting skeletal muscle at the L3 vertebral level and extracting features of the muscle. However, L3 is not visible on most RT planning scans. In this work we use transfer learning to re-train an existing model, originally for segmenting muscle at L3, to segment muscle at the T12 vertebral level. The use of transfer learning significantly reduces the number of training images required. To show prognostic value, mean skeletal muscle attenuation (SMA) at L3 and T12 were compared both directly ans in survival models. Material and Methods An existing model was available, pre-trained to segment the muscle compartment at L3 (Green et al, ESTRO 2019). Transfer learning was used to fine-tune the model using manual delineations at T12 (training n=16, validation n=2, unseen=10). To ensure no bone remained, a threshold of 226 HU was applied to the CT scan, expanded isotropically by 0.5 mm and the resulting mask excluded from the segmentation. Dice score and Distance-To-Agreement (DTA) were calculated for the unseen patients. 208 oesophago-gastric (OG) cancer patients were available for testing prognostic value. In this cohort, L3 had been previously segmented and validated. Segmentations at T12 were generated and visually assessed, to determine the failure rate. SMA was extracted and compared for patients with successful L3 and T12 segmentations by calculating their correlation and performing a paired t-test. Finally, prognostic value was investigated in Kaplan-Meier curves (split on median SMA) and multivariate Cox models including either L3 or T12 SMA controlling for performance status, age and sex. Results Transfer learning with 16 segmentations resulted in a mean Dice score of 0.75(σ=0.05) and mean DTA of 0.56(σ=0.40) cm in the unseen patients. Of the 208 scans in the OG cancer cohort 191 segmentations at T12 were successful (Fig 1). SMA at L3 and T12 were strongly correlated (R=0.80), but mean SMA was different (25 HU vs 34 HU, p<0.001), Fig 1. Kaplan-Meier curves for L3 and T12 showed significant differences in survival when split on median density, Fig 2. Multivariate Cox models identified performance status and skeletal muscle density as predictive of overall survival. SMA at both L3 and T12 were found to be prognostic (p=0.03 and p<0.01, respectively) with hazard ratios 0.98 and 0.97 per HU showing increased muscle density is beneficial. Conclusion We demonstrated a fully automated method for sarcopenia assessment at T12. Transfer learning with a small training set resulted in accurate muscle segmentation. Analysis on a cohort of OG cancer patients shows that SMA at L3 and T12 are correlated and are both prognostic for patient outcome. Automating skeletal muscle segmentation at T12 provides a significant step forwards in exploring the prognostic value of sarcopenia in larger cohorts of patients treated by RT.