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High body mass index (BMI) has been identified as a factor in reduced abdominal compression effectiveness, however, does not consider body composition or fat distribution. Patients with higher levels of abdominal subcutaneous adipose tissue (SAT) may have reduced effectiveness of abdominal compression, due to attenuation of the compression force. Quantification of SAT on cross-sectional imaging, including diagnostic MRI, is used as a biomarker in metabolic conditions such as diabetes. This work quantified SAT percentage fat fraction (%FF) using MRI acquired on a MR-Linac and identified the optimal point for quantification of abdominal SAT for radiotherapy motion management. The impact of SAT levels on reduction of respiratory motion with abdominal compression for radiotherapy was also investigated.

Eight participants (6 patients and 2 healthy volunteers) were imaged on a 1.5T MR-Linac as part of an institutional study. T2-weighted 3D MRI and cine-MRI were acquired with and without an abdominal compression belt. External contour and SAT were segmented retrospectively on a single slice at three points: the junction of vertebral levels T12/L1, L1/2, and L2/L3. The inner and outer boundaries for SAT were defined as the external border of skeletal muscle, and the external contour respectively. Images were segmented and evaluated by an experienced therapeutic radiographer (RTT). %FF (SAT volume relative to external contour volume) was calculated. Respiratory motion, defined as craniocaudal motion of the central apex of the liver from highest to lowest point was measured on coronal cine-MRI averaged across three breathing cycles. Bivariable correlation analysis was used to evaluate the relationship between %FF and change in amplitude with compression at each vertebral level, as well as the relationship between %FF and BMI.

Median BMI was 27.9 (range, 25.3 – 38.0), and all participants were considered overweight or obese. Seven out of eight participants saw a reduction in peak-to-peak liver apex motion with abdominal compression (Table 1). One patient volunteer saw an increase of 0.62 mm from 5.12 mm (BMI 26.1, %FF 41.5). Change in motion amplitude was strongly correlated with initial motion (r = 0.85). %FF showed strongest correlation with BMI at the level of T12/L1 (r = 0.61). %FF at T12/L1 and L2/3 had a moderate negative correlation with amplitude change with abdominal compression (Figure 1). 

Single-slice SAT measurements on MR images acquired on the MR-Linac are feasible. SAT %FF demonstrated a stronger relationship with amplitude change with abdominal compression than BMI, particularly at the level T12/L1.  In patients with smaller initial motion (≤ 5 mm), care should be taken when using abdominal compression as increases in liver motion are possible. This work will be validated on a larger cohort as part of ongoing work.