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Compression belts are an innovative solution to reduce target and OAR motion for patients with liver cancer. However, the widespread adoption of this equipment remains limited. This work aims to evaluate the treatment uncertainties of an abdominal compression belt utilising the IGRT verification data for patients with liver cancer.

Evaluation of patients treated from January 2020-October 2022 was approved. Before the planning CT scan, patients were coached for compression and exhale breath-hold (EBH). A contrast-enhanced EBH CT scan and free-breathing (FB) 4DCT were acquired. Patients were treated in FB with a compression belt (CIVCO RT) and vacbag. Daily CBCTs were registered to the EBH CT and matched to bone with a soft tissue adjustment. For a subset of patients, a post-treatment CBCT was taken. All online CBCT shifts measured the uncorrected interfraction errors and intrafraction errors. All independent offline CBCT shifts measured the matching variability. Statistical analysis evaluated if patient demographics were associated with interfraction systematic shifts (∑).

Of 76 patients with liver malignancies treated, 34 cases were reviewed, resulting in 217 pre-treatment CBCTs and 53 post-treatment CBCTs Table 1.

At CT, 33 (97%) patients achieved compression, 26 (76%) patients achieved EBH and 33 (97%) achieved a 4DCT. For 15 fractions re-setup was required and on 13 a repeat CBCT was taken as match time exceeded 10 minutes. Table 2 summarises the calculated mean (M), ∑ and random (σ). There were 64 (29%) interfraction shifts  >10mm. There was 1 (2%) intrafraction shift >5mm. There were 4 (2%) occasions when the matching variability >5mm. Analysis to assess patient demographics and interfraction ∑ showed that patients with no EBH scan had a larger long ∑. Additionally, in 11 patients there was a moderate positive relationship between BMI and the long ∑, Table 1.

The compression belt achieves a stable set-up for liver patients and over 97% of patients can tolerate it. Almost 1/3 of interfraction shifts are greater than 10mm. This is not concerning for patients provided shifts are corrected with imaging and our centre will continue to CBCT daily. Larger ∑ in the long were seen in patients with no EBH scan and in patients with a higher BMI. Intrafraction errors were low (<2mm), highlighting the compression belt is a good solution for maintaining a stable position during treatment delivery. Thus, the post-treatment CBCTs were removed after 10 patients ensuring IGRT doses are ALARP. The interobserver variability was low, but although matches were performed independently, they were not completed blind from online registrations. This may have increased match agreement. The initial findings in Table 2 indicate treatment uncertainties are low and our CTV-PTV margin of 5mm appears suitable when using the compression belt in liver patients with daily CBCT; however, more patients in the analysis are required for conclusive PTV calculations.