Standardization of Immobilization for Reproducible Set-Up for Patients Under 18 Years Treated with Proton Beam Therapy in the Thoracic Region

Objectives

To review positioning and immobilisation for patients treated with PBT in the thoracic region for sarcoma and lymphoma. To evaluate daily set up error from imaging data, comparing immobilisation methods.

Methods

A retrospective analysis of patients ≤18 years treated with PBT in the thoracic region at University College London Hospitals from December 2021 to present date was undertaken. Diagnosis, age, general anaesthetic requirement, treatment site, positioning and immobilisation were recorded. Image verification data including daily set up error, additional imaging and dosimetric evaluation of positional discrepancies were captured and analysed from ARIA record and verify system.

Results

Twenty patients, with a mean range of age of 13 years (range 5-18y), were treated. Five patients were treated under general anaesthesia. All patients were immobilised in either a long vacuum bag, short vacuum bag or head and shoulders thermoplastic shell with personalised Moldcare® cushion. Individual and population set up error means and standard deviation were calculated and compared by immobilisation type, general anaesthesia status and treatment site. Due to small population, no statistically significant differences in set-up errors were noted for all variables. Overall population translation means ≤1mm, standard deviation ≤0.3; rotational means ≤0.2°, standard deviation ≤0.75. Shoulder position was the most frequent cause for patient re-positioning, followed by correction of yaw. Patients immobilised in head and shoulders thermoplastic shells did not show reduced set-up error compared to those immobilised in a vacuum bag. This was attributed to patient specific factors. Patients ≥10 years treated under general anaesthesia had greater set-up error than those treated awake.

Conclusion

The optimal immobilisation method is determined primarily by treatment site and beam arrangement for treatment delivery. Patient factors such as comfort and compliance influenced set-up reproducibility and should be given due consideration, when selecting immobilisation, in all cases.