ESTRO 2024 - Abstract Book

S2027

Clinical - Paediatric

ESTRO 2024

radiotherapy centres. Volumetric Modulated Arc Radiotherapy (VMAT) is able to deliver TBI within resource constraint settings with a standard LINAC and provide better sparing of lungs and kidneys. The Department of Clinical Oncology at University Malaya Medical Centre is the first centre in Malaysia to deliver TBI with a VMAT technique. The aim of this report is to provide an overview of VMAT TBI implementation in our centre.

Material/Methods:

The TBI service planning started in May 2023 and delivered treatment for the first paediatric patient in October 2023. Acquisition of 3D simulation images was done with Philips Brilliance CT simulator with the patient lying supine in a knee-bent position on a vacuum-lock bag. The headrest and thermoplastic shell were fitted on the vacuum-lock bag. Two sets of images were acquired with different setup position; 1) head-first (HF) position and 2) feet-first (FF) position. After simulation in HF position, manual rotation of the patient and vacuum-lock bag for scanning in a feet-first (FF) position was done. Two separate origins were identified for each scan direction. Both HF and FF image sets were imported and stitched using the MIM Maestro® system version 7.0.6 to produce a single image set. The stitched image was then used for delineation of target volumes and organs at risk. Organs at risk were lens, lungs, and kidneys with 3mm contracted to produce planning risk volume (PRV). Radiotherapy treatment was planned with Monaco version 6 treatment planning system delivered with Elekta VersaHD LINAC, with a prescribed dose of 12 Gy in 6 fractions, treating twice a day over 3 days. Each isocentre was limited to 20cm cranio-caudal field length. Robustness plans were performed to quantify dose inhomogeneities due to set-up errors. Pre-treatment verification and patient-specific quality assurance (PSQA) was done using a two-dimensional ionisation chamber array MatriXX Evolution and Gafchromic EBT3 film. During the treatment, in vivo dosimetry was performed using Gafchromic EBT3 film at the junction of the HF and FF fields. The patient was positioned at the origin with verification with fast CBCT with online correction. The patient was then moved to the cranial-most isocentre for verification and treatment. After completion of cranial and thorax radiotherapy, the thermoplastic shell was removed to enhance patient comfort. Verification and treatment were then done downwards for the remainder of the body. Following treatment at the pelvic isocentre, the vacuum bag was rotated on the couch to treat the lower limbs. In our experience, total treatment time including verification of each isocentre ranged from 60-90 minutes. A few challenges were identified in our setting. As patients needed to be treated in HF and FF position, they had to be manually rotated while on the vacuum bag. This may be particularly challenging with adult patients. Other centres have adopted a rotating couch-top which would ease the positioning of patients and minimise set-up errors but would incur additional costs. The images scanned in the HF and FF positions needed to be stitched prior to planning. VMAT planning is more labour- and time-intensive than TomoTherapy planning due to the need for multiple isocentres and complex IGRT procedures. As there were limited centres delivering TBI with VMAT technique, we outlined our planning protocol based on literature available. The results of the PSQA and dosimetry verification were within the tolerance limits. Initially, we began treatment delivery from pelvis prior to moving upwards towards the cranium. However, we found that patients became uncomfortable wearing the thermoplastic shell for extended periods. Thus, we then first verified the user origin at the pelvis and then proceeded to the cranium for further verification and treatment. Currently, we conclude that taking the downward rather than upward approach is most comfortable for paediatric patients and reduced treatment time. Results: