(P 321) IDENTIFYING RISK FACTORS FOR REDUCTION OF PROTON THERAPY RE-PLANS FOR PAEDIATRIC PATIENTS WITH PELVIC SARCOMAS

Objective: Proton beam therapy (PBT) is a precise form of radiotherapy used as treatment for local control for paediatric pelvic sarcomas. PBT is very sensitive to changes in anatomy and tissue density which can affect dosimetry resulting in under-coverage of the target volume or over-dosage of critical organs. Strategies such as image guidance, dosimetric assessment and replanning during treatment is necessary to ensure safe and effective delivery of PBT. The replanning process takes about 1-2 weeks and may therefore cause delays or gaps during PBT treatment.
We reviewed the replanning rates and risk factors for patients with pelvic sarcomas treated at our institution.

Methods: 13 pelvic sarcoma patients < 18 years treated between December 2021 to April 2023 were identified from our institutional database. Patients’ medical records were reviewed and analysed for the reasons and timing of replans, weight charts, dietetic records and overall treatment time.

Results: Demographics are described in Table 1. The median age was 14 years (range 1-15).
The cohort comprised of patients with Ewings (n=9), Rhabdomyosarcoma (n=3) and synovial sarcoma (n=1). Patients received doses of preoperative (n=3), definitive (n=9) or post operative PBT (n=1) given over 5 ½ to 6 weeks.
5 patients (38.5%) required replanning. 4 patients required one replan, these were due to separation gain identified on the cone beam CT (CBCT) on Fraction 1 (Fr1) of PBT. The separation gain varied from 0.6 – 1.2cm, resulting in the clinical target volume (CTV) being under dosed. The replans were initiated at week 1 (n=2), week 2 (n=1) and week 3 (n=1). 1 patient required two replans, in week 1 and 4. These were due to tumour shrinkage, causing small bowel to exceed recommended dose tolerance.
The mean weight change from planning CT scan (pCT) to Fr1 was 2.48kg in the replan group vs. 1.28kg in the non-replan group (p=0.2). The mean weight change from pCT to replan CT was higher at 2.98kg/5.88% of total body weight (range 1.3-4.4kg/2.9-10.5%) in the replan group (Table 2).
Dietician input occurred in 3 patients (60%) in the replan group after the weight gain was observed, in week 1 (n=2) and week 3 (n=1). After dietician input their weight varied by a maximum of 1.3kg, 1.2kg, 0.9kg during the remainder of their PBT compared 2.2kg, 1.3kg, 3.1kg prior to dietician input respectively.
Dietician input in the non-replan group occurred in 4 patients (50%), 3 of these saw a dietician prior to starting PBT. This was prompted by clinician due to fluctuations with weight during chemotherapy.
The were no unexpected treatment breaks and the length of treatment time was maintained in the replan group.

Conclusion: This review shows that weight changes are often seen alongside contour changes identified on CBCT. The majority of patients required a replan due to changes seen before the start of PBT. Weight gain of as little as 2.9% resulted in the need for a replan, highlighting the importance of maintaining weight between the pCT and starting PBT as well as throughout the treatment course.
The planning process takes 3-4 weeks from pCT to starting PBT, ideally this interval would be reduced but due to the complexity of PBT planning and timing of chemotherapy this is often not possible.
37.5% of patients in the non-replan group v’s 0% in the replan group had met a dietitian prior to the start of PBT. Our results suggest that early engagement of a dietician during the PBT planning process may be crucial in reducing replans due to proactive patient education and monitoring. We are working towards implementing dietician reviews at the first PBT clinic appointment for our patients and twice weekly weights. This will be re-audited after implementation to assess if this reduces weight changes and the number of replans.
Future work is being undertaken to explore the impact of weight changes from chemotherapy such as hydration regimens and utilisation of steroids prior to and during PBT.