ESTRO 2023 - Abstract Book

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ESTRO 2023

prostatectomy group. However, a perfect contour alignment is not suggested as it may lead to an overdose of OARs or under-dose to target.

PO-2326 Evaluating the use of planning PET in radiotherapy treatment planning for rectal cancer

Y. Gao 1 , G. Blackman 2 , T. Richards 2 , M. Hawkins 3 , D. Brand 3

1 University College London , Department of Medical Physics and Bioengineering, London, United Kingdom; 2 University College London Hospitals, Oncology, London, United Kingdom; 3 University College London, Department of Medical Physics and Bioengineering, London, United Kingdom Purpose or Objective Radiotherapy for rectal cancer is typically contoured using clinical, endoscopic, CT and MRI data. Staging PET usage is infrequent, so a planning FDG-PET scan, examining tumour metabolic activity, might aid both staging and the contouring process. University College London Hospital (UCLH) has been using planning FDG-PET and here we examine its influence on treatment pathways. Additionally, given novel total neoadjuvant strategies, we examine planning FDG-PET activity post neoadjuvant chemotherapy. Materials and Methods From the UCLH treatment planning system (TPS), 327 consecutive rectal adenocarcinoma patients were identified (radiotherapy dates: 13/02/2015–20/04/2022). Those who underwent planning FDG-PET prior to long (45–53.2Gy in 25–28 fractions, n=112) or short (25–30Gy in 5 fractions, n=12) course radical neoadjuvant rectal radiotherapy without staging FDG-PET were included (n=124). Patient demographics, disease characteristics and radiation treatment parameters were retrieved from health records and the TPS. We hypothesised firstly that planning FDG-PET would alter 20% of patients' treatment pathways: change of radical intent or new metastases. Secondly, that neoadjuvant chemotherapy, prior to radiotherapy, might cause zero biological activity on planning FDG-PET. Results Baseline characteristics are summarized in Table 1. The cancer pathway was altered in four patients (4/124, 3.2%, 95%CI: 1.3–8.0%) due to previously unknown metastases on planning FDG-PET (liver metastases, n=4). This was statistically significantly less than the hypothed 20% treatment pathway alteration rate (proportion z-test, p<0.001). Regarding FDG- PET scans with zero biological activity, this occurred in zero (0/109, 0%) patients without neoadjuvant chemotherapy and two (2/15, 13.3%, 95%CI: 3.7–38%) who received it (Fisher’s Exact, p=0.014).

Conclusion At our centre, planning FDG-PET scans have unexpected findings, altering treatment strategy, in a low number of patients. With regards to patients having total neoadjuvant therapy, zero FDG-PET activity was observed more frequently than those not receiving neoadjuvant chemotherapy, although the rate was still low.