ESTRO 2023 - Abstract Book

S755

Monday 15 May 2023

ESTRO 2023

(BgRT) is a new therapeutic modality that accurately delivers radiation to target tumours using PET emissions originating from a volume named the biological tracking zone (BTZ). BgRT can be delivered by a new generation of linear accelerators equipped with PET detectors (RefleXion Medical, Hayward, California). Due to the particularly high PSMA uptake by metastatic RCC, these tumours may be ideal for BgRT treatment. This study investigates the feasibility of BgRT for patients with metastatic RCC. Materials and Methods Eighty-three consecutive patients that underwent PSMA PET/CT for RCC staging purposes at our institution between 2014 and 2020 were reviewed. Gross tumour volumes (GTV) were delineated on the CT component of the PET/CT scan by using the standardised uptake value (SUV) distribution on the PET component as a guide. BTZs were generated as the GTV plus a disjoint outer margin expansion of the GTV of of 5 mm / 10 mm / 20 mm. Tumours were deemed suitable for BgRT if there was no physiological or adjacent tumour avidity inside the margin expansion and if the normalised SUV (nSUV), defined as the ratio between SUVmax inside the GTV and SUVmean inside the margin expansion, was larger than a specified nSUV threshold. The determination was repeated for nSUV threshold 3, 4, and 5. Results Of the 83 patients screened, 47 patients had metastatic disease, resulting in 136 GTVs (1 to 22 per patient), of which were mostly in lung and bone (40% and 30% of all GTVs). An illustration of the disease distribution in one patient is shown in Fig. 1. GTV volumes ranged from 0.1 cm ³ to 152 cm ³ (median GTV volume = 6.8 cm ³ ). SUVmax inside GTVs ranged from 1 to 69 (median SUVmax = 11). The proportion of tumours suitable for BgRT classified per anatomical site (abdominal (ABDO), lymph nodes and soft tissue (LN&ST), bone, and lung) is shown in Fig. 2 for a range of nSUV thresholds and the three margin expansions considered. With nSUV ≥ 3, 66% / 63% / 41% of all GTVs were suitable for BgRT within 5 mm / 10 mm / 20 mm from the tumour. Bone metastases had the highest rate of BgRT suitability (50% to 98% of bone metastases were suitable for BgRT depending on the margin expansion and the nSUV threshold considered). Tumours unsuitable for BgRT had PET avidity inside the margin expansion due to the presence of the kidney or the liver. Conclusion 63% of all tumours were found suitable for BgRT within 10 mm of the tumour. Therefore, BgRT for metastatic RCC patient is feasible. Presence of another tumour, liver, or kidney near the tumour may affect BgRT delivery.