Feasibility of biology-guided radiotherapy using PSMA-PET to boost to dominant intraprostatic tumour

Biology-guided radiotherapy (BgRT) delivers dose to tumours triggered from positron emission tomography (PET) detection. Prostate specific membrane antigen (PSMA) PET uptake is abundant in the dominant intraprostatic lesion (DIL). This study investigates the feasibility of BgRT to PSMA-avid subvolume in the prostate region.

Patients enrolled in the prospective randomized trial ProPSMA at our institution were included (ID: ANZCTR12617000005358). Gross tumour volumes (GTVs) were delineated on the PET component of a PET/CT scan from a standardized uptake value (SUV) threshold technique. Suitability for BgRT requires a strong signal-to-background ratio with a surrounding tissue free of significant PSMA uptake. The signal-to-background ratio was quantified from the calculation of the normalized SUV (nSUV), defined as the ratio between SUVmax within the GTV and SUVmean inside a 3D margin expansion of the GTV. The PSMA distribution surrounding the tumour was quantified as a function of the distance from the GTV.

In this cohort of 84 patients, 83 primary tumours were included. Prostate volume ranged from 19 cm³ to 148 cm³ (median = 52 cm³; IQR = 39 cm³ - 63 cm³). SUVmax inside the prostate was between 2 and 125 (median = 19; IQR = 11 - 30). More than 50% of GTVs generated with threshold between 25%SUVmax (median volume = 10.0 cm³; IQR = 4.5 cm³ - 20.0 cm³) and 50%SUVmax (median volume = 1.9 cm³; IQR = 1.1 cm³ - 3.8 cm³) were suitable for BgRT by using nSUV ≥ 3 and a margin expansion of 5 mm.

It is feasible to identify GTVs suitable for BgRT in the prostate. These GTVs are characterized by a strong signal-to-background ratio and a surrounding tissue free of PSMA uptake.