Impact of 68Ga-PSMA PET/CT on radiation treatment planning of prostate cancer patients

Aim This study aimed to assess the impact of 68 Ga-PSMA PET/CT on radiation treatment (RT) planning in prostate cancer patients with salvage (sRT) or definitive (dRT) radiotherapy. Methods 38 patients (27 sRT, median PSA 0.79 ng/ml (range 0.06-12.1); 11 dRT, median PSA 4.35 ng/ml (range 1.55-55.5) underwent 68 Ga-PSMA PET/CT before RT. Influence of 68 Ga-PSMA PET/CT on the extent of planning target volume (PTV) and addition of PET-based boosts were assessed. Median follow up was 12 months (range 3-24). Results 68 Ga-PSMA PET/CT showed positive findings in 23/38 patients (8/23: local recurrence (LR), 11/23: nodal metastasis, 1/23: LR and nodal, 2/23: solitary bone metastasis, 1/23: oligometastatic nodal/ bone metastases). In sRT primary PTV was changed in 16/27 patients extending the PTV to the lymphatic drainage (10/16), PSMA-positive LR (3/16), bone metastases (2/16) and both nodal/bone metastases (1/16). PET-based increase of primary PTV was 116%. PET-based boosts were administered in 19/27 patients (8/19: local, 10/19: nodal, 1/19: both), median boost volume was 31.3 cm 3 (range 17.2-80.2) (local) and 19.7 cm 3 (range 3.0-109.3) (nodal). PTV was changed in 1/11 (9%) of dRT patients (extension of primary PTV to the lymphatic drainage (RT volume of 644.5 cm 3 ), additional nodal boost (volume of 2.7 cm 3 , 23.1 Gy)). All patients showed biochemical response (mean PSA decrease 88.8 +/- 14.0%). Nadir PSA was reached 10 months (range 1-17) after end of RT (median 0.07 ng/ml, range 0.002-3.96). Within a median 12 months follow-up (range 3-22/8-24 in sRT/dRT), median PSA was 0.05 ng/ml (range 0.002-8.5) (sRT) and 0.26 ng/ml (range 0.02-2.68) (dRT). Conclusions 68 Ga-PSMA PET/CT influenced sRT planning in almost 63% and dRT in 9% of patients by change of PTV and additional boosts.