Radical radiation therapy for oligorecurrent prostate cancer is considered to improve both overall and disease-specific survival [5, 6]. However, there is no consensus on imaging modality to evaluate the indications for radical radiation therapy such as SABR. Recent report has shown the diagnostic utility of 177Lu-PSMA SPECT; however, 68Ga-PSMA PET is likely to be more useful because of the better spatial resolution and sensitivity of PET . In this article, bone scintigraphy and SPECT/CT did not accurately detect the recurrent lesion. We considered that the first bone metastasis was osteoblastic metastasis, but the second bone metastasis was bone marrow metastasis rather than osteoblastic metastasis. Therefore, 99mTc-HMDP did not accumulate in the second metastasis, and abnormal accumulation was observed in the first osteoblastic metastatic lesion despite SABR was performed. On the other hand, in 68Ga-PSMA PET, the tracer specifically binds to the PSMA which is expressed in viable prostate cancer cells and the spatial resolution is better than SPECT [1–3, 7, 8].
There are several unique aspects in this article. First, 68Ga-PSMA PET may be superior to bone scan in evaluating the therapeutic effect after radiotherapy. As far as we know, this is the first report to evaluate the effectiveness of SABR by performing both bone SPECT/CT and 68Ga-PSMA PET/CT. We consider that 68Ga-PSMA PET may be a useful biomarker for assessing the therapeutic effect of SABR for osseous oligometastasis. Second, abnormal accumulation of 99mTc-HMDP on bone scintigraphy and SPECT/CT does not necessarily mean relapse when SABR is given to bone metastases. In a case of oligometastasis, local control of metastatic lesion is an important prognostic factor [4–6]. Therefore, accurate assessment of the effectiveness of SABR is mandatory in considering re-irradiation or predicting prognosis. 68Ga-PSMA PET may become a useful biomarker for predicting the post-treatment course of oligometastasis.
There are several limitations in this study. First, the optimal timing of 68Ga-PSMA PET after SABR has not yet been determined. In addition, there are few well-documented reports on PSMA PET-guided SABR for oligometastasis of CRPC . A large-scale prospective study may warrant the feasibility and effectiveness of PSMA PET-guided SABR. Second, a single case report cannot be generalized to others without further scientific validation, however, this is the first report showing that PSMA PET/CT helps diagnose marginal recurrence of bone metastases treated with SABR.
In conclusions, the implementation of PSMA ligand PET may substantially improve the diagnostic accuracy of detecting osseous oligometastasis even after SABR for metastatic CRPC.