Can Ga-68-PSMA PET/CT scan change radiation treatment decisions?

A diagnostic technique is valuable only insofar as it is able to change treatment decisions. A small Australian study claims that GA-68-PSMA PET/CT scans can in about half the cases.

Thomas Shakespeare diagnosed 54 patients using a Ga-68-PSMA PET/CT. He selected patients with any of the following characteristics:

• ·      Equivocal results on bone scan, CT or MRI
• ·      Negative bone scan, CT or MRI, but reason to question those findings
• ·      Suspected as having oligometastatic PC (1-3 nodal or distant metastases)
• ·      If post-primary treatment, PSA<10 and no detected metastases

The potential decisions to be made were whether to…

• ·      pursue curative primary IMRT in 15%.
• ·      pursue salvage IMRT in 33%.
• ·      pursue radiation of oligometastases after primary RT or salvage RT in 50%.
• ·      determine response to systemic therapy in 2%.

After PSMA-PET diagnosis, the treatment plans changed as follows:

• ·      Observation: 50%-> 19%
• ·      IMRT (primary or salvage): 33% -> 28%
• ·      Oligometastatic treatment: 9% -> 37%
• ·      Systemic therapy (ADT and/or chemo) only: 7% -> 17%

When conventional imaging was negative, PSMA-PET was also negative in 32% of cases, but was positive in 46% of cases. When conventional imaging gave equivocal results, PSMA-PET was split pretty evenly – negative in 7 cases (13%), positive in 5 (9%).

The PSMA-PET had little effect on the primary/salvage IMRT decision. Most of its effect was in detecting oligometastases for palliative treatment, and the remainder in detecting patients who were found to be poor candidates for any radiation therapy.

The radiation treatment of oligometastases continues to be controversial, with the most recent data showing little or no curative benefit. Although Dr. Shakespeare is careful to label the treatment of oligometastatic prostate cancer as “palliative,” he elsewhere writes: “In our study, potentially curable patients were found to be incurable, and potentially incurable patients were found to be curable.”  While it’s true that 5 extra patients (9%) were given systemic (non-curative) therapy only, there do not seem to be any who were discovered to be “potentially curable.” If Dr. Shakespeare were to follow US standard-of-care protocols (i.e., no non-palliative radiation treatment of oligometastases) and treat those with oligometastases with observation or systemic therapy only, there would have been little change in treatment decisions: 3 of the 18 patients (17%) originally planned for curative radiation would have been assigned to observation or systemic treatment.

There is no question that PSMA-PET scans are more accurate than conventional imaging, but it remains an open question as to whether that enhanced accuracy will change radiation therapy decisions as it is currently practiced in the US enough to justify the increased cost.