Can gallium-68-PSMA PET/CT scans change radiation treatment decisions?

A diagnostic or prognostic technique is valuable only insofar as it is able to change treatment decisions. A small Australian study claims that gallium-68-PSMA  ([68Ga]PSMA) PET/CT scans could do this in about half the cases examined.

Shakespeare evaluated 54 patients using a [68Ga]PSMA PET/CT scan. He selected patients with any of the following characteristics:

  • Equivocal results on bone scan, CT scan, or MRI
  • Negative bone scan, CT scan, or MRI, but reason to question those findings
  • Suspected as having oligometastatic prostate cancer (one to three nodal or distant metastases)
  • A PSA level < 10 ng/ml post-primary treatment and no detected metastases

The potential decisions to be made were whether to …

  • Pursue curative primary IMRT in 15 percent of the patients
  • Pursue salvage IMRT in 33 percent of the patients
  • Pursue radiation of oligometastases after primary or salvage radiation therapy in 50 percent of the patients
  • Determine response to systemic therapy in 2 percent of the patients

After [68Ga]PSMA PET/CT evaluation, the treatment plans changed as follows:

  • Observation: 50 percent → 19 percent
  • IMRT (primary or salvage): 33 percent → 28 percent
  • Oligometastatic treatment: 9 percent → 37 percent
  • Systemic therapy (ADT and/or chemotherapy) only: 7 percent → 17 percent

When conventional imaging was negative, [68Ga]PSMA PET/CT was also negative in 32 percent of cases, but was positive in 46 percent of cases. When conventional imaging gave equivocal results, [68Ga]PSMA PET/CT was split pretty evenly — negative in 7 cases (13 percent), positive in 5 (9 percent).

The [68Ga]PSMA PET/CT scan result 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 percent) 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: 3/18 patients originally planned for curative radiation (17 percent) would have been assigned to observation or systemic treatment.

There is no question that [68Ga]PSMA PET/CT 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 they are currently practiced in the US enough to justify the increased cost.

Editorial note: This commentary was written for The “New” Prostate Cancer InfoLink by Allen Edel.

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