Just a week of FDG PET/CT for treatment response in advanced melanoma

Cancer immunotherapy has helped transform the standard of care for many malignancies, but not all patients respond to therapy, and the treatment can cause severe adverse events. Typically, immunotherapy patients are imaged around three months after starting treatment to monitor their progress.

The research team led by Michael D. Farwell, an associate professor of radiology at the Hospital of the University of Pennsylvania hypothesized that by using F-fluorodeoxyglucose (FDG) PET/CT, which is one of the most common and readily available ways to test for cancer, patients could experience metabolic changes in tumors after a week on immunotherapy.

The study is published in Clinical Cancer Research, a journal of the American Association for Cancer Research (AACR).

To test their hypothesis, the researchers recruited 21 patients with advanced melanoma scheduled to initiate pembrolizumab. As part of the trial, patients were required to have at least one measurable lesion and could not have received previous anti-PD-1 or anti-PD-L1 therapies. FDG PET/CT imaging was performed on each patient within four weeks prior to their start of therapy and then at about one week after the first dose of pembrolizumab.

FDG activity for each lesion was measured using the maximum standardized uptake value. For the purposes of the study, an MF was defined as a greater than 70% increase in tumor standardized uptake value and an MR as a greater than 30% decrease in tumor standardized uptake value. An MF or MR was identified in 55% of the patients who responded to treatment (6 out of 11) and 0% of patients who did not respond (0 out of 8). An MF or MR also correlated with longer survival, with 83% of the MF-MR group seeing an overall survival of three years compared to 62% in the group with stable metabolism.

The researchers concluded that since tumors pass through a stable metabolism phase between MF and MR responses, it will be key to identify if a tumor with stable metabolism is in fact responding but is in between reaction phases and that some potential solutions for that include layering on companion studies such as blood tests, a CD8 PET scan, or serial FDG PET/CT imaging to better plot out the change over time.

“While the results need to be validated, this has the potential to be broadly applicable and offer physicians the ability to deescalate therapy or avoid surgery in patients who are responding, identify non responders who may need an escalation of therapy, and to be used in phase I clinical trials to test if a therapy is working,” Farwell explained.

Reference: One-week FDG PET/CT predicts treatment response in advanced melanoma; Clinical Cancer Research