PET may spare some patients with rhabdomyosarcoma a marrow biopsy

Before chemotherapy or surgery, every patient in an international study underwent two checks for cancer in the bone marrow: a standard sample and an FDG-PET scan. The study included more than 300 people with rhabdomyosarcoma, a rare cancer of muscle tissue that accounts for about 60% of soft-tissue sarcomas in children. The results suggest that a scan could spare many patients the needle procedure.

Bone marrow sampling has, until now, been considered the gold standard for determining whether the tumor has spread to bone marrow. But it is painful and reports what is happening at the puncture site; if tumor cells are scattered unevenly through the marrow, sampling can miss them. FDG-PET offers a body-wide image: cancer cells consume more energy than healthy cells and accumulate the weakly radiolabeled sugar, allowing a scanner to reveal suspicious tissue. Patients receive only a small amount of radiation, according to the researchers.

The study found that three-quarters of the patients had no bone marrow involvement. Among the remaining quarter, sampling and FDG-PET both indicated involvement in around 70% of cases, while 30% of positive diagnoses were based solely on PET imaging. In one case, sampling alone found involvement that PET did not detect.

The proposed workflow is straightforward: use FDG-PET first, and avoid sampling when the scan shows no FDG-avid marrow foci. An abnormal scan would still need confirmation with bone marrow sampling or magnetic resonance imaging, because injuries and inflammation can also produce a positive result. The researchers describe a positive scan as a strong indication, not a final diagnosis.

So what changes in practice? For patients with a negative FDG-PET scan, the findings suggest that about three-quarters could likely avoid a painful marrow biopsy. The study supports changing the order of tests rather than abandoning sampling: PET would screen the body first, while sampling or MRI would remain available to verify suspicious areas. The study's figures come from a retrospective analysis, and the researchers recommend this approach for the future.