A Scan Can Spare Most Kids a Painful Bone Marrow Biopsy

The needle goes in at an angle through skin and muscle, into the hip bone, until it reaches the marrow inside, and children with a rare cancer called rhabdomyosarcoma have often had to endure it twice, once on each side, just to find out whether the disease has spread there. A scan already used to find the tumor itself, it turns out, can often replace that needle. A clear image, a large international study found, is trustworthy enough on its own to close the question, but a suspicious one still needs a second look, because bruised or inflamed marrow can light up almost exactly the way cancer does. That asymmetry, more than any single number in the results, is the discovery a research team spent years gathering enough patients to prove.

Rhabdomyosarcoma is rare, but it is the most common soft-tissue cancer in children and teenagers, arising from tissue that would otherwise go on to become muscle. Whether the disease has reached the bone marrow changes everything about staging and treatment: three-year, event-free survival runs just 14% to 17% for children with marrow or bone involvement, compared with 34% to 37% for those whose metastases spare it.

The historical standard has been the bone marrow aspirate and biopsy, sampled from both hip bones on the assumption that a needle anywhere in the marrow will catch any cells that have spread there. Rhabdomyosarcoma doesn’t always infiltrate marrow evenly, though, and a biopsy only tests the two spots where the needle actually goes in, so patchy disease elsewhere in the skeleton can be missed. The scan works differently: patients get a small dose of a radioactive sugar tracer called FDG (short for fluorodeoxyglucose), and because cancer cells are hungry for fuel, they soak up more of it than healthy tissue does and light up on the resulting image, the same principle behind PET imaging described in the National Cancer Institute’s own patient guide to rhabdomyosarcoma. The same PET scan, in other words, that first spots the tumor can in principle survey the entire skeleton for the same telltale glow, without a needle anywhere near the hip.

Getting a biopsy in a child usually means general anesthesia, plus the ordinary risks of bleeding or infection that come with any needle procedure, and a study tracking repeated bone marrow procedures under anesthesia in children with cancer found real, if usually minor, complications along the way, so doctors have wanted a less invasive alternative for years. Nobody had tested the swap in a large enough group of children with this specific cancer, though, until an international team pooled records from patients treated across Europe and North America.

Putting the Scan to the Test

The researchers, working through a consortium called INSTRuCT, gathered records on 313 children and young adults with rhabdomyosarcoma from 18 hospitals, everyone of whom had both a bone marrow biopsy and a PET scan done before starting chemotherapy. Of those, 301 had tumors that lit up clearly enough on the scan to be usable for the main comparison, called FDG-avid tumors in the paper’s own terms. Because every patient in the group had both tests, the team could compare them head to head rather than relying on separate studies with different patients.

Simone Hettmer, a pediatric oncologist at University Medicine Halle in Germany and the study’s senior author, has watched the limits of the needle approach up close. “Bone marrow sampling has, until now, been considered the gold standard for determining whether the tumor has spread to a patient’s bone marrow. However, this procedure is painful and only provides information about the situation at the site of the puncture. Patchy bone marrow involvement may therefore be missed,” she says.

Across the 301 patients with FDG-avid tumors, the scan and the biopsy agreed almost every time: both flagged marrow disease in 54 patients, and both came back clear in 222, for an overall match rate that left the scan looking almost as trustworthy as the needle it might replace. The scan caught marrow disease in another 24 patients whose biopsy missed it, and it detected 98% of every case biopsy itself confirmed, a sensitivity high enough that a clear PET result carried a 100% chance of truly being clear, at least in this group. The needle still won exactly once: a single patient had tumor cells on biopsy that the scan never picked up. The tradeoff sits on the other side of the ledger, where the scan was less disciplined about calling something abnormal that was not: specificity came in at 90%, meaning a meaningful share of positive results reflected something other than cancer, most likely inflammation, injury, or ordinary marrow activation rather than tumor itself. Even genuine marrow disease was not always straightforward to picture, since about a third of the FDG-avid cases involved only a handful of scattered spots, one to five foci, rather than a single obvious mass.

Where the Needle Still Wins

The two positive groups were not clinically identical, either: children whose marrow disease was confirmed by both the scan and biopsy had a hazard of death nearly seven times higher than children with no marrow disease at all, while children flagged by the scan alone carried a hazard roughly two and a half times higher, a gap the researchers link to more extensive disease rather than to the test itself. It is an association, not a randomized comparison, and the study cannot say why concordant cases trend more severe, only that they do.

Timing mattered too: when the scan was performed after chemotherapy had already started, sensitivity for marrow disease fell to 86%, and when it was done only after the tumor itself had already been surgically removed, sensitivity dropped further, to 60%, though both of those subgroups were small enough that the estimates carry wide uncertainty. The biopsy side of the comparison had its own weak spot: sampling was frequently inadequate by published standards, with a share of specimens either one-sided or too short a core to trust fully, which raises a separate question about how solid the historical gold standard really was to begin with.

What Changes at Diagnosis

The practical upshot, the authors argue, is a reordering of the diagnostic sequence rather than the end of the biopsy altogether: scan first, and only reach for the needle, or an MRI, when the scan itself raises a flag. That flag still needs following up, since a positive image alone was accurate only 90% of the time in this cohort, but a negative one, done correctly and covering the whole body before treatment starts, appears to be close enough to conclusive. Reineke Schoot, a pediatric oncologist at the Princess Máxima Center in Utrecht and the study’s co-senior author, frames it as a shift rather than a shortcut. “A positive FDG-PET scan is a strong indication of bone marrow involvement but still requires confirmation. Conversely, patients with a negative FDG-PET scan can likely avoid a painful bone marrow biopsy in about three-quarters of cases,” she says. “Our study suggests that this approach is truly practice-changing,” Schoot adds.

Rhabdomyosarcoma is a small enough disease that a single retrospective study rarely settles anything for good, and prospective confirmation, testing the scan-first approach as it happens rather than after the fact, is the obvious next step before it becomes routine practice everywhere. For now, the more immediate change is smaller and more concrete: the next child who needs staging for this cancer may walk out of the imaging suite already done, with no second procedure and no second dose of anesthesia, unless the picture on the screen says otherwise.

FAQ