Desmoplastic Small Round Cell Tumor: Diagnosis and Therapeutics

Desmoplastic small round cell tumor (DSRCT) is a rare, aggressive cancer that grows primarily in the abdominal cavity and overwhelmingly affects young men, typically in their teens to early thirties. Diagnosing it depends on a combination of tissue biopsy, a characteristic pattern under the microscope, and confirmation of a specific genetic translocation that fuses two genes called EWSR1 and WT1. Treatment remains difficult because no single therapy reliably cures the disease, but multimodal approaches that combine intensive chemotherapy with aggressive surgery have pushed five-year survival from roughly 5% in earlier decades to around 25% in more recent cohorts. The biology of this tumor is slowly yielding new therapeutic angles, from androgen receptor blockade to antibody-based strategies, though none has yet changed the standard of care.

Who Gets DSRCT and How It Shows Up

DSRCT is strikingly skewed toward young males. In most reported series, the male-to-female ratio is about four to one, and the average age at diagnosis is in the late twenties.¹ The reason for this sex disparity is not fully understood, though androgen receptor expression in the tumor cells is one hypothesis researchers are exploring.

Patients almost always present with advanced disease. The tumor seeds itself across the peritoneum, the lining of the abdominal cavity, forming multiple masses that do not clearly originate from any single organ.² Symptoms are vague at first: abdominal pain, bloating, a feeling of fullness, sometimes a palpable mass. Because these complaints overlap with far more common conditions, DSRCT is often discovered only once imaging reveals a large dominant mass, frequently over five centimeters, surrounded by many smaller deposits studding the omentum and peritoneal surfaces.¹ Liver and lung metastases are common at diagnosis or shortly afterward. Rare cases arise outside the abdomen, in locations such as the parotid gland, the chest, or the pelvic soft tissues, which can delay recognition even further.³

Making the Diagnosis

The diagnosis of DSRCT rests on three pillars: what the tumor looks like under the microscope, how its cells stain with antibody-based tests, and whether the defining genetic rearrangement is present.

Microscopic Appearance

Under the microscope, DSRCT has a distinctive look. Well-defined nests of small, round, blue-staining cells sit embedded in dense fibrous tissue, the “desmoplastic stroma” that gives the tumor its name.⁴ The contrast between the tightly packed tumor cell nests and the abundant scar-like tissue surrounding them is one of the first visual cues for a pathologist. However, this pattern is not always textbook-perfect. Some cases show cells that are larger than expected or have unusual growth patterns, which can create confusion with other small round cell tumors.³

Immunohistochemistry

What makes DSRCT unusual among tumors is that its cells simultaneously express markers from three different tissue lineages: epithelial, mesenchymal, and neural.⁵ In practical terms, the tumor stains positive for keratins (an epithelial marker), desmin (a muscle marker, often in a characteristic dot-like pattern), and sometimes markers associated with nerve tissue. This triple-positive profile narrows the diagnostic field considerably, though it is not absolute. Occasional cases show atypical staining that can lead pathologists astray, particularly when the tumor arises in an unexpected location.

The EWSR1-WT1 Fusion Gene

The gold standard for confirming DSRCT is detection of a specific chromosomal rearrangement, a translocation between chromosomes 11 and 22, written as t(11;22)(p13;q12). This fuses part of the EWSR1 gene with part of the WT1 gene, creating a hybrid protein that drives the tumor’s growth.⁶ This fusion is found in virtually all confirmed DSRCT cases and is not seen in other tumor types, making it a definitive diagnostic marker. Molecular testing by fluorescence in situ hybridization (FISH) or reverse transcription PCR is now standard when DSRCT is suspected. In ambiguous cases where the tissue looks unusual or the staining profile is atypical, finding this fusion gene settles the diagnosis.

Imaging

CT scanning is typically the first imaging study and reveals the characteristic pattern of peritoneal and omental masses without a clear organ of origin. MRI can help delineate the relationship between tumor deposits and adjacent structures when surgery is being planned. PET-CT, which detects areas of high metabolic activity, adds information about the extent and stage of disease that CT alone may miss.⁷ PET-CT is also useful for evaluating whether chemotherapy is shrinking metabolically active deposits before surgery.

Diagnostic Pitfalls

Because DSRCT belongs to the broader family of “small round blue cell tumors,” it can be confused with several other cancers, including Ewing sarcoma, rhabdomyosarcoma, lymphoma, and neuroblastoma. A study evaluating diagnostic accuracy across over a hundred cases found real-world examples of DSRCT being mistaken for Ewing sarcoma and vice versa, partly because both tumors involve EWSR1 gene rearrangements, just with different fusion partners.⁸ In one case from that series, a DSRCT involving the uterus and omentum was initially mistaken for a high-grade endometrial stromal sarcoma because its location pointed toward a gynecologic primary. Keratin staining, which is characteristic of DSRCT, helped flag the correct diagnosis, but the case illustrates how clinical context can mislead even experienced pathologists. The practical message is that molecular confirmation of the EWSR1-WT1 fusion should be pursued whenever DSRCT is in the differential, especially if the tumor arises in an atypical site.

First-Line Treatment

DSRCT demands a multimodal treatment strategy, and the backbone is systemic chemotherapy followed by surgery to remove as much visible tumor as possible. The most widely used regimen is the P6 protocol, originally developed for Ewing sarcoma-family tumors, which alternates high-dose cycles of alkylating agents with other chemotherapy drugs.⁹ The goal of upfront chemotherapy is to shrink the tumor burden enough to make surgical debulking feasible.

Complete or near-complete cytoreductive surgery, meaning removal of all visible tumor, is the single most consistent predictor of longer survival. A large retrospective analysis showed that the combination of chemotherapy that achieves a measurable response and complete surgical cytoreduction has improved five-year overall survival to roughly 25%, up from about 5% in earlier treatment eras.¹⁰ That improvement is real but sober: the vast majority of patients still relapse, and long-term cures remain exceedingly rare. Surgery for DSRCT is often extensive, sometimes requiring removal of portions of bowel, omentum, and peritoneal surfaces in a single operation.

Radiation Therapy

Radiation is used after surgery to try to control microscopic residual disease in the abdomen and pelvis. Whole abdominopelvic radiation therapy (WART or WAPI) has been the traditional approach, but it comes with serious toxicity. One series found that most patients experienced significant gastrointestinal side effects, and over three-quarters developed severe drops in platelet counts.¹¹ A third of patients in that series went on to develop small bowel obstructions, a serious long-term complication.

Intensity-modulated radiation therapy (IMRT) has been introduced as a refinement. By shaping the radiation beams more precisely, IMRT delivers roughly a quarter less radiation to the bones of the spine and pelvis compared with conventional techniques, which translates into less suppression of blood cell production.¹² Even with this improvement, abdominal recurrence remains common, and radiation has not been shown to meaningfully change overall survival on its own. It is best understood as one component of an aggressive multimodal strategy rather than a standalone treatment.

The HIPEC Question

Hyperthermic intraperitoneal chemotherapy, or HIPEC, involves bathing the abdominal cavity in heated chemotherapy solution during or immediately after cytoreductive surgery. The idea is that direct contact between the chemotherapy and the peritoneal surfaces, combined with the heat, may kill residual tumor cells that surgery missed. HIPEC is used at some specialized centers for DSRCT, and one study showed that patients who achieved complete or near-complete cytoreduction and received HIPEC had a median survival of over five years, compared with roughly two years for those with substantial residual disease after surgery.¹³

However, whether HIPEC itself adds survival benefit beyond what thorough surgical cytoreduction alone achieves is genuinely unclear. A review of the available evidence found no clear survival advantage from adding HIPEC after cytoreductive surgery.¹⁴ The challenge in answering this question is the rarity of DSRCT, which makes randomized trials nearly impossible. What is clear is that completeness of cytoreduction matters enormously regardless of whether HIPEC is added.

Targeted Therapies Under Investigation

Standard chemotherapy buys time but rarely provides lasting control, so researchers have been looking for vulnerabilities in DSRCT’s biology that could be exploited with targeted drugs.

Pazopanib

Pazopanib is an oral drug that blocks the growth of new blood vessels feeding a tumor. In a study of patients with advanced DSRCT, roughly 60% achieved some degree of clinical benefit, mostly in the form of disease stabilization rather than tumor shrinkage. Median progression-free survival was about five and a half months, and median overall survival was close to sixteen months.¹⁵ These numbers are modest, but for patients who have progressed through first-line chemotherapy, a drug that holds the disease steady for several months is meaningful.

Androgen Receptor Blockade

The strong male predominance of DSRCT and the discovery that many tumors express the androgen receptor have led to interest in hormone-blocking therapies. Early work showed that about a third of DSRCT samples expressed androgen receptors at high levels, and that the receptor was functional: male hormones stimulated growth of DSRCT cells in the lab.¹⁶ In a small clinical series, six patients with androgen receptor-positive tumors were treated with combined androgen blockade. Three achieved clinical benefit, and all three responders had normal testosterone levels before treatment, whereas the non-responders already had low testosterone from prior therapy.¹⁷ More recent laboratory work with enzalutamide, a potent androgen receptor blocker used in prostate cancer, has shown that it slows DSRCT cell growth and shrinks tumors in mouse models, reinforcing the idea that this pathway matters. Clinical trials testing androgen-directed therapy in DSRCT are underway or in planning.

HER2-Directed Antibody-Drug Conjugates

A newer line of investigation involves HER2, a protein better known as a target in breast cancer. Researchers found that DSRCT tumors express HER2 and tested antibody-drug conjugates, drugs that use an antibody to deliver a toxic payload directly to tumor cells, in DSRCT models. Both trastuzumab deruxtecan and trastuzumab emtansine showed activity in patient-derived xenograft, cell line, and organoid models of DSRCT.¹⁸ These are preclinical findings and have not yet been tested in patients with DSRCT, but they represent a potentially important avenue given that these drugs are already approved for other cancers and could move into DSRCT trials relatively quickly.

Second-Line Chemotherapy Options

When first-line chemotherapy fails, options are limited. One emerging combination is trabectedin plus irinotecan. Preclinical testing in a patient-derived xenograft model showed that irinotecan and trabectedin together produced complete responses that were maintained through the end of the experiment, outperforming either drug alone as well as other agents tested.¹⁹ In early clinical use, two heavily pretreated young patients received this combination as third-line therapy. One achieved stable disease and the other had a complete tumor remission, with limited toxicity.²⁰ Two cases are far from proof, but the alignment between preclinical results and early clinical observations makes this combination worth watching as it enters formal trials.

Why Immunotherapy Has Struggled in DSRCT

Checkpoint inhibitors have transformed treatment for several cancers, but DSRCT appears to be a particularly tough target for the immune system. Multi-omic profiling of DSRCT tumors has shown that the immune microenvironment is dominated by dysfunctional T cells and a scarcity of the antigen-presenting cells that normally help the immune system recognize and attack cancer.²¹ Spatial analysis of the tumors confirms that they are “immune-cold,” meaning immune cells are sparse and largely excluded from the tumor nests themselves.²² The abundant desmoplastic stroma surrounding the tumor nests may act as a physical and chemical barrier that keeps immune cells out. This helps explain why single-agent checkpoint inhibitors, which work by releasing the brakes on existing immune responses, have shown little activity in DSRCT.

One approach designed to work around this problem is radioimmunotherapy. DSRCT cells express a surface protein called B7H3, which can be targeted by antibodies. A phase I study tested intraperitoneal delivery of a radioactive antibody directed against B7H3, called 131I-omburtamab, in patients with DSRCT or other B7H3-expressing tumors involving the peritoneum.²³ By delivering radiation directly to the tumor surface through the peritoneal cavity, this strategy bypasses the need for a functional immune response and instead uses the antibody as a guidance system for localized radiation.

Monitoring Disease After Treatment

Because recurrence is the norm rather than the exception in DSRCT, ongoing surveillance is critical. Standard monitoring relies on serial CT or PET-CT scans, but an intriguing complementary approach involves liquid biopsy: detecting tumor-specific DNA fragments circulating in the blood. In one genomic case study, researchers identified the exact breakpoint of the EWSR1-WT1 fusion in a patient’s tumor and then designed a personalized assay to detect that specific DNA rearrangement in blood samples. Over three years of follow-up after surgery, the assay detected no circulating tumor DNA, a finding that tracked with clean imaging scans and good clinical condition.²⁴ This is a single case and not yet a validated clinical tool, but the principle is appealing. Detecting the unique EWSR1-WT1 fusion rearrangement in blood could eventually offer a highly specific early-warning system for recurrence, one that might catch rising disease before it becomes visible on imaging.

Long-Term Toxicities of Aggressive Treatment

The treatments that extend survival in DSRCT are punishing. For patients who live long enough, the cumulative toll of intensive chemotherapy, extensive surgery, HIPEC, and whole abdominal radiation can produce serious chronic problems. A study of long-term complications after cytoreduction and HIPEC found that some patients required intravenous nutrition for extended periods, with a median duration of over eight months, due to gastrointestinal damage. Complications requiring additional surgery included bowel obstructions, severe stomach motility problems, and hemorrhagic cystitis.²⁵

Adding whole abdominal radiation on top of surgery and HIPEC compounds the risk. One group described the full sequential protocol as resulting in severe side effects, underscoring that this treatment path demands careful patient selection and a treatment center experienced in managing these complications.²⁶ The dilemma is real: the most aggressive approaches offer the best chance at longer survival, but they exact a heavy physical cost. For patients and families, conversations about goals of care and quality of life are an essential part of treatment planning, not an afterthought. Young patients who achieve remission may face years of managing nutritional support, bowel dysfunction, and other sequelae of the therapies that saved their lives.