The incidence of desmoid tumors is 0.03% of all neoplasms, and less than 3% of soft tissue tumors, and hence is an extremely rare disease with an onset frequency of about 2–4 per 1-million people. Approximately 10–20% of all cases occur in the abdominal cavity, as in our present cases, whereas 80–90% occur in the limbs, neck, shoulders, chest, and abdominal wall (14–16).
Desmoid tumors often occur in the abdominal cavity of patients who have risk factors, such as FAP, experience of pregnancy and childbirth, and a history of abdominal surgery. However, our patients had none of the above risk factors. In patient 1, sudden left abdominal pain triggered the diagnosis. MRI displayed intratumoral hemorrhage, suggesting that the abdominal pain was owing to rapid tumor-capsule extension. To date, there has been only 1 reported case of a desmoid tumor that was discovered owing to intratumoral hemorrhage (17). In Patient 2, right hydronephrosis was detected on abdominal ultrasonography. There are relatively many reports of desmoid tumors that cause ureteral obstruction, and most of these are found in advanced cases at the time of diagnosis (18). In patient 2, the tumor arose near the right ureter on the proximal side of the ileum mesentery, which caused right hydronephrosis when the tumor was still relatively small.
The differences between desmoid tumors and GISTs in diagnostic imaging are characterized by their site of occurrence, shape, contrast effects, and tumor content. GISTs originate from the gastrointestinal tract and are characterized mainly as a well-defined circular or lobulated tumor with a heterogeneous contrast effect, sometimes with necrotic vessels and cysts inside. On the other hand, desmoid tumors originate from outside of the gastrointestinal tract, often have a morphologically invasive form, and have relatively uniform contrast effects and rarely have necrosis or cysts inside (19). In the imaging analyses of patient 1, the tumor had a well-defined circular shape. The inside of the tumor was heterogeneous on imaging, accompanied by necrosis and cyst formation, which were very different from the previously reported imaging characteristics of desmoid tumors. On the other hand, in patient 2, the mesenteric tumor had a uniform contrast effect, with an irregular rim shape and invasive growth, which was consistent with the characteristics of desmoid tumors. FDG-PET of patient 2 displayed slight FDG accumulation in the main lesion, indicating negative malignancy. FDG accumulation is generally low in desmoid tumors (20) because it reflects tumor burden and cell proliferation, and is hence useful in differentiating malignancies. Based on the above findings, patient 1 was initially diagnosed as having GIST and patient 2 was suspected as having a desmoid tumor.
The NCCN Guidelines suggest that careful observation of histologically confirmed desmoid tumors is a treatment option only for slow-growing cases without functional restriction. Fiore et al. reported that half of the cases of “wait and see” were free of recurrence for 5 years. (21) However, there are no precise treatment guidelines specific to intra-abdominal desmoid tumors. Surgery may be the first choice for patients with rapidly growing tumors or those with the risk of losing organ function owing to invasion of the tumor to other organs. As in patient 2, intra-abdominal desmoids with obstructive uropathy often require ureteral resection. If desmoid tumors are suspected before surgery, a treatment strategy including ureteral resection is necessary (22).
Macroscopically, the appearance of the tumors from the 2 patients were completely different. In patient 1, the tumor showed necrosis, cysts, and hematoma, which are rare in desmoid tumors, and grew expansively. By contrast, in patient 2, the tumor was hard like a stone, and showed highly invasive growth. On the other hand, histologically, the tumors showed very similar characteristics. Desmoid tumors are characterized by much more abundant collagen fibrils compared with GISTs, which are also mesenchymal tumors, and by the orderly arrangement of spindle-shaped fibroblasts with low cell atypia and mitosis (23). Positive immunostaining for β-catenin assists in the diagnosis of desmoid tumors (24), and negative immunostaining for c-kit, CD34, and CD117 assists in clearly distinguishing them from GISTs. The tumors from our patients had all of the above characteristics. Ki-67 is an indicator included in the diagnostic criteria as a risk factor for malignancy and recurrence in GISTs as well as other neoplastic diseases, which suggests that the tumor in patient 1 has higher malignancy than the tumor in patient 2. However, in desmoid tumors, Ki-67 is not well documented as a prognostic or risk factor for recurrence (25, 26). This is thought to be because desmoid tumors do not have metastatic potential, even when cell proliferation is rapid. Complete resection without pathological remnants was considered important to achieve a low recurrence rate and a favorable prognosis (13, 27), and complete resection was performed in both of our patients.
The NCCN Guidelines state that if surgical resection significantly impairs function or the tumor is difficult to resect, Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), hormonal therapy, and systemic therapy mainly based on chemotherapy and molecular-targeted agents should be used. In addition, radiation therapy may be indicated if remnants are suspected after resection. However, radiation therapy for intra-abdominal desmoids has the risk of radiation enteritis and is hence not recommended by the NCCN Guidelines. Even if residual tumors are suspected in our patients, systemic therapy can be selected. It should be noted, however, that treatments other than surgical resection for desmoid tumors have not been established and are still in the experimental stage.
He et al. reported that incomplete resection (marginal status R1 or close to R1), large tumor diameters, tumors in the extremities, and younger onset are risk factors for recurrence (13). In patient 1, the tumor size was very large, which is considered to be a risk factor for recurrence, and careful follow-up is considered to be necessary.