Teaching Case

A 51-year-old woman with previous uterine leiomyoma and an incidental solid lesion found in Douglas’ pouch was addressed for PET-CT examination. She has a previous history of melanoma and thyroid carcinoma, with a negative follow-up.

Laboratory findings were in range, in particular normal levels of CA 19.9 and CA 125.

Contrast-enhanced (FDG)PET-CT was performed, confirming a peritoneal mass in the pouch of Douglas (maximum axial diameter of 85 mm – Figure 1) and a mesenteric mass in the left iliac fossa (36 mm – Figure 2 and 3). Both lesions were well-defined and circumscribed with mild homogeneous enhancement and mild FDG uptake (Figure 4). The Douglas’ pouch mass was adjacent to the sigmoid, with thin homogeneous walls. There were neither adnexal lesions, ascites, pathological nodes, nor evidence of peritoneal carcinomatosis. The patient has been lost during follow-up.

After three months, she presented at the emergency department due to abdominal pain. She underwent contrast-enhanced CT (CECT), revealing a dimensional increase of the peritoneal mass (92 vs 85 mm) with inhomogeneous enhancement, few non-enhancing necrotic areas, and some hyperattenuating hemorrhagic areas, without calcification or fatty attenuation (Figures 5 and 6). Laparoscopic surgical excision was then performed. Histopathological analysis revealed the diagnosis of leiomyosarcoma.

Leiomyosarcoma is a malignant neoplasm of smooth muscle cells, representing 10–20% of sarcoma diagnoses [1], more common in women [2]. The clinical presentation of leiomyosarcomas is associated with nonspecific symptoms [3]. The histological specimen is the reference standard for the final diagnosis [4].

On ultrasonography [US], it presents as a hypoechoic mass that can be heterogeneous due to necrotic areas and internal haemorrhages [2].

On the CECT scan, leiomyosarcoma is seen as a well-defined, heterogeneous, hypodense mass with possible central necrotic or cystic degeneration; it may show either homogenous or heterogeneous enhancement [2]. It’s important to investigate the infiltration into the adjacent muscles [5]. The presented cases, during the first PET-CT scan, did not show any typical appearance of leiomyosarcoma, considering the well-defined shape without hemorrhagic nor necrotic areas and with slight homogeneous enhancement.

On magnetic resonance imaging [MRI], the mass can show uniform low signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images. Areas of necrosis or cystic degeneration typically present as highly hyperintense on T2-weighted images. Heterogeneous enhancement of the mass is typically seen [2].

PET scans may be useful for high-grade sarcomas that typically show higher FDG uptake, while specificity decreases in the case of low-grade tumours [5].

Leiomyomatosis peritonealis is one of the possible differential diagnoses in a woman with previous history of uterine leiomyoma [4]. Leiomyomas, however, may present degeneration, oedema, or heterogeneous enhancement, making the differential diagnosis with leiomyosarcoma challenging [6]. Because during previous abdominal surgery, leiomyoma debris may have spread into the abdominal cavity, knowing the patient’s surgical history can be useful [7].

Desmoid tumour is another differential diagnosis: on CECT, it normally presents homogeneous attenuation similar to the muscle with a slight enhancement after the intravenous injection of contrast media. It has been reported an association with the Gardner Syndrome [8] .

Other masses to be considered are round-cell and pleomorphic subtype of liposarcoma, with a nonspecific appearance on CT and MRI, as well-circumscribed mass containing little or no fat, often showing internal haemorrhage and necrosis [8-10]. GIST, extremely rare in pelvis, are usually of varying density, and can show patchy enhancement [11]. Inflammatory pseudotumor may present both as well-defined or with irregular margins, most commonly with heterogeneous attenuation and enhancement due to central necrotic areas [9,10], while hyaline vascular Castleman disease can manifest as a round mass with hypervascularization and multiple branching feeding vessels on its surface [9].