Integrally calcified solitary fibrous tumor in the retroperitoneum: a case report and review of the literature

A 64-year-old Japanese man without any symptoms had a medical checkup, and abdominal ultrasonography revealed a mass of the right upper abdomen with acoustic shadow. He had hypertension and underwent appendectomy for appendicitis by McBurney’s incision at 13 years old. He was 162 cm tall and weighted 70 kg (body mass index, 26.8). His blood pressure, pulse, oxygen saturation, and body temperature were 146/76 mmHg, 66 beats/min, 97 %, and 36.2 °C, respectively. His abdomen was flat and soft without any palpable masses. Laboratory examinations revealed no remarkable abnormalities of complete blood count, inflammation, liver function, renal function, electrolytes, or coagulation system. Plain computed tomography showed a mass with extremely high densities; it was 6.5 cm in the longest diameter and located next to the undersurface of the right hepatic lobe and the right kidney (Fig. 1a, b). Contrast enhanced computed tomography did not reveal any other lesions in the chest and abdomen. Magnetic resonance imaging showed low signal intensities on both T1- and T2-weighted images (Fig. 1d, e). We considered that the lesion mainly consisted of calcification and diagnosed large peritoneal pearl body or sponge-induced granuloma due to the past appendectomy. Malignant fibrous histiocytoma, dedifferentiated liposarcoma, or extraskeletal osteosarcoma were considered as differential diagnoses although they were atypical or rare according to the imaging findings. We informed the patient about a low possibility of malignancy of the lesion and he wished observation. Two years after, the patient hoped surgical excision although the lesion showed a stable imaging in computed tomography (Fig. 1c).

We performed laparoscopic resection of the mass. We broke up adhesions between the omentum and the right upper abdominal wall and found a white and hard mass in the retroperitoneum over the adhesions (Fig. 2a). It was excised from the retroperitoneum using ultrasonically activated scalpel. Operative duration was 115 min and intraoperative hemorrhage was little. On gross examination, the excised specimen entirely looked like calcification (Fig. 2b). Division surface of the specimen mainly presented calcification and partially red soft tissues (Fig. 2c). Microscopically, small quantities of densely proliferated spindle cells were scattered in major quantities of hyalinizing collagen fiber (Fig. 3a). The spindle cells showed low cytological atypia and no mitosis. Immunohistochemistry showed positive staining of CD34 and vimentin and negative staining of Bcl-2, SMA, A-100, or p53 (Fig. 3b–g). Immunoreaction for Ki-67 was detected in fewer than 1 % of the neoplastic cells (Fig. 3h). The surgical margin was free from the tumor cells. We diagnosed morphologically and immunologically benign SFT in the retroperitoneum. The patient recovered without any postoperative complications, discharged 9 days after the surgery, and had no recurrence 1 year after the surgery.

SFT which is currently known as an uncommon soft tissue neoplasm was first reported as a kind of pleural neoplasms in 1931 [1] and was originally an exclusive diagnosis of pleural neoplasms [2]. Recent electron microscopic and immunohistochemical studies revealed that SFT derives from fibroblastic or myofibroblastic cells under the mesothelium [3, 4]. Thoracic SFT has an age-standardized incidence rate of 1.4 per million, typically occurs in the sixth and seventh decade, and affects both genders equally [5, 6]. Eighty percent of SFT occurs in the thoracic cavity while 20 % of that is found in extrathoracic regions including the retroperitoneum [7, 8]. Retroperitoneal SFT as in the present case is considered to be comparatively rare.

SFT can cause local pressure syndrome [9] and some reports described that SFT presented hypoglycemia due to the insulin-like growth factor II secretion from the tumor [10, 11]. In imaging studies, SFT typically presents well-defined and lobulated mass with well-enhancement [9]. Wignall et al. investigated characteristics of radiologic findings in 34 SFTs and revealed that 62 % of them had the lobulated contours, 9 % had the local invasions, 79 % had heterogenous enhancement patterns, and 65 % had strong contrast enhancement [12]. On magnetic resonance imaging, SFT typically exhibits hypo- or isointensity in T1-weighted image and heterogeneous hyperintensity in T2-weighted image [13], the differential diagnoses of SFT in the retroperitoneum include liposarcoma, leiomyosarcoma, lymphoma, schwannoma, neurofibroma, leiomyoma, myolipoma, angiomyolipoma, sympathetic paraganglioma, desmoid tumor, and epithelioid sarcoma [14]. In the present case, SFT was incidentally detected without any symptoms, imaging inspection provided extremely atypical findings as any retroperitoneal neoplasms including SFT, and thus a precise preoperative diagnosis was difficult. Non-neoplastic lesions such as peritoneal pearl body or foreign body granuloma due to retained surgical sponge in the past appendectomy were exclusively considered, but the imaging findings were not typical of them as well as of neoplastic disease [15]. Fine needle aspiration or biopsy might be difficult due to its marked calcification.

Microscopically, SFT shows multiplying spindle cells in the patternless appearance and the hemangiopericytoma-like appearance with prominent vascularity [16]. England et al. described high cellularity, high mitotic activity (more than 4 mitoses per 10 high-power field), pleomorphism, necrosis, and hemorrhagic changes as the criteria for morphological malignancy of SFT [17]. Those malignant features are observed in about 10 % of SFTs [18]. In immunohistochemistry, CD34 and Bcl-2 are especially useful for differentiating SFT from the other spindle cell neoplasia [9, 19]. Immunoreactivity for CD34 was observed in 98 % and that for Bcl-2 was confirmed in 90 % of SFTs [20]. Other immunological targets such as CD99, desmin, SMA, c-kit, S-100, EMA, CK, CD31, and inhibin are also used for specific diagnosis of SFT [21]. In the present case, the marked calcification was extremely unusual but the scattered tumor cells provided typical structures of SFT with positive immunostaining of CD34 and were considered to be morphologically benign.

The problem regarding malignancy of SFT is discrepancies of morphological malignancy and clinical malignancy [8, 22, 23]. The discrepancy may be partially due to malignant transformation with dedifferentiation of the tumor cells [24, 25]. Therefore, we should not trust morphological findings but immunohistochemistry for some relevant proteins may provide indications of SFT’s clinical malignancy. Takizawa et al. reported that patients with positive immunostaining of both CD34 and Bcl-2 had no recurrence [23]. Loss of immunoreactivity for CD34 and Bcl-2 was observed in the component with malignant transformation [24, 25]. Immunoreaction for p53 was not observed in benign SFT but much confirmed in morphologically and clinically malignant SFT [25]. The present case morphologically yielded typical benign features without immunoreactivity for p53 but can potentially cause a recurrence and should be carefully followed up.

Almost all patients with SFT underwent surgical excision. Many surgical doctors recommend complete surgical excision to obtain clear margins with careful long-term follow-up [8, 9, 16, 26]. There have been few reports concerning chemotherapy for SFT and thus its efficacy is not evident. Baldi et al. used anthracycline and ifosfamide for a patient with recurrent SFT and got partial response; cisplatin and gemcitabine, progressive disease [22]. We performed microscopically complete resection of the tumor and consider follow-up without adjuvant chemotherapy as reasonable.

The present case showed prominent calcification in preoperative imaging and the excised mass mostly consisted of calcification with scattered tumor cells. Calcification is not rare for various neoplastic or non-neoplastic lesions in the retroperitoneum such as lipoma, liposarcoma, schwannoma, germ cell tumors, lymphoma after chemotherapy or radiotheraphy, and peritoneal pearl body [43, 44]. Nakashima et al. suggested that the absence of calcification can predict malignancy of primary retroperitoneal tumor [45]. In SFT, calcification was reported to be observed in 3 of 34 cases (8.8 %) [12] and regarded as an indicator of malignancy [46]. However, those kinds of calcification are tiny, scattered, or partial in the lesion. There has been no report of integrally calcified SFT like the present case. The reason why SFT coexisted with such significant calcification is not apparent but the fact is very interesting for considering carcinogenesis or nature of SFT.

We experienced a case of integrally calcified SFT in the retroperitoneum. There has been no report of SFT with such prominent calcification. Retroperitoneal SFT is difficult to diagnose preoperatively, and careful follow-up after the excision is crucial because morphological malignancy does not necessarily correspond to clinical malignancy.

Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.