Malignant phosphaturic mesenchymal tumor of the pelvis: A report of two cases

Abstract

Tumor-induced osteomalacia (TIO) is a rare acquired form of hypophosphatemia commonly associated with phosphaturic mesenchymal tumors (PMTs) located in the bone or soft tissue. Resection of the tumor can cure osteomalacia. Fibroblast growth factor 23 has been identified as a major pathophysiological factor responsible for phosphaturia. The majority of PMTs are benign, and malignant PMTs are uncommon. Even in rare cases, the malignant transformation of PMTs is extremely uncommon. The current study presents two cases in which the patients succumbed to malignant PMTs that developed in the pelvis. The first patient was a 35-year-old female with a malignant PMT occurring as a synchronous double cancer associated with papillary thyroid carcinoma. Diagnosis was difficult, as the multiple uptake on positron emission tomography with 18F-fluorodeoxyglucose presented as pseudofractures mimicking the metastases of thyroid carcinoma. The patient succumbed to rapidly progressive lung metastases. The second patient presented with a pelvic tumor that had developed over 26 years. The patient was diagnosed with a benign PMT by open biopsy and a complete resection was performed. However, two years later, the tumor recurred and lung metastases were observed. The patient ultimately succumbed to respiratory failure due to relapsing lung metastases and disseminated intravascular coagulation. These two cases demonstrate the potential lethality of malignant PMTs and the malignant transformation of benign PMTs. Therefore, TIO patients must be followed up even if diagnosed with a benign tumor. Although TIO is an extremely rare disease, the possibility of malignant PMTs must be recognized.

Keywords: fibroblast growth factor 23; malignant; osteomalacia; phosphaturic mesenchymal tumor; transformation.

Figure 1

Case one: Positron emission tomography with ¹⁸F-fluorodeoxyglucose (FDG-PET) showing abnormal uptake in the (A) right acetabulum, (B) right lobe of the thyroid gland, (C) left first rib and (D) T2 vertebral body, which were considered to be multiple bone metastases from the thyroid cancer.

Figure 2

Case one: Plain radiograph showing an osteolytic lesion in the right acetabulum.

Figure 3

Case one: (A) Coronal and (B) axial T2-weighted magnetic resonance imaging (MRI) showing a mass with inhomogeneous intensity located in the right acetabulum.

Figure 4

Case one: Open biopsy of the acetabular lesion. (A) Histological examination revealed spindle and round cells, with the multinucleated giant cells in a collagenous matrix with capillaries (hematoxylin-eosin stain; magnification, ×400). In the immunological studies (B) the Ki-67 index was 20% and (C) the tumor cells were fibroblast growth factor 23-positive (indicated by the arrows; immunohistochemical stain; magnification, ×200). Based on these observations, the tumor was diagnosed as a malignant phosphaturic mesenchymal tumor (PMT).

Figure 5

Case two: Plain radiograph showing a bony destructive lesion, with calcification in the right ischium and ununited fractures in the shafts of the bilateral femora (Looser’s zones).

Figure 6

Case two: Contrast-enhanced computed tomography showing a homogeneously-enhanced mass in the pelvis.

Figure 7

Case two: (A) Coronal and (B) axial T1-weighted gadolinium-enhanced magnetic resonance imaging (MRI) revealing a heterogeneous intensity mass, with partial cystic change, compressing the rectum and bladder.