Phosphaturic mesenchymal tumor (PMT): exceptionally rare disease, yet crucial not to miss

A Rare Case of Phosphaturic Mesenchymal Tumor Distal Femur in a Young Female

Introduction

Phosphaturic mesenchymal tumors (PMTs) are rare bone neoplasms with diverse clinical presentations, often posing diagnostic challenges.

Case Report

We describe the case of a 37-year-old female schoolteacher with a PMT localized in the distal femur. Diagnostic indicators included hypophosphatemia, hyperphosphaturia, elevated fibroblast growth factor-23 levels, and clinical symptoms of osteomalacia. Surgical management involved tumor resection and limb salvage surgery with a megaprosthesis. The post-operative period was uneventful, leading to a stable discharge. On follow-up, the patient showed no signs of recurrence, regained full ambulation, remained pain-free, and resumed teaching comfortably.

Conclusion

This case highlights the importance of considering PMT in patients with unusual clinical symptoms, accompanied by hypophosphatemia, hyperphosphaturia, and osteomalacia, and demonstrates successful surgical management, leading to a favorable outcome.

Superficial acral calcified chondroid mesenchymal neoplasm harboring an FN1::FGFR2 fusion and review of the literature

Calcified chondroid mesenchymal neoplasm is a recently recognized bone and soft tissue entity primarily found in the extremities and the temporomandibular joint. This neoplasm is typically driven by the fusion of the FN1 gene with a kinase. In this case report, we provide a detailed account of a rare superficial calcified chondroid mesenchymal neoplasm located on the left big toe, characterized by an FN1::FGFR2 fusion. The tumor exhibited a peripheral collarette and consisted of large intradermal histiocytoid to epithelioid cells with no mitotic activity. These cells displayed fine chromatin and abundant pale eosinophilic cytoplasm, forming a swirling syncytium. They were interspersed with localized areas of glassy chondromyxoid matrix containing randomly mineralized calcific material and isolated osteoclast-like giant cells. RNA sequencing confirmed the presence of an FN1 (exon 29)::FGFR2 (exon 7) gene fusion. Our report emphasizes the importance for dermatopathologists to consider this entity when evaluating superficial lesions displaying mesenchymal, chondroid, and calcified attributes.

Phosphaturic mesenchymal tumor: Clinicopathological features with outcomes in 10 patients with review of literature

BACKGROUND

Phosphaturic mesenchymal tumors (PMTs) are rare mesenchymal tumors, associated with long-standing, non-specific but often debilitating symptoms in the affected patients. These tumors display characteristic histopathological features and in case, identified timely, can be a boon for patients, given an excision is completely curative.

AIMS

To evaluate the clinical and histopathological features of 10 PMTs, diagnosed at our institution, along with clinical outcomes in those patients.

MATERIALS AND METHODS

This was a retrospective study, wherein 10 PMTs, diagnosed from January 2013 to July 2022, were included.

RESULTS

The average age at the time of diagnosis was 40 years with an M:F ratio of 4:1. Clinical features included lumps, weakness, bone pain, difficulty in moving and walking, and pathologic fractures. The biochemical analysis showed normal serum calcium levels (average = 9.5 mg/dL), with low serum phosphorus (average = 2.2 mg/dL) and raised serum fibroblast growth factor 23 (FGF23) levels, in all the cases, wherever available. On histopathology, all tumors showed cells arranged in a hemangiopericytomatous pattern, including oval to short spindle forms. Multinucleate giant cells were present in nine tumors, and characteristic "grungy calcifications" was observed in eight tumors. Prominent pseudo cystic spaces were seen in eight tumors. A significant number of mitotic figures and tumor necrosis were not seen in any tumor. In five cases where follow-up was available, there was complete resolution of symptoms post-resection with no recurrence or metastasis. All those patients were free of disease until the last follow-up.

CONCLUSION

This constitutes the first largest comprehensive study on these rare tumors from our country. PMTs can be diagnosed based on certain histopathological features and correlation with clinicoradiological and biochemical findings. These are invariably benign neoplasms. Patients are relieved of their debilitating symptoms after adequate surgical tumor resection. Therefore, their correct and timely diagnosis is crucial.

Phosphaturic mesenchymal tumor with de novo liver metastases: a case report and literature review

Phosphaturic mesenchymal tumors (PMTs) are rare tumors that can cause tumor-induced osteomalacia (TIO) through overproduction of FGF23, a peptide hormone that causes renal phosphate wasting and reduced osteoblastic activity. The diagnosis of PMTs can be difficult to make as the presenting symptoms are non-specific. Although PMT is a rare entity, most cases are benign in nature, not requiring further intervention after surgery, as resection is typically curative. Here, we present a unique case of malignant PMT with de novo liver metastasis in a female patient who presented with TIO and underwent surgical resection of her primary lesion with subsequent regression of her liver metastasis. Moreover, we analyze a review of literature and discuss the importance of a timely diagnosis of this rare phenomenon. It is encouraged that providers strongly consider a diagnosis of PMT in patients with otherwise unexplained bone pain, fatigue, weakness, especially if accompanied with hypophosphatemia. Further studies are also warranted to identify prognostic factors that predict a PMT's malignant potential as they may help identify possible therapeutic targets.

Prolonged generalized osteomalacia associated with a sinonasal cavity phosphaturic mesenchymal tumor: A case report

Phosphaturic mesenchymal tumor (PMT) is a rare disorder primarily affecting the extremities. It is notable for its correlation with hypophosphatemic osteomalacia and high FGF23 serum levels, which results in renal phosphate wasting and clinical symptoms associated with low serum phosphorus. We presented a patient with a 5-year history of progressive osteomalacia who recently experienced a major pathological bone fracture. Laboratory findings showed a persistent low serum phosphate, normal calcium, elevated alkaline phosphatase activity, high parathyroid hormone levels, and increased renal excretion of phosphate. According to ultrasonography and nuclear imaging, there was no evidence of parathyroid adenoma. During further diagnostic assessment, a sinonasal cavity tumor was found and resected. Histologically, the tumor was composed of bland spindle cell proliferation in the background of a calcified matrix with foci of osteoid formation, hemangiopericytoma-like (HPC-like) vasculature, and osteoclast-like giant cells. Tumor cells showed variable positivity for SMA, but CD34, S100, CD99, Melan-A, p63, and desmin were all nonreactive. Regarding the clinical context, histological and immunohistological findings, a final diagnosis of tumor-induced osteomalacia (TIO) secondary to a PMT was made. After surgery, laboratory results returned to normal, clinical symptoms disappeared, and the patient did not experience a recurrence during a six-month follow-up.

A case of FN1-fused calcified chondroid mesenchymal neoplasm of the hand with novel FGFR3 partner gene

Calcified chondroid neoplasms with FN1::FGFR1 or FGFR2 fusions constitute a recently described category of mesenchymal neoplasms mostly encountered in the extremities and temporomandibular joint. Herein, we report a case of FNI1-fused calcified chondroid neoplasm of the hand with a novel FGFR3 fusion partner. The tumor exhibited a multilobulated growth pattern composed of epithelioid cells embedded in abundant stroma with myxoid, chondroid, and fibrous areas and scattered osteoclast-like giant cells. RNA sequencing revealed an in-frame fusion between Exon 31 of FN1 and Exon 3 of FGFR3, which was subsequently confirmed by reverse transcription-polymerase chain reaction. Our findings expand on the spectrum of potential fusion partners in FN1-fused calcified chondroid neoplasms.

Imaging characteristics of phosphaturic mesenchymal tumors

BACKGROUND

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome associated with phosphaturic mesenchymal tumors (PMTs). Localization of the causative tumor in these cases is an arduous task since the culprit lesions are usually small, slow-growing, and can be located almost anywhere from head to toe.

PURPOSE

To describe the morphological characteristics of histologically proven PMTs on various radiological modalities.

MATERIAL AND METHODS

After institutional ethical approval, this retrospective study analyzed 20 cases with a histopathological evidence of PMT. Various imaging characteristics of the tumors on available computed tomography (CT) and magnetic resonance imaging (MRI) scans were evaluated. Descriptive statistical analyses were conducted.

RESULTS

The tumors were located in diverse locations: lower extremities (n = 10); head and neck (n = 5); vertebral column (n = 3); pelvis (n = 1); and upper extremities (n = 1). Bone lesions seen on CT had variable morphology: sclerotic (n = 3/8, 37.5%); lytic (n = 3/8, 37.5%), and both lytic and sclerotic (n = 2/8, 25%) with presence of narrow zone of transition in all cases (n = 8/8) and amorphous internal matrix calcifications in 25% of cases (n = 2/8). Of the tumors, 68.4% (n = 13/19) were hypointense on T1 and all of them showed hyperintense signal on T2-weighted and STIR images (n = 19/19) and contrast enhancement (n = 16/16). Of the tumors, 66.7% (n = 6/9) showed restricted diffusion. DOTANOC PET/CT showed tumor uptake in all cases (n = 8/8).

CONCLUSION

PMTs may have variable and non-specific tumor appearances on various imaging modalities. However, in an appropriate clinical scenario and a background of suggestive biochemical work-up, the radiologist should keep a high index of suspicion.

Tumor induced osteomalacia from a peripheral mesenchymal tumour of the foot

Tumour induced osteomalacia (TIO) is a rare condition caused by peripheral mesenchymal tumours (PMT) which produce fibroblast-growth factor 23 (FGF23). FGF23 inhibits renal phosphate reabsorption leading to vitamin D resistant osteomalacia. The rarity of the condition and difficulty with isolating the PMT make diagnosis difficult, with delayed treatment leading to significant patient morbidity. We present a case of PMT of the foot with TIO, with a discussion on diagnosis and treatment.

Phosphaturic Mesenchymal Tumors: Rethinking the Clinical Diagnosis and Surgical Treatment

BACKGROUND

The diagnosis of phosphaturic mesenchymal tumors (PMT) is easily delayed clinically, and their surgical treatment is unstandardized. This study aimed to evaluate our experience in the diagnosis and treatment of PMT and provide a research basis for the accurate and standardized treatment of PMT.

MATERIALS AND METHODS

Twelve patients diagnosed with PMT in our department and who underwent surgical treatment were included in this study. Preoperative demographic and clinical information were recorded. CT, MRI, and technetium-99m (Tc99m)-octreotide PET/CT imaging techniques were used to evaluate the general conditions and lesion boundaries of the tumors. Surgical treatment was performed using radical resection and microwave ablation-assisted extended curettage according to the lesion location and size. Patients were strictly followed up with and evaluated for oncological prognosis, radiological results, bone healing, serum ion levels, limb function, and pain level; the occurrence of complications was also recorded.

RESULTS

Three patients underwent radical resection, and nine underwent microwave ablation-assisted extended curettage. The average duration of symptoms in this group was 1.5 years (9-35 months) before diagnosis. Serum phosphate and AKP levels returned to normal one and two weeks postoperatively, respectively. There was no apparent specificity in the pathological findings; however, the immunohistochemistry of FGF-23 was positive, and the original fracture sites were effectively healed during the follow-up. The limb function and pain scores were significantly improved. The MSTS score increased from 15.3 to 29.0, and the VAS score decreased from 5.3 to 0.4. All patients recovered, and 90% resumed their original jobs.

CONCLUSIONS

Accurate diagnosis and standardized surgical treatment are crucial to achieving a clinical cure for PMT. Combining clinical manifestations, biochemical examinations, imaging characteristics, and pathological findings is an effective way to diagnose PMT accurately. Radical resection and microwave ablation-assisted extended curettage are reliable surgical treatment methods for PMT.

Dorsal vertebral body tumor and non-compressive quadriparesis – A rare case report of a phosphaturic mesenchymal tumor

Background:

Phosphaturic mesenchymal tumors cause renal phosphate wasting leading to hypophosphatemia manifesting as bone pain and fractures. About 95% of these tumors involve the extremities and the appendicular skeleton, with spinal tumors being exceedingly rare. We describe a case of non-compressive quadriparesis, caused by a thoracic vertebral body phosphaturic mesenchymal tumor (PMT).

Case Description:

A 34-year-old gentleman presented with a 3-year history of gradually progressive quadriparesis, predominantly involving proximal muscles. Magnetic resonance imaging neither showed evidence of compressive lesions nor myelopathy. On routine biochemistry, he was found to have hypophosphatemia and phosphaturia, with serum phosphorus levels of 1.84 mg/dl, and fractional excretion of phosphorus >5%. A DOTATOC positron emission tomography scan suggested the possibility of a PMT in the D10 vertebral body with a corresponding hyperdense/sclerotic focus on non-contrast computed tomography. After instituting phosphate replacement therapy and complete surgical excision of the hyperdense focus, he made a full neurological recovery. His phosphate levels normalized without the need for supplements. Histopathological examination showed spindle cells with positive staining for Vimentin, a mesenchymal cell marker. At 5 years of follow-up, the patient continues to be asymptomatic with a full return to normal function and no residual weakness.

Conclusion:

PMTs involving the spine are a rare reversible cause of non-compressive quadriparesis. Early diagnosis and excision of the lesion correct the hypophosphatemia and result in complete neurological recovery.

Phosphaturic Mesenchymal Tumor

Phosphaturic mesenchymal tumors (PMTs) are neoplasms associated with tumor-induced osteomalacia. Patients typically present with pathologic fractures in the setting of chronic hypophosphatemic hyperphosphaturic osteomalacia, as well as gradual muscle weakness, bone pain, and difficulty walking. Because of their rarity and nonspecific symptomatology, phosphaturic mesenchymal tumors often go undiagnosed for years. Even when discovered on imaging, the tumors can be diagnostically challenging for radiologists. Phosphaturic mesenchymal tumors often tend to be small and can be located nearly anywhere in the body, and, therefore, can mimic many other tumors. This case highlights the imaging and pathologic markers of a phosphaturic mesenchymal tumor, often found in a patient with tumor-induced osteomalacia.

Persistent phosphaturic mesenchymal tumor causing tumor-induced osteomalacia treated with image-guided ablation

Phosphaturic mesenchymal tumors (PMTs) can present with vague symptoms of diffuse bone pain with pathologic fractures that often lead to a delayed diagnosis. We present a 60-year-old patient with a PMT that was persistently hypophosphatemic after resection, who was then successfully treated with cryoablation of the tumor. Tumor-induced osteomalacia (TIO) is a rare cause of hypophosphatemia characterized by vague symptoms of gradual muscle weakness and diffuse bone pain with pathologic fractures that often lead to a delayed diagnosis. This condition is usually caused by benign phosphaturic mesenchymal tumors (PMTs). Here, we present a case of persistent PMT after surgical resection treated with image-guided ablation. We present the patient's clinical examinations and laboratory findings (phosphorus, 1,25 (OH)₂D, FGF-23, Intact PTH). Representative histologic images of a PMT are also presented. A 61-year-old male was evaluated for persistent hypophosphatemia and presumed osteomalacia. Six years earlier, he underwent surgical excision of a left ischial mass after presenting with TIO. The pathology was consistent with a PMT; however, hypophosphatemia persisted suggesting incomplete resection. He was treated with calcitriol and phosphate salts. A PET Ga68 dotatate scan of the patient revealed an avid left ischial mixed lytic and sclerotic lesions with marked amount of radiotracer uptake, suggesting persistent tumor. The patient was resistant to re-excision of the tumor due to the extended recovery period from his prior surgery and was treated instead with cryoablation of the tumor. His biochemical findings of hypophosphatemia and elevated FGF23 resolved after the ablation and have remained normal for 5 months after surgery. In patients with TIO, wide surgical excision is the treatment of choice. When this is not possible, image-guided ablation is an alternative therapeutic option.

Extended Whole-body Ga-68 DOTATATE PET-CT in evaluating Tumour-Induced Osteomalacia: Case report and review of literature

Tumour-induced osteomalacia is a rare paraneoplastic syndrome that manifests as chronic hypophosphataemia, non-specific bone pain and muscle weakness. It is generally caused by phosphaturic mesenchymal tumour (PMT), which is uncommonly associated with synchronous tumours. However, diagnosis is often delayed for several years due to the rarity, indolent growing nature and non-specific symptoms of the disease, often resulting in an overlook by clinicians during assessments. The patient initially presented with hypophosphataemia and generalised skeletal pain with multiple atraumatic fractures. Blood tests revealed serum calcium levels at the upper limit and extremely low inorganic phosphate levels. Herein, we report a case where two synchronous PMTs from two different sites were detected by 'extended' whole-body Ga-68 DOTATATE PET-CT, leading to remission of the disease after complete surgical removal. Early detection and diagnosis of PMT neoplasm is crucial, as complete surgical resection of this tumour is the only definitive treatment currently known. Upon excision, this curable disease will result in complete resolution of symptoms and blood parameters, leading to remission of the disease which significantly improves the patient's quality of life. PMT often over-expresses somatostatin receptors (SSTR), predominantly subtype 2A, and Ga-68 DOTATATE PET-CT is a selective SSTR imaging that targets this characteristic over-expression in these tumours. The high diagnostic accuracy of Ga-68 DOTATATE PET-CT should be the primary imaging modality for full evaluation of this disease.

The phosphaturic mesenchymal tumor as a cause of oncogenic osteomalacia. Three cases and review of the literature

INTRODUCTION

The phosphaturic mesenchymal tumour (PMT) is a very uncommon cause of oncogenic osteomalacia (OO), which is a paraneoplastic syndrome with severe clinical osteomalacia. The PMT is a neoplasia that produces the fibroblast growth factor FGF23, resulting in reduced proximal tubular phosphate reabsorption leading to hyperphosphaturia and hypophosphatemia. Our aim is to present our experience and complications in diagnosis and treatment of PMT in three patients.

MATERIAL AND METHODS

We propose an observational, descriptive and retrospective study of three cases of OO secondary to PMT found in our database of bone and soft tissue tumours. The inclusion criteria were: symptoms related with OO, presence of hyperphosphaturic hypophosphatemia, elevated levels of FGF23 in blood and pathological diagnosis of PMT.

RESULTS

In all cases, the disease showed asthenia, non-specific bone pain, progressive functional weakness, and pathological fractures. The average delay time in diagnosis was 7 years. All presented with hyperphosphaturic hypophosphatemia, elevated levels of alkaline phosphatase as well as FGF23. The use of Octreoscan and PET-CT were essential to find the producing tumour and its subsequent biopsy. Treatment was surgery in two cases and one case was treated by CT-guided cryotherapy with neurophysiological control. Once the surgery was performed, the blood parameters normalized. There is no recurrence.

CONCLUSIONS

Phosphaturic mesenchymal tumor is a very rare entity as part of bone and soft tissue tumors, it may occur in both tissues. The phosphate-calcium homeostasis is altered due to high serum levels of FGF23 because of PMT. Delay in diagnosis is usual, leading to renal and skeletal comorbidities. To avoid this, knowledge of this entity together with high diagnostic suspicion are critical. Surgical treatment leads to normalization of serum levels and systemic symptoms.

Phosphaturic Mesenchymal Tumors from Head to Toe: Imaging Findings and Role of the Radiologist in Diagnosing Tumor-Induced Osteomalacia

BACKGROUND

This study aimed at evaluating the imaging findings of phosphaturic mesenchymal tumors and tumor-induced osteomalacia and assess the clinical and biochemical profiles of patients with tumor-induced osteomalacia.

MATERIALS AND METHODS

Imaging findings in six patients with tumor-induced osteomalacia and histopathologically proven phosphaturic mesenchymal tumors were evaluated. Clinical and biochemical profiles of these patients were also assessed.

RESULTS

Along with having a characteristic biochemical profile, patients with phosphaturic mesenchymal tumors also have certain imaging findings which can aid in the diagnosis such as increased uptake on DOTA PET-CT and homogeneous post-contrast enhancement on CT and MRI.

CONCLUSION

Patients with tumor-induced osteomalacia have characteristic symptoms, imaging and biochemical profiles. For radiologists, raising the suspicion of a phosphaturic mesenchymal tumor in patients with refractory hypophosphatemic osteomalacia as well as localizing the tumor on imaging is crucial, as complete excision of the tumor leads to resolution of the osteomalacia and the patient's clinical symptoms.

A Rare Case of Phosphaturic Tumor/Oncogenic Osteomalacia - Diagnostic Challenges and Management Algorithm

Introduction

Oncogenic osteomalacia is a rare paraneoplastic syndrome of acquired hypophosphatemic osteomalacia, resulting from a deficit in renal tubular phosphate reabsorption, in which fibroblast growth factor 23 seems to be implicated.

Case Report

A 42-year-old male presented with complaints of low back pain for the past 4 years and inability to walk for the past 3 years. Blood investigations showed hypophosphatemia and elevated serum alkaline phosphatase. Radiographs and magnetic resonance imaging revealed features of osteomalacia. DOTONAC PET-computed tomography (CT) showed a lesion in the greater trochanter suggestive of a phosphaturic mesenchymal tumor. CT-guided biopsy of the lesion was done which confirmed the diagnosis. The tumor was resected by intralesional methods (burring and cementation) with correction of hypophosphatemia by oral phosphorus supplementation. Patient clinically improved and was able to walk with support at the time of discharge.

Conclusion

Phosphaturic tumor was localized to be in the posterior aspect of the right greater trochanter. To find the tumor was a major diagnostic challenge involving various diagnostic modalities. The occurrence of such a condition is rare and it may take even years to correctly diagnose the disease.

Review of paraneoplastic syndromes in children

Paraneoplastic syndromes are defined as clinical syndromes that are not related to direct tumor invasion or compression but are secondary to tumor secretion of functional peptides/hormones or related to immune cross-reactivity with normal host tissue. Paraneoplastic syndromes have a wide range of presentations and can present before the primary malignancy or tumor recurrence is diagnosed. They can mimic non-neoplastic processes, making detection, diagnosis and treatment difficult. However, they can also provide clues to the presence of an underlying malignancy. In this paper, we reviewed a range of paraneoplastic syndromes that can occur in children including: (1) neurologic (opsoclonus-myoclonus, limbic, anti-N-methyl-d-aspartate [NMDA] and anti-Ma2 encephalitis and myasthenia gravis); (2) endocrine (neuroendocrine tumors, hypercalcemia, SIADH [syndrome of inappropriate antidiuretic hormone secretion], osteomalacia/rickets and ROHHAD [rapid onset of obesity, hypoventilation, hypothalamic dysfunction and autonomic dysregulation]); and (3) dermatologic/rheumatologic syndromes (hypertrophic osteoarthropathy and paraneoplastic pemphigus). Familiarity with these syndromes can aid in early diagnosis, treatment and imaging optimization.

PHOSPHATURIC MESENCHYMAL HEEL TUMOR PRESENTING WITH TUMOR-INDUCED OSTEOMALACIA

Objective

To help clinicians identify and treat patients with tumor-induced osteomalacia (TIO) resulting from a phosphaturic mesenchymal tumor, mixed connective tissue variant (PMTMCT).

Methods

Describe the history, presentation, laboratory findings, diagnostic studies, treatment, and literature review.

Results

A 58-year-old female with no significant past medical history presents with ongoing multiple bone pain for years. She had a bone scan showing multiple focal areas of increased uptake involving bilateral ribs, distal right tibia, and left femoral neck, representing previously healed fractures. Her bilateral lower-extremity magnetic resonance imaging showed stress fractures of the anteromedial cortex, right tibia, and the left femoral neck. Phosphorus was noted to be 1.9 mg/dL (normal range, 2.0 to 4.0 mg/dL), and alkaline phosphatase was 179 U/L (normal range, 25 to 110 U/L). Tubular maximum re-absorption of phosphate to glomerular filtration rate ratio was 0.438, which was low. An outside physician initiated patient on teriparatide, which showed bone mineral density improvement after 1 year, and then the teriparatide was stopped. Later, she developed a nontraumatic pubic ramus fracture; teriparatide was resumed. While on teriparatide, she developed several new rib fractures. Due to declining phosphorus levels, further investigation led to an elevated fibroblast growth factor 23 (FGF-23) level of 243 RU/mL (normal, <50 RU/mL). TIO was strongly suspected, and a nuclear medicine positron emission tomography/computed tomography trunk with ⁶⁸Ga-1,4,7,10-tetraazacyclododecane 1,4,7,10-tetraacetic acid tyrosine-3-octreotate (i.e., ⁶⁸Ga-DOTATATE) showed a right heel soft-tissue nodule. Fine-needle aspiration biopsy was performed, confirming PMTMCT, positive for FGF-23 mRNA. After surgery, her symptoms resolved and her phosphorus normalized.

Conclusion

TIO is a rare paraneoplastic syndrome characterized by bone pain, muscle weakness, and fractures associated with persistent hypophosphatemia. Clinicians now have new imaging tools to help identify and treat patients with PMTMCT.

Phosphaturic mesenchymal tumors: what an endocrinologist should know

Tumor-induced osteomalacia (TIO), also known as "oncogenic osteomalacia", is a rare cause of osteomalacia. TIO often has an insidious onset characterized clinically by progressive muscle weakness and bone pain with fractures. The hallmark biochemical finding is a persistent low serum phosphorus concentration due to renal phosphate wasting. The vast majority of cases of TIO result from production of the phosphaturic hormone fibroblast growth factor 23 (FGF23) by a histologically distinctive mesenchymal tumor, termed "phosphaturic mesenchymal tumor" (PMT). Circulating FGF23 induces internalization of renal sodium/phosphate co-transporters resulting in reduced proximal tubular phosphate reabsorption. FGF23 also inhibits production of 1α,25-dihydroxyvitamin D which is inappropriately low or normal in the context of hypophosphatemia. Diagnosis is often delayed owing to the rarity of the condition and an underappreciation for the role of phosphorus as a cause for the constellation of symptoms. Primary treatment for TIO is identification of the offending tumor and surgical removal. However, these tumors are notoriously difficult to find, precluding the opportunity for a curative surgery in many. In such cases, phosphate and calcitriol therapy is used to improve symptoms and heal the osteomalacia. Recently, molecular genetic studies have shown recurrent genetic events in PMT, including the novel fusions FN1-FGFR1 and less commonly FN1-FGF1. These fusion events are hypothesized to result in autocrine/paracrine signaling loops within the tumor, spurring tumorigenesis. This review will cover the clinical features, imaging characteristics, pathologic features, molecular genetic aspects, and therapy of PMT, with a brief discussion of other neoplasms that may cause TIO.