The objectives of this observational study are to assess the long-term safety and long-term effectiveness of burosumab in patients with TIO who are being treated with burosumab as prescribed by their physician and to monitor the course of the underlying phosphaturic mesenchymal tumor (PMT) overtime in patients with TIO irrespective of their treatment status.

Enrolled patients may or may not be treated with commercially available burosumab during the TIO DMP at the discretion of their treating physician. Given the observational nature of the TIO DMP, specific treatments or supportive management will not be provided as part of the study.

Inclusion Criteria:

• Have a clinical diagnosis of TIO based on the presence of an underlying PMT (confirmed by imaging) AND/OR historical documentation. Note: For adult patients with TIO in whom the causative PMT has never been located, and all pediatric patients, documented evidence of negative genetic testing for other hereditary hypophosphatemic disorders is necessary
• For patient safety, all participating female patients of child-bearing potential must be willing to have pregnancy tests prior to certain assessments performed as part of the DMP
• Be willing to provide access to prior medical records including tumor pathology reports and biopsy slides, imaging, biochemical, and diagnostic, medical, and surgical history data, if available
• Be willing and able to provide informed consent after the nature of the study has been explained, and prior to any research-related procedures
• Be willing and able to comply with the study visit schedule and study procedures

Exclusion Criteria:

• Have a clinical diagnosis of TIO deemed to be caused by a tumor other than a PMT
• Serious medical or psychiatric comorbidity that, in the opinion of the Investigator, would present a concern for patient safety or compromise the ability to provide consent or comply with the study visit schedule and study procedures
• Less than 1 year of life expectancy (for any cause) in the opinion of the Investigator
• Concurrent enrollment in a clinical trial without prior approval from the TIO DMP Sponsor
• Undergoing treatment with burosumab for an unapproved indication

May include patients who have undergone complete tumor resection and continue to have biochemical/clinical evidence of disease, patients with tumor identified, or patients in whom causative tumor has not been identified and who have been diagnosed with TIO based on biochemical/clinical symptom profile.

Patients may be treated with burosumab, or phosphate and active vitamin D metabolites/analogs, as prescribed by a physician, or may be untreated, at any time during the TIO DMP.