Radiographically Occult Latent Radiogenic Osteosarcoma Uncovered on Tc-99m Methylene-diphosphonate Bone Scintigraphy
A 70-year-old male presented with mild left hip pain and was found to have an elevated serum alkaline phosphatase (ALK) level of 770 units/L. Unfortunately, due to a language barrier, a complete medical history could not be obtained. In light of the patient's elevated ALK level, the patient was referred for a whole-body Tc-99m MDP bone scan. The planar whole body images [] of the Tc-99m MDP bone scan demonstrated intense diffuse uptake extending from the left femoral head to the proximal shaft of the femur. Initially, we felt this type of appearance could be compatible with early Paget's disease given the markedly elevated ALK, despite the absence of apparent structural deformities.\nTo confirm the initial diagnosis, the patient was sent directly for an X-ray []. However, the pattern of multifocal sclerosis (cotton wool appearance) that would be expected of Paget's disease was not present.[] Instead, the radiograph demonstrated three previously unreported fixation screws, used to repair a femoral neck fracture (later discovered to have occurred 1 year ago). Surrounding the screws were poorly defined osteopenic, potentially lytic lesions in the femoral neck and femoral head []. Correlation of the Tc-99m MDP bone scan with the X-ray changed the initial diagnosis from suspected Paget's disease to a differential of malignancy/metastasis, reparative phase of osteonecrosis, osteomyelitis, or reactive osteolysis to the fixation screws. Though simple loosening of the fixation screws could have explained the patient's mild hip pain, the activity demonstrated by the bone scan was much too diffuse and intense to support this diagnosis.[]\nTo evaluate for metastatic disease, a computed tomography scan of the chest, abdomen, and pelvis was ordered the following day. Evaluation of the scan of the left hip was limited due to streak artifact from the screws; however, it was still possible to appreciate some mixed lytic/sclerotic changes in the native bone [Figure and ]. Even though these changes raised the suspicion of osseous malignancy or metastasis, they were not pathognomonic, as hardware loosening, osteopenia, and degenerative changes could have a similar appearance.[] However these findings in combination with the bone scan findings, provided enough suspicion of a pathology sinister enough to refer the patient for surgical hardware revision with potential biopsy.[]\nSurgical exploration of the left hip revealed a grossly abnormal appearance of the left proximal femur with areas of hemorrhagic bone surrounding lytic lesions. The fixation screws were removed and curettage of the suspicious lesion was performed. Histologic study of the biopsy [] demonstrated sarcomatous, spindle shaped cells with adjacent tumor osteoid production; represented by eosinophilic, amorphous, fibrillary deposits between individual or small aggregates of tumor cells. These histological findings were diagnostic of osteosarcoma.[]\nIn context, the presence of osteosarcoma was highly unlikely as there have been no case reports of primary osteosarcomas occurring in a site of a fracture that was surgically repaired 1 year prior. Additionally, the distal femur would be a far more likely site for a primary osteogenic sarcoma to occur.[] However, upon further interrogation of the patient's past medical history, it was discovered that the patient had prostate cancer 11 years ago. Given the standard of care at that time, one could surmise that he was treated with 60-70 Gy of targeted external beam radiation to the prostate.[] The dose that he received and the time frame since receiving his radiation would be compatible with radiogenic osteosarcoma as there is a latency period of about 10 years.[] The repaired stress fracture 1 year prior was potentially pathologic from the malignancy, but was most likely missed due to the slightly atypical radiographic appearance on the plain film X-ray performed in the surgeon's office.