The cause and clinical significance of central tumor photopenia on thallium scintigraphy of pediatric osteosarcoma of the extremity

OBJECTIVE. The objectives of our study were to determine whether central tumor photopenia on thallium-201 (²⁰¹Tl) scintigraphy of primary osteosarcoma results from central tumor necrosis or dense central tumor ossification and to determine the relation of this finding to tumor response to chemotherapy and to patient survival. MATERIALS AND METHODS. After the institutional review board approved our study and waived the need for patient or parental consent, two radiologists independently reviewed ²⁰¹Tl scans, conventional radiographs, and MR images of 57 patients obtained at diagnosis of extremity primary nonmetastatic osteosarcoma to detect the presence of central tumor photopenia on ²⁰¹Tl scintigraphy and estimate outer tumor ossification versus inner tumor ossification and enhancement. The dynamic enhanced MRI parameters dynamic vector magnitude (DVM) and k_ep (measure of the exchange rate between plasma and extracellular fluid space) were compared for outer tumor versus inner tumor, and the relation among ²⁰¹Tl scintigraphy, conventional radiography, MRI, and the dynamic enhanced MRI parameters was analyzed. We examined whether central tumor photopenia on ²⁰¹Tl imaging was related to histologic response or to patient survival. RESULTS. Thirty-three patients (58%) had central tumor photopenia on ²⁰¹Tl imaging that was not associated with central tumor ossification (p = 0.8) or with the difference between outer tumor and inner tumor contrast enhancement (p = 0.4). Central tumor photopenia on ²⁰¹Tl scintigraphy was significantly associated with an increasing difference between outer tumor DVM and inner tumor DVM (i.e., outer tumor DVM minus inner tumor DVM) (p = 0.05), an increasing difference between outer tumor k_ep and inner tumor k_ep (i.e., outer k_ep minus inner k_ep) (p = 0.01), and an increasing outer k_ep-inner k_ep ratio (p = 0.02). We found no relation between central tumor photopenia and histologic response (p > 0.2). Older patients (age, > 13 years) with central tumor photopenia were least likely to survive, whereas younger patients (age, < 13 years) without central tumor photopenia were most likely to survive (p = 0.07). CONCLUSION. Central tumor photopenia on ²⁰¹Tl scintigraphy of primary osteosarcoma is unlikely to reflect central ossification but may be due to central necrosis reflected by higher outer tumor DVM and k_ep than inner tumor DVM and k_ep and may be negatively associated with survival in older patients. Prospective studies are needed to determine the value of this information in planning treatment.