Quantitative evaluation of thallium-201 uptake in predicting chemotherapeutic response of osteosarcoma

Thallium-201 has been shown to be useful in predicting tumour viability in patients undergoing neoadjuvant chemotherapy for osteogenic sarcoma. Early studies relied upon qualitative assessment of analog images to obtain predictive results. Recently, the lesion to normal tissue uptake ratio of 201Tl has been used in evaluating bone and soft tissue sarcomas. This study attempts to quantitate changes in tumour to normal tissue ratio following chemotherapy. Eight consecutive patients with classical osteosarcoma received standard preoperative chemotherapy with a combination of cisplatin, adriamycin and high-dose methotrexate. 201Tl gamma scintigraphic images were obtained both before and after chemotherapy. The average counts taken over the tumour divided by that from the contralateral normal tissue area yielded a tumour-to-normal tissue (T/N) ratio. The percentage change in the T/N ratio before and after preoperative chemotherapy was correlated with the percentage of tumour necrosis from pathological section. The median post-chemotherapy T/N ratio was 1.85 (range 0.5-7.7). The median percentage change in T/N ratio after chemotherapy was -58% (range +26% to -83%). The median percentage of necrosis from pathological section was 80% (range 0%-95%). There was a good correlation between the percentage of tumour necrosis and the percentage change in T/N ratio (rank correlation coefficient r = 0.84, P = 0.0085). Quantitative assessment of changes in 201Tl uptake by osteosarcoma correlates well with tumour necrosis after preoperative chemotherapy. This method may be used to predict response to chemotherapy at an earlier stage, enabling the clinician to consider alternative chemotherapeutic regimens or salvage surgery.