A direct measurement of strontium-89 activity in bone metastases

The total absorbed dose after systemic administration of 89Sr has been determined by measuring directly its activity in bone metastases. Autoradiography was performed on sections of bones obtained from patients treated with 89Sr to study the pattern of deposition. Discs of 5 and 8 mm diameter were cut from metastatic sites and normal bone. The beta-ray activity was determined with a scintillation counter, which was calibrated using similar bovine cancellous bone discs, onto which a known activity of 89Sr was transferred by pipette. From the activity measured, the initial activity (at the time of 89Sr administration) was calculated. The absorbed dose was estimated using the methodology described in NCRP Report No. 58. The estimated initial activity of 89Sr in the bone metastases varied from 2.3 to 240 MBq kg-1, with a mean value of 31 +/- 27 MBq kg-1. The total absorbed dose ranged from 1.3 to 64 Gy, with a mean of 18 +/- 16 Gy. The average total dose to normal bone sites was 1.1 +/- 0.4 Gy. The metastases to normal bone dose ratio in individual samples varied from 8 +/- 4 to 40 +/- 25. These estimates are in agreement with those obtained previously by indirect methods.