Relationship between tumor volume and quantitative values calculated using two-dimensional bone scan images

The bone scan index (BSI) is calculated from a whole-body bone scan image; it shows the tumor burden in bone as a percentage of total skeletal mass. It has been used to determine the prognosis and to assess treatment effects; however, little has been reported on whether the BSI calculated using a two-dimensional image can accurately evaluate the three-dimensional spread in tumor volume. We investigated the relationship between tumor volume and BSI using Monte Carlo simulation (MCS). We simulated a gamma camera and constructed a voxel phantom based on an anthropomorphic phantom computed tomography (CT) image and gamma rays emitted from each part according to technetium-99m-labeled methylene diphosphonate (^99mTc-MDP) uptake (bone 1, soft tissue 0.2, tumor 2-32). We constructed bone scan images from the obtained counts and analyzed them using the BSI calculation software. The BSI increased with increased tumor uptake (two- to 32-fold). However, there was not always a significant difference between change in BSI and tumor uptake of eight times or greater than that of bone. When BSI was calculated with a tumor having an uptake of four-to-eight times higher than that of bone, the BSI was consistent with tumor volume, but decreased to about half the tumor volume when tumors were in the thoracic spine (Th-spine) segment. The BSI can be a good indicator of tumor volume in most segments, even though it is affected by the tumor's ^99mTc-MDP uptake. Nevertheless, values calculated from the Th-spine should be interpreted carefully.