99mTc HMDP single photon emission tomography coupled with CT scanning

Bone scintigraphy with 99mTc-HMDP is a routine examination for the detection of bone metastases. The protocol involves a planar whole-body scan with few anatomical landmarks hence the use of hybrid imaging particularly in cases of indeterminate focus. However the addition of computed tomography (CT) increases patient irradiation. The aim of this study was to assess the additional effective dose delivered by CT in an adult oncology population of 43 patients. We calculated the effective whole-body doses from scintigraphy and CT which averaged 4.22 mSv and 3.07 mSv respectively. The mean total effective dose was 7.29 mSv below diagnostic reference levels but exceeded in 12 patients. The percentage of effective dose from CT to total dose delivered by hybrid imaging ranged from 9.06 to 64.95% with a mean of 39.72%. The ratio was higher for the abdominal-pelvic region (45.68%) than for the thoracic region (39.97%). This dose is not negligible. However when the benefits of hybrid imaging are taken into account the extra radiation is still justifiable.