Intraoperative gamma probe-directed biopsy of asymptomatic suspected bone metastases

Radioisotope bone scanning is frequently used in staging malignancies. However, false-positive results are common, and biopsy is usually required. In the absence of plain radiographic abnormalities or local symptoms, localization of the area of abnormal tracer activity at the time of open rib or sternum biopsy may be difficult. It often requires resection of a large portion of one or more ribs or the sternum to assure that biopsy of the target area was performed. In this setting, a small gamma probe underwent evaluation as an aid to precise intraoperative localization of increased tracer activity in the target bone.

Ten patients with asymptomatic suspected osseous chest metastases by radioisotope bone scanning but with normal plain radiographs underwent open biopsy of 13 ribs and 1 sternum. Six to 12 hours before operation, each received an intravenous injection of 28 mCi of technetium-99m oxidronate. The hand-held, pencil-sized gamma probe in a sterile sleeve was used to localize the area of greatest activity in the target bone, once the bone was exposed through a small incision. Biopsy of a 3-cm length of rib or portion of sternum was performed. In the first two rib biopsies, an intraoperative radiograph with a radiopaque marker on the rib confirmed that the correct rib was selected for biopsy. Intraoperative radiographs were not done on later cases.

The mean ratio of hot spot activity on the targeted rib to background counts on adjacent ribs was 1.65 +/- 0.22 (range, 1.35 to 2.05), and the difference was easily discernible intraoperatively. The ratio of hot spot activity on the sternum was somewhat lower (1.22), but the target area was still easy to detect. An abnormal diagnosis to account for the increased tracer activity was found in each of the 13 ribs and 1 sternal biopsy in all 10 patients: metastatic squamous cell carcinoma (1 rib), metastatic prostatic adenocarcinoma (1 rib), lymphoma (2 ribs), localized hypercellular marrow (1 rib), medullary fibrosis/Paget's disease of the bone (2 ribs), localized fibrosis/granulation tissue (1 rib), enchondroma (3 ribs), and chondroma (2 ribs, 1 sternum). The difference in background counts to hot spot activity was best with injection of the tracer 6 hours before operation.

The intraoperative use of gamma counting is an easy, highly accurate aid (100% sensitivity) to localize areas of abnormal radioisotope uptake in suspected asymptomatic rib and sternal metastases. Use of this technique obviates the need to obtain intraoperative localizing radiographs to confirm accurate rib identification, thereby decreasing operative time.