NEW YORK—Researchers at Memorial Sloan-Kettering Cancer Center have developed a method of quantifying bone involvement in patients with androgen-independent prostate cancer and have found that the resulting bone scan index (BSI) correlates with patient survival. In contrast, simply counting the number of bone lesions present did not provide useful prognostic information.
“At present, because of the difficulty in measuring bone disease, prostate cancer patients with bone-only disease are generally excluded from prostate cancer trials,” Paul Sabbatini, MD, said in an interview with Oncology News International. “Since you can’t measure their disease, you can’t tell whether a patient is getting better or worse with treatment.”
Yet, he pointed out, bone-only disease is the most common prostate cancer presentation, and these patients usually have earlier disease, “when you would really like to test new strategies,” than those with measurable metastases, such as lymph node or soft tissue involvement. “Our ultimate goal,” he said, “is to develop a way to make bone disease measurable via serial bone scans that can be compared objectively over time.”
Calculating the BSI
To come up with a BSI, the radiologist visually estimates the percent involvement of each bone, based on a patient’s bone scans. “This is less laborious than you might think,” Dr. Sabbatini said. “Usually, you have a total of seven or eight lesions. You might note, for example, that 20% of the clavicle is involved. That percentage is then multiplied by the clavicle’s percentage of the total weight of the skeleton.”
These proportions are already known from a published “reference man” that shows the proportional weight of about 120 bones in the average human skeleton. “When this is done for each bone lesion, you come up with a percentage of total bone involvement, termed the BSI,” he said. The higher the BSI, the greater the bone involvement (see Figure).
In a so-called superscan in which almost the entire skeleton shows tumor involvement, the BSI is 50% to 60%. Serial BSI measurements would not be useful in these patients, Dr. Sabbatini said, and these patients typically also have lymph node or liver disease that can be measured in therapeutic trials. (Even in a “superscan,” he noted, the BSI is not 100%, because of the allowance for bone marrow and for areas of bone where tumor does not go.)
In their study, presented at the American Society of Clinical Oncology 1997 meeting, the investigators calculated baseline BSI in 191 prostate cancer patients who had progressive disease after primary hormonal therapy. These patients were enrolled in a randomized phase III study of oral liarozole vs predni-sone, and survival data were available.
The patients were stratified into three equal groups based on baseline BSI: less than 1.4% (63 patients), 1.4% to 5.1% (65 patients), and more than 5.1% (63).
In a multivariate analysis controlling for other prognostic factors such as baseline PSA, age, LDH, and treatment arm, BSI was shown to be independently predictive of survival. Among those with the lowest baseline BSI, median survival was 18.3 months vs 15.5 months and 8.1 months, respectively, in the two groups with higher baseline BSI (P = .0079).
Further studies are planned to define the prognostic significance of serial BSI measurements in prostate cancer patients with bone lesions, Dr. Sabbatini said. “Although we did not have serial bone scans for the dataset used in the current study, we did have survival data, and we thought it was important first to be able to say definitively that BSI correlates with survival, and we were able to do that. The next step is to look at serial scans.”
