Predicting Bone Scan Positivity (tumor spread to bone)
Physicians often order periodic bone scans to check for metastases in patients with an increasing PSA, and a biochemical recurrence after radical prostatectomy, but most scans negative. A group of physicians from Memorial Sloan Kettering Hospital in Manhattan, NY studied patient characteristics in an attempt to build a predictive model for a positive bone scan.8
They identified all patients with detectable PSA after radical prostatectomy and analyzed the following features at the time of each bone scan for association with a positive scan: preoperative PSA, time to biochemical recurrence (BCR), pathologic findings of the RP, PSA before the bone scan (so-called “trigger PSA”- see note below), PSA kinetics (PSA doubling time, PSA slope, and PSA velocity), and time from biochemical PSA recurrence to bone scan. The results were incorporated into a predictive model. Note: this nomogram is applicable only to a man who has developed biochemical failure after radical prostatectomy and has received no other prostate cancer therapy.
There were 414 bone scans performed in 239 patients with biochemical recurrence of PSA and no history of androgen deprivation therapy. Only 60 (14.5%) were positive for metastases. Nomogram 9 was constructed and used for predicting the bone scan result. Trigger PSA, PSA velocity, and slope were associated with a positive bone scan. A highly discriminating nomogram could be used to select patients according to their risk for a positive scan. Omitting scans in low-risk patients could reduce substantially the number of scans ordered.8
Nomogram 9 is based on 414 bone scans of 239 patients observed at MSKI. Each scale position corresponds to points (top axis). Point values for all predictor variables are determined consecutively and are summed to arrive at the total point value. This value is plotted on the total point axis, and directly below it is the predicted probability of a positive bone scan.
Clinical scenario 9: Using Nomogram 9, a patient who was negative for ECE, SVI, and LNI (lymph node involvement) but who had a preoperative PSA of 10.1 (7 points), a radical prostatectomy specimen Gleason score of 7 (5), a PSA of 8 at the time of bone scan performance (58), a PSA velocity of 3 ng/mL/month (10), and a PSA slope of 0.2 (10). would have 90 total points. This predicts a 7% probability of a positive bone scan. A different man, who also was negative for ECE, SVI, and LNI, had a preoperative PSA of 8.1, a radical prostatectomy specimen Gleason score of 9, a PSA of 63 at the time of bone scan performance, a PSA velocity of 5 ng/mL/month, and a PSA slope of 0.5 would have 137 points, which predicts a 92% probability of a positive bone scan.
Note: To determine “trigger PSA”, use current serum PSA and calculate PSA slope and velocity from the last three PSA values, the third of which is the trigger PSA. (PSA velocity is in units of ng/mL/month and PSA slope is in units of log [ng/mL]/month) [The slope is calculated as the difference in the 2 log PSA values divided by the time between the readings in months]. Tools for the calculation of PSA velocity and slope are available at www.nomograms.org.
8. Dotan ZA, Bianco FJ Jr, Rabbani F, et al: Pattern of prostate-specific antigen (PSA) failure dictates the probability of a positive bone scan in patients with an increasing PSA after radical prostatectomy. Journal of Clinical Oncology, Vol 23 1962-1968, 2005.