Prostate-specific antigen spikes with ¹³¹Cs brachytherapy. Is there a difference with other radioisotopes?

There is a suggestion that a dose-rate effect exists for the prostate-specific antigen (PSA) spike after brachytherapy. ¹³¹Cs is a newer radioisotope with a half-life of 9.7 days that is being used for prostate brachytherapy. There is no published data on the PSA spike with this radioisotope and the goal of this study was to quantify PSA spikes with ¹³¹Cs and compare it with published data for other isotopes.

We have been maintaining a prospective database for all patients treated with ¹³¹Cs prostate brachytherapy at our institution. We selected patients for whom followup PSA was available for at least 24 months. The PSA spike was defined as an increase of 0.2 ng/mL, followed by a decline to prespike level.

One hundred twenty-three patients had monotherapy, whereas 32 had external beam radiation therapy followed by a brachytherapy boost. Median followup was 36 months and mean numbers of PSAs obtained were 7. Forty-six (29.7%) patients had a PSA spike. The mean time and duration for the PSA spike were 12.5 and 8.8 months, respectively. The mean magnitude of increase and mean PSA value at increase were 0.63 and 1.56 ng/mL, respectively.

The incidence of a PSA spike in our series is consistent with reported numbers for other radioisotopes. The occurrence of the spike at 12.5 months appears to be at the early end of the spectrum reported for (125)I, but the duration and magnitude are similar to other radioisotopes.