Indications for and the significance of seminal vesicle irradiation during 3D conformal radiotherapy for localized prostate cancer.

PURPOSE

To evaluate the use of pretreatment prostate specific antigen, Gleason score, and clinical stage as predictors of the risk of seminal vesicle involvement in patients with clinically localized prostatic cancer, and to determine the impact of excluding the seminal vesicles on the dose received by surrounding normal tissues.

METHODS AND MATERIALS

An empirically derived equation combining the preoperative prostate specific antigen and Gleason score was applied to 188 patients treated with radical prostatectomy, for whom pathologic evaluation of the seminal vesicles was available. High and low risk groups for seminal vesicle involvement were defined using this equation. The observed risks of seminal vesicle involvement was compared to the predicted risk using the preoperative prostate specific antigen, Gleason score or clinical stage alone or using the empirical equation. Dose-volume histograms for five patients treated using six-field conformal radiotherapy were compared including and excluding the seminal vesicles.

RESULTS

Using the empirically derived equation, a low risk group of 109 patients was identified with a calculated risk of seminal vesicle involvement of < or = 13% and an observed incidence of 7.3%. Among the high risk group of 79 patients, which included all patients with a calculated risk > 13%, 37% had seminal vesicle involvement (p < 0.001 low vs. high risk). Twenty percent of the rectal volume received on average above 86% of the total dose for the five plans which included the seminal vesicles compared to 68% for the five plans excluding the seminal vesicles. The doses to 40% of the rectal volume were 64% and 37% if the seminal vesicles were included and excluded, respectively. The dose to the bladder and femoral heads was also decreased but to a lesser extent.

CONCLUSION

The empirical formula predicts risk of seminal vesicle involvement with a higher degree of significance for a larger number of patients than either Gleason score, clinical stage, or prostate specific antigen alone. Based on an analysis of our first 100 patients treated with definitive conformal therapy alone, approximately 47% of those patients could have been treated excluding the seminal vesicles. Excluding the seminal vesicles may allow us to go to a higher total dose with less rectal toxicity.