Potency probability following conformal megavoltage radiotherapy using conventional doses for localized prostate cancer.

PURPOSE: Impotence is a familiar sequela of definitive external beam radiation therapy (EBRT) for localized prostate cancer; however, nerve-sparing radical prostatectomy (NSRP) has offered potency rates as high as 70% for selected for patients in several large series. To the authors' knowledge, age and stage-matched comparisons between the effects of EBRT and NSRP upon the normal age trend of impotence have not been performed. Herein, we report the change in potency over time in an EBRT-treated population, determine the significantly predisposing health factors affecting potency in this population, and compare age and stage-matched potency rates with those of normal males and prostatectomy patients. METHODS AND MATERIALS: Our results are obtained from a retrospective study of 114 patients ranging in age from 52 to 85 (mean, 68) who were diagnosed with clinical stages A-C C (T1-T4N0M0) prostate cancer and then treated conformally with megavoltage x-rays to 6500-7000 cGy (180-200 cGy per fraction) using the four-field box technique. Information concerning pre-RT potency, medical and surgical history, and medications was documented for each patient as was time of post-RT change in potency during regular follow-up. The median follow-up time was 18.5 months. RESULTS: The actuarial probability of potency for all patients gradually decreased throughout post-RT follow-up. At months 1, 12, 24, and 36, potency rates were 98, 92, 75, and 66%, respectively. For those patients who became impotent, the median time to impotence was 14 months. Factors identified from logistic regression analysis as significant predictors of post-EBRT impotence include pre-EBRT partial potency (p < 0.001), vascular disease (p < 0.001), and diabetes (p = 0.003). Next, an actuarial plot of potency probability to patient age for the EBRT-treated population was compared to that obtained from the Massachusetts Male Aging Study of normal males. The two curves were not significantly different (logrank test, p = 0.741) between ages 50 and 65. Finally, potency probability after follow-up of 1 year or more in EBRT-treated patients was stratified by age and substratified by clinical stage and then compared to similarly stratified potencies for patients treated with NSRP. The prostatectomy data were derived from the pooled data of six large (total n, 952), independent series conducted at academic centers. For patients older than 70 years, 79.1% of EBRT patients and 32.9% of NSRP patients remained potent after treatment. For patients with stage B2 disease, 75.0% of EBRT patients and 49.3% of NSRP patients remained potent after treatment. Overall EBRT patient potency was 76.1% vs. 66.2% for NSRP patients. CONCLUSIONS: 1) By 36 months after completion of EBRT for localized prostate cancer, fully one-third of all patients becomes impotent; however, for patients younger than 70 years, the probability of impotence does not depart significantly from that for normal males. 2) In the EBRT-treated population, pre-EBRT partial potency, vascular disease, and diabetes are the most significant predispositions to the development of impotence. Patients with these predispositions, though, do not become impotent significantly earlier than other patients. 3) When comparing age and stage-stratified potency rates for EBRT and NSRP patients, potency is roughly equal for both modalities for most age and stage groups; however, for patients older than 70 years or with stage B2 disease, EBRT offers notably higher posttreatment potency rates than NSRP. Thus, for the treatment of localized prostate cancer, EBRT may not affect the normal age trend of impotence in younger patients and may induce impotence less frequently than NSRP in older patients or in patients with later stage disease.

Radiation-induced DNA double-strand break frequencies in human squamous cell carcinoma cell lines of different radiation sensitivities.

DNA neutral (pH 9.6) filter elution was used to measure radiation-induced DNA double-strand break (dsb) frequencies in eight human squamous cell carcinoma cell lines with radiosensitivities (D0) ranging from 1.07 to 2.66 Gy and D values ranging from 1.46 to 4.08 Gy. The elution profiles of unirradiated samples from more radiosensitive cell lines were all steeper in slope than the profiles from resistant cells. The shapes of the dsb induction curves were curvilinear and there was some variability from cell line to cell line in the dose-response for the induction of DNA dsb after exposures to 5-100 Gy 60Co gamma-rays. There was no relation between the shapes of the survival curves and the shapes of the dose-responses for the induction of DNA dsb. At low doses (5-25 Gy), three out of four of the more sensitive cell lines (D less than 2.5 Gy) had larger initial break frequencies than the more resistant lines (D greater than 3.0 Gy). Although the low-dose (5-25 Gy) elution results were variable, they do suggest that DNA neutral elution will detect differences between sensitive and resistant tumour cells in initial DNA dsb frequencies.