Potency rates following external beam radiation therapy vary widely, from 23% to 73% depending on the type of radiation delivered and length of follow-up.53-59 Some authors have noted that potency was retained in a greater percentage of patients who had a higher level of sexual functioning prior to radiation treatment.53'60-62 This observation correlates with the findings from the radical prostatectomy data, which also show that preoperative sexual functioning is a significant predictor of postoperative impotence.
Three-dimensional conformal radiotherapy (3D-CRT) differs from conventional external beam radiotherapy in its treatment planning and delivery technique. The former obtains images of the patient's internal anatomy by computed tomography scans in the treatment position, and then delivers a more precise radiation dose to spare more normal tissue. The goal is to irradiate at higher doses with less morbidity. Studies have been contradictory concerning the ability of 3D-CRT to offer patients higher potency rates versus traditional therapy. Beard et al. found 20% complete impotence in the conformal group as compared to 44% in the whole pelvis radiation group.52 This suggests that larger radiation fields negatively affect post-treatment erectile function. Other data suggest that there is no difference in the risk of impotence between conformal therapy and conventional radiotherapy.58'63 One study by Zelefsky etal. found a higher incidence of erectile dysfunction with higher doses of 3D-CRT.64 At 5 years, 68% of patients who received ^75.6 Gy were impotent compared to 52% who received lower doses.64
It appears that potency rates gradually decrease as the number of years post-treatment increases. Wilder etal. found that the potency preservation rate decreased with each year after treatment from 100%, to 83%, to 63% at 3 years after treatment.65 Several other studies have reported similar results.57,59,66,67 The question remains at what point in time do patients after 3D-CRT become impotent? The median time to impotence after conformal radiotherapy varies between 14 and 19 months.64,66 This implies that studies that report potency rates after radiotherapy to the prostate at or before 1 year are reporting falsely elevated figures.
Four to eight years after traditional radiotherapy to the prostate sexual function appears to stabilize. Previously it was discussed that the rate of impotence increases as a function of time after radiotherapy. How does this rate change over extended periods of time and does it always increase? Fransson etal. reported that sexual function at 4 years predicts what sexual function will be at 8 years after radiotherapy.68
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