6507 Background: External beam radiotherapy (EBRT) is a commonly used for treatment of clinically localized prostate cancer. Yet, secular trends in the delivery of this highly technical therapy have received little attention. Methods: Using data from the linked Surveillance, Epidemiology, and End Results (SEER) Medicare program, we evaluated trends in five EBRT quality measures among 23,018 patients age 65 or older diagnosed from 1994 to 2002 with clinically localized prostate cancer and treated with primary EBRT. Using tumor registry data from SEER and Medicare claims, we excluded 6,956 patients who received brachytherapy and 7,009 patients who received combination EBRT and brachytherapy. We identified treating radiation oncologists via unique physician identification numbers reported on claims and obtained board certifications from the AMA Masterfile. We assessed the five EBRT quality measures proposed by a RAND expert panel that were amenable to analysis using SEER-Medicare data: 1) use of conformal radiotherapy treatment planning; 2) use of high-energy (=10MV) photons; 3) use of custom immobilization; 4) radiation oncologist board certification; and 5) completion of two follow-up visits with a radiation oncologist in the year following therapy. Results: As shown in the table , conformal radiotherapy increased over the study period. Approximately one-third of patients received consistent follow-up from a radiation oncologist in the year following therapy. Notably, however, claims data revealed that 80% of patients completed at least two follow-up visits with either urologists or radiation oncologists. Conclusions: Conformal radiotherapy is now routine for elderly men with localized prostate cancer. Observed practice patterns deviate from the RAND metric for patient follow-up, suggesting that this measure merits clarification. Future research should examine whether variation in receipt of these quality measures affects important clinical outcomes. No significant financial relationships to disclose. [Table: see text]