BACKGROUND: The benefit of adding radiation therapy after excision of ductal carcinoma in situ (DCIS) is widely debated. Randomized clinical trials are underpowered to delineate long-term outcomes after radiation. METHODS: The authors of this report constructed a Markov decision model to simulate the clinical course of DCIS in a woman aged 60 years who received treatment with either of 2 breast-conserving strategies: excision alone or excision plus radiation therapy. Sensitivity analyses were used to study the influence of risk of local recurrence, likelihood of invasive disease at recurrence, surgical choice at recurrence, and patient age at diagnosis on treatment outcomes. RESULTS: The addition of radiation therapy was associated with slight improvements in invasive disease-free and overall survival. However, radiation therapy decreased the chance of having both breasts intact over a patient's lifetime. Radiation therapy improved survival by 2.1 months for women who were diagnosed with DCIS at age 60 years but decreased the chance of having both breasts by 8.6% relative to excision alone. The differences in outcomes between the treatment strategies became smaller with increasing age at diagnosis. Sensitivity analyses revealed a greater benefit for radiation with an increased likelihood of invasive recurrence. The decrement in breast preservation with radiation therapy was mitigated by an increased likelihood of mastectomy at the time of recurrence or new breast cancer diagnosis. CONCLUSIONS: The current analysis quantified the benefits of radiation after excision of DCIS but also revealed that radiation therapy may increase the likelihood of eventual mastectomy. Therefore, the authors concluded that patient age and preferences should be considered when making the decision to add or forgo radiation for DCIS. Cancer 2011;. © 2011 American Cancer Society.