Background: Many patients undergoing breast cancer therapy require management of side effects after treatment is completed. Adherence to survivorship recommendations, however, is suboptimal. In this study, we aimed to compare patient enrollment in a symptom intervention trial during vs. immediately following completion of radiation treatment (RT). Methods: An automated weekly query of breast radiation oncology electronic patient-reported outcomes (ePRO) was generated to identify patients with moderate or greater fatigue, the primary eligibility criterion for an ongoing randomized clinical trial testing a symptom-based intervention. In a substudy of the ongoing trial, 12 radiation oncologists were assigned to have their potentially eligible patients undergoing RT receive a direct portal message trial invitation. Mid-study, the protocol was amended to include patients post-treatment (defined as completing ePRO questionnaires within 4 weeks of their last treatment). Messages were sent from the symptom-intervention trial team (Integrative Medicine), and we compared responses and enrollments between the two treatment periods using a two-sample, two-sided proportion test. Reasons for non-enrollment were categorized. Results: From 12/19/22 to 3/13/23, 361 patients completed ePROs in the post-treatment period, of which 112 (31%) screened positive for fatigue compared to 394 of 1041 (38%) in the on-treatment period (09/05/22-03/13/23). Among 112 patients receiving a portal message post-treatment, 28 patients (25%) responded compared to 44 of 100 patients (44%) receiving the same message while on-treatment (p=0.004). Subsequently, 15 patients (13%) enrolled post-treatment vs. 25 patients (25%) on-treatment (p=0.03). Among those not enrolling in post-treatment (n=97), 84 (87%) did not respond to the initial portal message, 4 (4%) did not answer the follow-up phone call after expressing interest, 5 (5%) declined, and 5 were deemed ineligible. Non-enrollment on-treatment (n=75) included 56 patients (75%) that did not respond, 6 (8%) that did not answer the follow-up phone call, 2 (2.6%) that declined, and 11 that were deemed ineligible. Conclusions: Engaging patients with breast cancer in a symptom-based intervention trial was more successful during RT than immediately afterward. Although oncologic treatment itself can be time-consuming and cognitively burdensome for patients, it may provide an early window for engagement in symptom-based interventions by cognitively linking a new commitment with an existing one. This study highlights the need for further research into optimal timing of engagement in survivorship for patients with breast cancer.