Polysubstance use (PSU), particularly opioid-involved and stimulant-involved PSU, is a growing issue in the USA. PSU increases the risk of negative health consequences, including infectious diseases, worsening physical and mental health conditions, and overdose-related deaths. These consequences occur in the context of varying health risk behaviours, substance-related preferences, and treatment engagements among people with PSU. To inform improvements in prevention, harm reduction, and substance use disorder (SUD) treatment, additional research is needed to comprehensively understand the current context and drivers of PSU preferences, motivations, and behaviours.

Herein, we describe the protocol for a prospective cohort study designed to capture detailed patterns, profiles, and trajectories of PSU, with the aim of comprehensively examining the drivers of PSU behaviours and SUD treatment utilisation. Adults (ages 18–75; n=400) who engage in PSU will be recruited from healthcare institutions, an established participant database maintained by an adjacent SUD research team, and online advertisements. Study assessments will capture dynamic patterns, choice preferences, and motivators of PSU via behavioural economic (BE) measures, detailed Timeline Follow-Back (TLFB) interviews, and self-administered surveys. The assessment timeline will include a baseline survey and TLFB interview, weekly TLFB interviews for 4 weeks post-baseline, and follow-up surveys and TLFB interviews at 4-, 8-, and 12-months post-baseline.

The study is funded through the National Institutes of Health Helping to End Addiction Long-term (HEAL) initiative and was approved by the University of Michigan Medical Institutional Review Board. Findings will be disseminated to academic, clinical, and community partners through the Michigan Innovations in Addiction Care through Research and Education programme. Results from this study will inform actionable and practical insights relevant to the delivery of personalised care in the context of PSU.

Over the past decades, interest in second breast-conserving therapy (BCT) has increased due to, among others, advances in radiotherapy techniques. Preoperative partial breast irradiation (PBI) is an experimental treatment for patients with low-risk primary breast cancer. This approach can downstage the tumour and may possibly reduce toxicity and improve cosmetic outcomes compared with postoperative radiotherapy. This study aims to evaluate the feasibility of single-dose preoperative PBI and second breast-conserving surgery (BCS) for patients with an ipsilateral recurrent breast event (IRBE) after previous BCT.

The REPEAT trial is a multicentre, prospective, single-arm trial investigating ablative single-dose preoperative PBI in patients with an IRBE. Eligible patients are ≥50 years, have a unifocal non-lobular invasive breast cancer ≤2 cm, Bloom-Richardson grade 1 or 2, oestrogen receptor-positive, human epidermal growth factor receptor 2-negative and clinically negative axillary lymph nodes. The study plans to enrol 25 patients. Radiotherapy planning will involve the use of CT and MRI in the treatment position. Single-dose PBI of 20 Gy to the tumour and 15 Gy to the surrounding 2 cm of breast tissue will be delivered using a conventional or MR-guided linear accelerator. Tumour response will be monitored preoperatively using MRI and liquid biopsies to identify biomarkers for evaluating radiosensitivity. BCS will be performed 3 (±one) weeks post PBI. The primary endpoint is the incidence of grade 2 or higher treatment-associated acute toxicity within 90 days. Secondary endpoints include the evaluation of acute (grade 1) and late toxicity, radiologic and pathologic response, mastectomy rate, patient-reported outcomes, cosmetic outcome, local, regional and distant recurrence rates, survival outcome and biomarkers in liquid biopsies and tumour tissue. Patients will be followed up to 5 years after PBI.

Ethical approval from the Medical Research Ethics Committee of the Amsterdam UMC has been obtained (NL85983.018.24). The results will be disseminated via peer-reviewed academic journal and presentation at conferences. In addition, summaries will be shared with the participating patients.

The trial was registered prospectively on October 11^th 2024 at clinicaltrials.gov (NCT06640881).