The factors predicting the long-term response to cabergoline are not well known, particularly in more invasive prolactinomas. This study aimed to evaluate the rate of good response (GR) (normoprolactinemia and ≥ 50% tumor shrinkage) at the one-year follow-up and to identify predictors of GR in patients with large invasive prolactinomas.

This retrospective single-center study included 38 patients (32 males, 6 females; mean age 41.9 ± 12.8 years) with invasive macroprolactinomas (baseline median prolactin: 2530 (1726–5451) ng/mL, mean longest tumor diameter: 36.4 ± 12.8 mm) who were followed for at least one year under cabergoline therapy. Prolactin levels and tumor volume changes were analyzed at early and late follow-ups. Patients were classified as good responders (GRs) or poor responders (PRs) based on their first-year outcomes.

At the first year, 17 patients (44.7%) achieved a GR. Baseline parameters were comparable between groups, but GRs had significantly higher normoprolactinemia rates (70.6% vs. 23.8%,p = 0.004) and greater tumor shrinkage (57.3 ± 15.6% vs. 41.9 ± 21.9%,p = 0.02) at 3–6 months. An early tumor shrinkage cut-off rate of 44.9% (88% sensitivity, 62% specificity) or an early prolactin cut-off level of 30 ng/mL (75% sensitivity, 77% specificity) were identified as good predictors of GR in the first-year. All GRs maintained normoprolactinemia and ≥ 50% tumor shrinkage, while 33.3% and 42.9% of PRs failed to meet these goals, respectively, at the last visit (p = 0.009, p = 0.002). Three of PRs required surgery during follow-up.

Approximately half of invasive prolactinoma patients achieved the composite goal within one year of cabergoline therapy. Early follow-up studies, rather than baseline characteristics, are strong predictors of treatment success.