Objective. To evaluate whether tumor shrinkage or GH and IGF-I levels achieved after 3 months predicted tumor shrinkage after 12 months of octreotide-LAR (LAR) treatment.

Patients. 67 patients with de novo acromegaly (33 women, 34 men; age 20–82 yrs) receiving LAR at a dose of 20 mg mg/q28d for three months. Final LAR dose was 10 mg/q28d in 4, 30 mg/q28d in 39 and 20 mg/q28d in 24 patients.

Design. Analytical, observational, open, retrospective.

Outcome measures. Percent change in GH and IGF-I levels and tumor volume after 3 and 12 months of therapy. Stepwise regression and ROC analysis were used to calculate the optimal cutoff to predict 12 months tumor shrinkage at 12 months.

Results. The percent tumor shrinkage after 12 months was significantly correlated with GH, IGF-I and tumor volume at 3 months and with the dose of LAR administered between 3–12 months. There was no correlation with gender, age, baseline GH levels and tumor volume. In a stepwise regression analysis, percent tumor shrinkage after 3 months was the best predictor of tumor shrinkage after 12 months (t=5,92; p<0.0001), followed by GH levels after 3 months (t=2,86; p=0.0056). To predict 50% tumor shrinkage after 12 months, the best cutoff point of tumor shrinkage at 3 months was 22,1% [sensitivity (95% CI) = 85,5% (71,2–95,4); specificity = 83,3% (65,3–94,3)] while that of GH levels after 3 months was 7,8 µg/l [sensitivity = 70,3% (53,0–84,1); specificity = 93,3% (79,0–99,0)].

Conclusion. Tumor shrinkage achieved after 3 months of LAR treatment at 20 mg/28d predicted tumor shrinkage at 12 months provided that dosages were changed according with individual patients requirement.