Non-functioning (NF) accidentally detected PitNETs (PIs) are common findings of CT/MRI scans currently. Data concerning their behavior vary, and some PIs will potentially experience a size change over time that might become clinically relevant. Objective: We aimed to evaluate CT-related PIs diameters following 3 aspects: a cross-sectional analysis based on the age’ groups at first PI diagnosis and on the gender distribution and a longitudinal analysis in PIs with <0.5 cm versus ≥0.5 cm as the largest diameter at baseline. Methods: A retrospective, real-life, multi-centric study in adults with NF micro-PIs was performed. Results: 208 subjects (92.79% females) were included (average age 43.18 ± 12.58 y). The mean largest diameter (between transversal and longitudinal diameters) was 0.55 ± 0.16cm, and 44.71% of the PIs were located on the right part of the pituitary gland. The patients were divided into 10 age-groups (21 to 70 y), and no difference was observed between the mean largest diameters of these groups (p = 0.693). Females and males had a similar age at PIs diagnosis (p = 0.353), transverse diameter (p = 0.910), longitudinal diameter (p = 0.229), and PI location (p = 0.368). Serial CT scans (2 to 4 per patient) included a median (Q1–Q3) of 20 (12–36) months for the second CT scan, 58 (36–84) for the third CT scan, 78 (53–118) for the fourth CT scan, and a total follow-up between baseline and last CT scan of 40 (13–72) months. The initial largest diameter correlated with the diameter change between the baseline and the last CT (r = −0.575, p = 0.000). The largest PI diameter was similar between each serial CT scan (p = 0.840). According to the cut-off value of 0.5 cm diameter (for initial largest tumor diameter), group A (N = 78, 37.50%, <0.5 cm) and group B (N = 130, 62. 50%; ≥0.5 cm) had similar age at baseline (43.83 ± 12.72 versus 42.79 ± 12.54 y, p = 0.565) and PIs locations (right, left, median). The largest PI diameter remained smaller in group A versus B amid the second CT scan (p = 0.000) and the last CT scan (p = 0.009). The largest diameter change from the first to the last CT scan showed an increase in group A (median of +0.10 cm, p = 0.000) and a decrease in group B (median of −0.01 cm, p = 0.002); this diameter change was different in group A versus B (p = 0.000). The cumulative probability of tumor-growth-free survival showed different survival functions (log rank p = 0.000): group A exhibited a more gradual decrease versus B; at 60 months, the cumulative probability was 0.32 ± 0.08 for group A versus 0.75 ± 0.07 for B. During follow-up, all PIs remained NF, and no hypopituitarism was noted (as limits, we mention that dynamic hypopituitarism testing was selectively performed). Conclusions: NF micro-PIs in adults showed a similar age and sex distribution. During follow-up, PIs with a largest diameter < 0.5 cm increased after a median of 40 months but remained <1 cm, while in PIs with ≥0.5 cm, the largest diameter decreased. This highlights a lower predictability in tumor behavior than expected, particularly in larger micro-PIs that, overall, remained without relevant clinical implications after surveillance.
