ABSTRACT: In situ breast cancer represents an important subset of breast neoplasms and is frequently detected through organized screening programs. We analyzed incidence trends, clinicopathologic characteristics, and treatment patterns of in situ breast tumors recorded in the Reggio Emilia Cancer Registry from 2000 to 2023. A total of 1,543 in situ breast tumors were identified, representing 13.3% of all registered breast cancers during the study period. Age-adjusted incidence rates per 100,000 were calculated using the 2013 European Standard Population, and the annual percentage change (APC) was estimated to assess temporal trends. Clinicopathologic features, including histology and hormone receptor status, were collected. Surgical management and demographic characteristics were also analyzed. Overall, in situ breast tumors showed a modest upward trend over the study period; however, this increase was not statistically significant (APC 0.7; 95%CI -0.6 to 2.2). A modest decline was observed in 2022–2023 following the COVID-19 pandemic, which significantly affected screening uptake in 2020–2021. Age distribution revealed that 8.5% of cases occurred in women under 45 years and 8.5% in women over 75 years. The majority (83%) were diagnosed in women aged 45-74, the primary target of screening programs, including 19.7% in women aged 45-49 years, 69% aged 50-69, and 11.3% aged 70-74. Histologically, the vast majority (93.5%) were ductal in origin. Hormone receptor analysis showed that 36.5% were estrogen receptor-positive (ER+) and 25.9% progesterone receptor-positive (PR+). Breast-conserving surgery was performed in 74.7%, while 19.1% underwent mastectomy. Sentinel lymph node biopsy (SLNB) was performed in 46.2% of cases, whereas axillary lymph node dissection was uncommon (3.3%). Women of foreign nationality represented 6.6% of the cohort. In situ breast tumors in our Cancer Registry demonstrated a modest, non-significant increase in incidence over time and predominantly affected women within the screening age range. Most cases were ductal and managed with breast-conserving surgery, reflecting current clinical practice. These findings highlight the sustained impact of screening programs and the importance of ongoing surveillance of early breast neoplasia, especially in the context of healthcare disruptions such as the COVID-19 pandemic.

Impact statement: From 2000 to 2023, in situ breast cancers in the Reggio Emilia Cancer Registry showed a modest, non-significant increase, predominantly ductal and mainly treated with breast-conserving surgery within screening ages.

Key words: In situ breast cancer; incidence trends; screening; breast-conserving surgery.