Breast cancer incidence and survival by subtype, stage at diagnosis and socioeconomic deprivation among young women in the Community of Madrid, Spain.

Breast cancer (BC) is the most common malignancy in women worldwide and has a significant impact on younger populations. This study analyses incidence and survival by grade of histological differentiation, molecular subtype, stage at diagnosis and socioeconomic deprivation in young women diagnosed with BC in 2018 in the Community of Madrid (CM) followed up to 2023. Data from invasive BC cases in women aged 20-49 were obtained from the Population-Based Cancer Registry of the CM. Descriptive analyses were conducted for sociodemographic and tumour characteristics. Crude, age-specific and age-standardised incidence rates were calculated. For survival analysis, observed, net and age-standardised net survival using the international cancer survival standard weights were estimated at 1, 3, and 5 years. Flexible parametric models were adjusted to determine differences in the risk of death by molecular subtype. In 2018, 1049 invasive BC cases were registered among 1,432,392 women aged 20-49. The age-standardised BC incidence rate computed was 66 cases/100,000 women-year. Luminal B was the most frequent subtype with 22.9 cases/100,000 (95% CI: 20.6-25.4). Stages I and II had the highest age-standardised incidence rates. Women in less deprived areas showed the highest crude incidence rate: 100.3 cases/100,000 women-year. The 5-year observed survival was 94.8% (95%CI: 93.2-95.9). Poorly differentiated tumours (grade III), triple negative subtype and stage IV at diagnosis had the lowest survival estimates. No significant differences in survival were observed across deprivation status. This study offers comprehensive epidemiological insights into BC incidence and survival in young women in Madrid, offering support for clinical decision-making and prognosis assessment.