Impact of underweight on patients receiving curative-intent anticancer therapy: analysis of a large oncology database in Saudi Arabia

Background The clinical effects of underweight status on short-term treatment-related morbidity in patients receiving curative-intent anticancer therapy remain unclear. Methods We conducted a retrospective observational cohort study using a comprehensive oncology database, including 2,504 consecutive patients with histologically confirmed malignancies treated between March 1 and June 30, 2020, at major cancer centers and outpatient clinics across Saudi Arabia. Patients who received curative-intent outpatient systemic anticancer therapy were eligible. Underweight (body mass index [BMI] < 18.5 kg/m²) and normal-weight patients were compared with respect to baseline characteristics, treatment patterns, and 30-day morbidity. Multivariable logistic regression was used to identify factors independently associated with morbidity. Results In total, 787 patients met the eligibility criteria and were included in the analysis. Underweight patients were more frequently male ( P < 0.001), whereas age ( P = 0.764) and Eastern Cooperative Oncology Group (ECOG) performance status ( P = 0.152) were similar between the BMI groups. Cancer stage distribution and curative treatment approach (neoadjuvant vs. adjuvant) did not differ significantly. Underweight patients were more likely to receive intravenous therapy ( P = 0.005) and less likely to receive hormonal or targeted therapy. In unadjusted analyses, underweight patients had higher 30-day morbidity, with increased hospitalization ( P = 0.023), emergency department visits ( P = 0.034), and chemotherapy dose reductions ( P = 0.004). After multivariable adjustment, underweight BMI was not independently associated with morbidity, whereas poorer ECOG performance status and treatment class were the strongest predictors. Intensive care unit admission rates did not differ between the groups. Conclusions Underweight status was associated with increased unadjusted short-term treatment-related morbidity among patients receiving curative-intent anticancer therapy. However, this association was largely explained by functional status and treatment-related factors. These findings highlight the importance of early risk stratification and proactive supportive care rather than BMI alone to optimize treatment tolerance in this vulnerable population.