Simple SummaryA large proportion of older adults are not fit for oncological treatments due to frailty and comorbidities. To aid in the decision-making of whom to provide active oncological treatment to, we used G8-screening and comprehensive geriatric assessment in patients at risk of frailty. We studied the added value of muscle measurement with computed tomography (CT) at the third lumbar vertebra level in these potentially frail ≥ 75-year-olds. In 58 patients with advanced or metastatic solid tumors, a higher 3-month mortality rate and poorer nutritional status and functioning were noted among those with low muscle mass, independent of other predictive factors. Most patients with low muscle mass were allocated to best supportive care only. A poorer 2-year survival among 21 patients treated with curative intent was noted in those with low muscle mass. Muscle mass assessment alongside geriatric assessment can thus help oncologists identify patients at increased risk of severe toxicities and with little benefit from oncological treatments.AbstractAs patients with solid (non-hematological) cancers and a life expectancy of <3 months rarely benefit from oncological treatment, we examined whether the CT-determined loss of muscle mass is associated with an impaired 3-month overall survival (OS) in frail ≥75-year-old patients with cancer. Frailty was assessed with G8-screening and comprehensive geriatric assessment in older adults at risk of frailty. The L3-level skeletal (SMI) and psoas (PMI) muscle indexes were determined from routine CT scans. Established and optimized SMI and PMI cut-offs were used. In the non-curative treatment group (n = 58), 3-month OS rates for normal and low SMI were 95% and 64% (HR 9.28; 95% CI 1.2–71) and for PMI 88%, and 60%, respectively (HR 4.10; 1.3–13). A Cox multivariable 3-month OS model showed an HR of 10.7 (1.0–110) for low SMI, 2.34 (0.6–9.8) for ECOG performance status 3–4, 2.11 (0.5–8.6) for clinical frailty scale 5–9, and 0.57 (0.1–2.8) for males. The 24-month OS rates in the curative intent group (n = 21) were 91% and 38% for the normal and low SMI groups, respectively. In conclusion, CT-determined low muscle mass is independently associated with an impaired 3-month OS and, alongside geriatric assessment, could aid in oncological versus best supportive care decision-making in frail patients with non-curable cancers.