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© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2021, Rationale: Cancer is associated with muscle wasting. However, optimal protein intake has not been determined, limiting the efficacy of nutritional interventions. This systematic review aims to assess the effect of protein intake on muscle mass of patients with cancer types with high prevalence of sarcopenia during treatment, in longitudinal studies. Methods: MEDLINE, CINAHL, and Scopus databases were searched following PRISMA guidelines. Longitudinal studies written in English, including adults with high sarcopenia prevalence cancer diagnosis, submitted to (chemo)radiotherapy, with assessment of protein intake and muscle changes during treatment, published until 4 October 2020 were included. Studies including supplementation with substances, such as n-3 fatty acids, specific amino acids, or proteins, were excluded. Study appraisal was independently conducted by two reviewers, and a qualitative research synthesis was performed. Results: Overall, 575 records were identified, of which, eight studies were included (one randomized clinical trial and seven uncontrolled before and after studies). Patients with head and neck (n = 5), lung (n = 2), and esophageal cancer (n = 1) were included, comprising a total of 554 participants. The studies presented heterogeneous methodologies, objectives, and methods to assess body composition. Overall, participant groups with a mean protein intake below 1.2 g/kg presented muscle wasting, with one exception, while those reporting a mean intake above 1.4 g/kg, maintained muscle during treatment. Conclusions: Our findings show that protein intakes below 1.2 g/kg, even when within the recommendations, have been associated with muscle wasting during treatment. Only intakes above 1.4 g/kg have been associated with muscle maintenance. High-quality research is needed to establish an optimal dose response.