Background: Diet intervention in oncology is currently under investigation. Nevertheless, some cancer patients are increasingly using various diet interventions with the hope to optimize the anti-tumor response or to control the side effects of oncological treatment. Thus, well conducted prospective trials are required. In order to design such trials, we conducted a prospective study in order to estimate(a) the number of patients already using diet interventions on their own and (b) the percentage of patients who would agree to follow a restrictive diet in combination to antitumoral treatment within a prospective trial, if offered by their oncologist. Methods: Metastatic RCC patients seen at a single institution as outpatient clinic between April and May 2018, were offered to take part in an anonymous feasibility questionnaire. Questions included current treatment status for RCC, gender, height, and weight and explored (a) self diet restriction practices and (b) willingness to take part in trial(s) that would evaluate various diet interventions: fasting, ketogenic, vegetarian or vegan diets (each of them being explained). The proposed diet intervention was supposed to be for a minimum of 3 months. Results: Overall, 119 patients filled out the questionnaire. Median age was 61,5 years, 79(66.3%) were under systemic treatment. At time of data collection, 7.5% patients reported ketogenic diet practices, 0.9% fasting, none vegan or vegetarian diet. Regarding willingness to take part in “diet therapeutic trials”, 42.0% of patients claimed to agree to participate in a ketogenic diet trial, 29.4% of patients would agree to take part in a trial exploring fasting, 28.5% a vegetarian diet trial, and 14.2% of patients a vegan diet trial. No link between being under treatment and willingness to participate in trial was observed (p = 0.365). Conclusions: For the first time, we report feasibility of diet intervention within an homogenous population of patients with RCC. Almost half of patients are really keen in participating, with more interest for ketogenic diet and fasting. Such study raises the question of the feasibility of such trials, and obviously would make randomized trials very challenging.