Because there was a well-known link between cancer and myositis, cancer screening was done at diagnosis. Researchers sought to share the results of the cancer screening technique and determine if using PET/CT to detect cancer-associated myositis (CAM) in a large cohort of myositis patients from a single center over a 10-year period was reliable. Except for inclusion body myositis, all patients diagnosed with any kind of myositis were included in the retrospective observational analysis. The cancer screening method was tailored to the patient’s clinical and serological data, with PET/CT serving as the primary test for detecting occult cancer (OC). Procedures resulting from a positive PET/CT scan were recorded. The median with the interquartile range was used to examine qualitative data expressed as percentages and quantitative data expressed as the median with the interquartile range. The reliability of PET/CT for CAM diagnosis was estimated using a ROC curve. A PET/CT scan was performed on 77 out of 131 patients for OC screening. The area under the curve ROC for CAM diagnosis was 0.87 (0.73–0.97) in individuals with myositis at disease onset when PET/CT was used. Invasive procedures did not result in problems in 7 (9%) individuals who did not have a final cancer diagnosis. After a median follow-up of 3.3 years (1.7–6.7), patients who were not evaluated for OC did not acquire cancer. Therefore, individualized cancer screening strategies should be used. PET/CT during the onset of myositis appeared to be an effective way to rule out CAM. The technique does not seem to do patients any more harm due to the additional testing required to clarify inconclusive data.