A 34-year-old man (ethnicity unknown) with a history of childhood eczema and asthma presented with an ASPS of the left thigh (10 cm) in June 2015. He underwent neoadjuvant radiotherapy of the left thigh from July to August 2015 (45 Gy in 25 fractions of 1.8 Gy in 3D conformal radiotherapy), followed by a large resection including the left vastus intermedius, anterior rectus, and part of the vastus lateralis muscles 7 weeks later (October 2015). Surgery was classified as R0. The extension work-up revealed multiple bilateral pulmonary micronodules that were managed via active surveillance. In January 2016, the progression of these micronodules led to the introduction of the first-line systemic treatment with sunitinib at a dosage of 37.5 mg per day, 3 out of 4 weeks. Co-medications included on-demand inhaled fluticasone/salmeterol and salbutamol. The initial tolerance of the treatment was good, marked by grade 1 hand–foot syndrome, grade 1 nail toxicity (streaks), and grade 1 diarrhea. In May 2016, 4 months after the first dose of sunitinib, the patient experienced painful swelling in the surgical scar area. Magnetic resonance imaging (MRI) showed diffuse and marked edema of the anterior compartment of the thigh characterized by high signal intensity on T2-weighted images, without nodular lesions, circumscribing a central core, and without any bone signal abnormality. The increased visibility of the intermuscular fascia and the convergence of normal muscular fibers (black-hole effect), in contrast with the displacement seen in tumors, were other characteristics suggestive of myositis. Therefore, antiangiogenic treatment was discontinued, and the symptoms rapidly resolved within a few days. Three weeks after the discontinuation of sunitinib, the inflammatory findings completely disappeared. However, a stony-hard zone < 10 cm developed. Six weeks after the diagnosis, plain radiograph showed well-circumscribed ossifications in the soft parts of the external face of the left thigh, extending > 12 cm. This finding was compatible with a MOC diagnosis. In July 2016, 2 months after the diagnosis, computed tomography (CT) showed an extensive calcified mass > 12 cm in height in the anterior muscle compartment, which was continuous with the cortical bone through periosteal ossification in some places. Finally, on MRI, the mass was hypointense without enhancement with gadolinium on T2-weighted images, corresponding to a calcified mass. The continuation of favorable clinical outcomes was confirmed. Five months after the diagnosis of MOC, the patient remained asymptomatic. A plain radiograph showed global ossification of the mass corresponding to a typical MOC fully incorporated into the femoral cortical bone.