Spontaneous tumor regression was first reported by Cole and Everson in 1956  and defined as partial or complete involution of a malignant tumor in the absence of the application of a specific therapy . Spontaneous tumor regression is a rare phenomenon, with an incidence rate of 1 out of every 60,000–100,000 patients with cancer, most commonly seen in malignant melanoma, neuroblastoma, and cancer of the kidney .
Thymic carcinoma is a rare carcinoma of the thymus arising in the thymic epithelium, with an incidence of ~ 0.02 per 100,000 person-years [6, 7], and represent > 1% of thymic malignancies . Spontaneous tumor regression is reportedly more frequent in certain malignancies, such as renal cell carcinoma, neuroblastoma, and malignant melanoma , whereas it is extremely rare in thymic carcinoma, with only 1 reported case in the English literature .
Although it is difficult to determine the scientific and definitive mechanism of the spontaneous regression of cancer, immune mechanisms are considered to play a central role [10, 11] in the observed regression of renal cell carcinoma and malignant melanoma. Other possible mechanisms, which affect spontaneous regression, are as follows: tumor necrosis , growth factor, cytokine changes , apoptosis , psychological factors , genetic and epigenetic factors , and induction of benign differentiation . In this study, preoperative CT revealed an exophytic heterogeneously enhancing mass exhibiting some areas of necrosis. Some common features of thymic carcinomas include large and highly aggressive anterior mediastinal mass with rapid progression and areas of necrosis . In our case, the resected specimen mostly comprised necrotic components. The phenomenon of spontaneous regression may be related to rapid enlargement because of its highly aggressive characteristics, which could have caused a disorder of the vascular supply, thereby leading to necrosis. Furthermore, this might cause inflammation around the tumor, thereby causing the chest pain.
Surgical excision is the gold standard treatment for resectable thymic carcinoma [3, 4]. Clinicians should recognize the possibility of spontaneous regression among patients with thymic carcinomas; surgical resection is needed even if an anterior mediastinal tumor regresses spontaneously.