SR of malignant tumors is a rare phenomenon defined as the partial or complete disappearance of the tumor without any appropriate treatment [1]. SR of malignant tumors is commonly reported in neuroblastoma, renal cell cancer, lymphoma, and hepatic cancer [3, 4]. SR among thoracic malignancies is most frequent in primary lung cancer [4], although few cases of SR of thymic epithelial tumors were also reported [5, 6]. In terms of the histological type of lung cancer with SR, Iwanaga et al. reported that adenocarcinoma accounted for 45%, followed by squamous cell carcinoma (20%) and SCLC (20%) 4.

A literature review of SR in SCLC, including our case, is shown in Table 1 [7–16]. The median age was 70 years (ranging from 55 to 83 years), with three males and eight females. SR was pathologically proven only in this study. Of the nine patients whose prognostic information was available, three patients survived for more than 5 years, and notably two patients survived for more than 10 years. In general, the prognosis of patients with SCLC is poorer than that of patients with non-SCLC, with a median survival rate of 2–4 months when not treated [17, 18]. The 5-year survival rate of SCLC is reported to be below 7% [19]. Given that six patients in Table 1 had no treatment and only two underwent surgical resection, the prognosis of SCLC with SR cases might be better than that of SCLC without SR. Although the tumor has completely regressed, such as other histologic subtypes, a careful attention to metastasis or regrowth should still be given [20]. A long-term follow-up also is warranted.

SR of small cell carcinoma was reported in the esophagus, bronchus, and parotid gland [21–23]. In all cases, SR was observed after a diagnostic biopsy. The mechanism of SR is unclear, but factors that stimulate an immune response, including infection, trauma, surgery, and blood transfusions, may be involved [1]. In this case, active fibrosis, inflammation, and lymphocyte infiltration were observed around the regressed tumor. These findings implied that the SR in this case was caused by some immunological response after the CT-guided PTNB conducted 29 days before surgery. The lymphocytes around the tumor were CD4/8 positive T cells, which play an important role in cancer cytotoxicity. Previous reports have suggested that cytotoxicity by T cells may be associated with SR [12]. The cell-mediated immunity induced by PTNB may be a possible cause of SR in this case. We are unsure why this small residual tumor existed on this site; however, it can be speculated that the fibrotic lesion was associated with a decreased blood flow. Although some cases have radiologically confirmed partial or complete SR, this is the first and rare case that histologically verified SR, with inflammation and infiltration of lymphocytes around the tumor. Analyzing its underlying mechanisms may lead to more effective treatment for cancer.