Spontaneous regression of colorectal liver metastasis with coexisting IgG4-rich inflammatory pseudotumor-like changes: a case report.

Spontaneous regression of colorectal liver metastasis (CRLM) is an extremely rare phenomenon with only a few reported cases. However, the underlying mechanism remains unclear. Infection-triggered immune reactivation and inflammation have also been proposed to be potential contributors. IgG4-related disease (IgG4-RD) is a systemic fibroinflammatory condition characterized by IgG4-positive plasma cell infiltration and fibrosis, and rarely manifests in the liver as an inflammatory pseudotumor (IPT). We herein report a rare case of a 71-year-old man with possible spontaneous regression of CRLM accompanied by coexisting IgG4-rich IPT-like changes. Eight months after the laparoscopic Hartmann's operation for rectal cancer, imaging revealed hepatic lesions in segments S5 and S7, which were diagnosed as CRLM. Partial hepatectomy revealed necrotic adenocarcinoma in the S7 lesion, with IgG4-rich IPT-like inflammatory changes. Postoperative evaluation revealed markedly elevated serum IgG4 levels and IgG4-positive plasma cell infiltration in the mediastinal lymph nodes, fulfilling the diagnostic criteria for systemic IgG4-RD. Twenty months postoperatively, the patient remained recurrence-free without adjuvant chemotherapy or steroid therapy. This case report describes the coexistence of spontaneous tumor regression and IgG4-related hepatic IPT within the same lesion and provides insight into the clinicopathological spectrum linking tumor regression and IgG4-RD.