The pathogenesis of intracranial growing teratoma syndrome: proliferation of tumor cells or formation of multiple expanding cysts? Two case reports and review of the literature

Growing teratoma syndrome (GTS) is a rare clinical phenomenon defined as the paradoxical growth of mature teratoma components during or after chemotherapy. The mechanism of this phenomenon is not well understood. We present two cases of pineal mixed germ cell tumors that exhibited the similar course to GTS and speculate its pathogenesis. The first case was accompanied by slightly elevated alpha-fetoprotein (8.8 ng/ml; normal < 6.6 ng/ml). The tumor rapidly grew from 1.5 to 2.7 cm in diameter within 4 weeks. Despite this rapid preoperative growth, thorough pathological investigation found only mature teratoma components along with multiple micro- and macro-cysts. The other case was diagnosed as a pure germinoma based on biopsy and serological examinations. During three courses of chemotherapy, this tumor presented a honeycomb-like appearance on magnetic resonance (MR) images and an exceptionally rapid enlargement. Second-look surgery confirmed the histological diagnosis of a mature teratoma. In both cases, meticulous pathological examination of all whole tumor sections revealed no malignant histological features, and the MIB-1 labeling indices were too low to account for the extremely rapid tumor growth. Instead, both MR images and histological findings demonstrated a predominant formation of multiple cysts. We speculate that this paradoxical growth might not be tumorous proliferation but instead the formation and expansion of multiple cysts inside mature teratoma components and that the presence or absence of growth might be a subsidiary phenomenon. Our hypothesis appears consistent with the characteristic radiological findings of GTS reported in the literature.