Molecular Profiling of Thymic Malignancies

Thymic malignancies are rare intra-thoracic epithelial tumors that may be aggressive and difficult to treat when in an advanced stage (Girard, 2009). The current histo-pathologic classification distinguishes thymomas (types A, AB, B1, B2, B3) and thymic carcinoma (WHO, 2004) based upon the morphology of epithelial cells (with an increasing degree of atypia from type A to thymic carcinoma), the relative proportion of the non-tumoral lymphocytic component (decreasing from types B1 to B3), and resemblance to normal thymic architecture (WHO, 2004). Tumor invasiveness as evaluated by the Masaoka staging system is a major prognostic indicator (Masaoka, 1981).

The most significant prognostic factor in thymic tumors is whether or not the disease may undergo complete resection (Girard, 2009; Kondo, 2003). Surgery is the mainstay of treatment. After surgery, thymomas have a tendency towards local and regional recurrence. By contrast, thymic carcinomas are highly aggressive tumors with frequent systemic involvement at time of diagnosis and poor prognosis despite multimodal treatment including surgery, radiotherapy and chemotherapy (Masaoka, 1981; Kondo, 2003). Novel strategies are needed.

Over the past years, significant efforts have been conducted to dissect the molecular pathways involved in the carcinogenesis of thymic malignancies (Girard, 2009). Research is hampered by the rarity of the tumor, evolution of histo-pathologic concepts, and a lack of established cell lines and animal models. Insights in the biology of thymic tumors have also been made following anecdotal clinical responses to targeted therapies.

The most relevant molecular alterations for clinical practice are KIT activating mutations in thymic carcinomas, which have been found in about 9% of cases. EGFR and RAS mutations have also been identified in thymoma and thymic carcinoma but are much rarer and of unknown therapeutic significance in this setting.

Author: Nicolas Girard, M.D.

Last updated: February 17, 2012

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