Phenotypic and Genotypic Appraisal of Pulmonary Mucinous Adenocarcinoma: A Retrospective Study

Abstract

Introduction: Lung cancer is the second most common cancer diagnosed in the world. According to GLOBOCAN 2020, it is approximately 11.4% of all malignancies and accounts for one in five cancer deaths worldwide. Several driver gene mutations are known in lung adenocarcinoma including Epidermal Growth Factor Receptor (EGFR), Kirsten rat sarcoma viral oncogene homolog(KRAS), v-raf murine sarcoma viral oncogene homolog B1(BRAF), Human epidermal growth factor receptor2 (HER 2), anaplastic lymphoma kinase (ALK), Ros Protooncogene 1, Receptor Tyrosine Kinase (ROS1) and rearranged during transfection (RET). Among these, KRAS mutation has been classically defined as bearing a negative prognostic factor. They are described to have unfavourable survival rates as compared with KRAS wild- type tumours. However, its real significance remains controversial due to heterogeneity amongst studies. Aim: The aim of this study was to assess the histomorphology, immunophenotype, EGFR and KRAS mutation profile in invasive mucinous and mixed mucinous/non mucinous type lung carcinoma. Materials and Methods: This retrospective study was done at a tertiary cancer centre in the state of Kerala, South India to identify the frequency of EGFR and KRAS mutations in invasive mucinous or mixed mucinous/non mucinous type lung carcinoma diagnosed during the period June 2017 to March 2019. Immunohistochemical assays for CK7, CK20, TTF-1, Napsin-A, CDX-2 and p63 were done on all cases. EGFR mutation analysis for exons 18, 19, 20, 21 were done using ARMS PCR and KRAS mutation analysis for codons 12, 13, 61, 146 by Sanger sequencer. Descriptive statistics was used wherever applicable. Results: Out of a total of 290 patients of pulmonary adenocarcinoma during the two year study period, twelve cases (4.1%) were diagnosed as invasive mucinous or mixed mucinous/non mucinous types based on histomorphology and immunophenotype, on both lung tissue biopsies as well as material from metastatic sites. Of the twelve, nine (75%) were pure mucinous and three (25%) were mixed mucinous and non mucinous. All the twelve cases were subjected to molecular testing for both EGFR and KRAS. Three cases (25%) showed KRAS mutations in codon 12. EGFR mutation (TKI resistant T790M mutation) was found only in a single case which had a histology of mixed mucinous and non mucinous carcinoma and a coexistent mutation in KRAS (c12 G>C) gene. Conclusions: Our study corroborates the described fact that pulmonary mucinous carcinoma differ from the conventional adenocarcinoma in terms of its histology, immunoprofile and molecular profile. KRAS mutation appears to be the main pathogenetic factor in this variant. In the current day practice, there is no demonstrable effective therapy for KRAS mutant lung adenocarcinoma and more trials are needed to identify newer mutations that can modify treatment outcomes.