Topic Modeling and Semantic Similarity-Based Evaluation of Biomedical Text Abstracts Generated by LLM

This study investigates how large language models (LLMs)—ChatGPT, Gemini, and DeepSeek—preserve thematic consistency in biomedical text summarization. A dataset of 56,000 cardiovascular texts was constructed, including 8,000 original PubMed abstracts and 48,000 LLM-generated summaries. After domain-specific preprocessing, topic modeling was performed using Latent Dirichlet Allocation (LDA), Latent Semantic Analysis (LSA), and Non-negative Matrix Factorization (NMF) across topic sizes (k = 10–50). Thematic consistency was evaluated using C_v coherence scores. In addition, semantic similarity between original and generated texts was assessed using SBERT-based cosine similarity to capture meaning preservation beyond lexical overlap. The results indicate that abstractive summaries, particularly those generated by Gemini and ChatGPT, achieve coherence levels comparable to or higher than original abstracts under LDA and LSA, while extractive summaries exhibit greater variability. For NMF, original abstracts show more stable trends, whereas ChatGPT’s extractive summaries remain competitive at higher topic sizes. Overall, coherence tends to decrease with increasing topic numbers in LDA, whereas LSA and NMF benefit from larger topic sizes. These findings highlight the importance of combining coherence and semantic similarity for a more comprehensive evaluation of LLM-based biomedical summarization.