Extracting Quality-of-Life Information of Patients Diagnosed With Breast Cancer From Health Care Online Forum Posts Using Open-Source Large Language Models: Algorithm Development and Evaluation Study.

Background: Quality-of-life (QoL) questionnaires are an established instrument designed to assess overall well-being and QoL of patients. They are important in predicting the outcome of the disease and understanding the needs of individual patients. However, their repeated collection imposes a substantial burden on both patients and clinical professionals. Many patients seek emotional support and mutual exchange in online communities for peer support, where they frequently share detailed descriptions of symptoms and treatment experiences, addressing topics covered in QoL questionnaires. The emergence of large language models (LLMs) uncovers potential for automatic extraction of relevant QoL information from patient-generated text. Objective: The aim of this study is to evaluate and compare various open-source LLMs and optimization approaches for automated extraction of QoL information from forum posts. Methods: The dataset consisted of 840 English-language posts from patients with breast cancer recruited on Inspire online communities, manually annotated with sentence-level text spans indicating whether and where posts contained information relevant to 53 QoL questions from standardized questionnaires. Eleven open-source LLMs were evaluated in a zero-shot setup under 2 input conditions: post-only and post with additional context. For the GPT-OSS-20B model, additional experiments assessed the impact of chain-of-thought prompting, instruction optimization, few-shot prompting, simultaneous all-questions prompting, and parameter-efficient fine-tuning. For correctly classified yes and no instances, the overlap between model-generated evidence and human-annotated spans was evaluated. Results: Across 11 evaluated LLMs, Qwen3-14B achieved the highest macro Conclusions: Automated extraction of QoL information from patient-generated text using open-source LLMs remains a challenging task. While prompt optimization techniques failed to improve baseline zero-shot performance, parameter-efficient fine-tuning with LoRA significantly increased accuracy. However, current models still struggle to reliably detect explicit symptom expressions in heavily imbalanced data, too often predicting the majority class "not in the text." Before such tools can be successfully integrated into clinical practice, future research must prioritize strategies to capture these minority-class signals.