Google open-sources MedGemma 1.5, a 4B-parameter medical AI model. It processes 3D CT/MRI scans, whole-slide pathology, and chest X-rays with precise annotations, and extracts data from medical records. The model shows major accuracy gains over its predecessor, including an 11% boost in 3D MRI classification and a 47% rise in pathology analysis F1 score, while maintaining parameter efficiency. It is a foundational model for developer fine-tuning, not a direct clinical tool.