Liquid biopsy, which analyzes tumor secretomes in biological fluids, allows us to not only diagnose cancer, but also evaluate the effectiveness of antitumor therapy, predict the prognosis of the disease, and select targeted therapy. One of the promising sources for identifying tumor markers using liquid biopsy is exosomes—small extracellular vesicles (sEVs) (30–150 nm in size) that are secreted by all types of cells, including tumor cells, to exchange information. It is known that during the maturation process, mainly biologically active proteins and non-coding RNA are packaged into exosomes, and tumor cells secrete significantly more exosomes than normal cells. Taking into account the involvement of microRNAs in the mechanisms of carcinogenesis, their high stability in EVs, and ease of detection, exosomal microRNAs are the most promising tumor markers for creating panels that can serve as a guide both for clarifying diagnostics and for making therapeutic decisions on effective cancer treatment, including breast cancer (BC). The purpose of this review is to summarize information on the shortcomings of modern methods for diagnosing early BC, the involvement of exosomal microRNAs in BC dissemination (impact on the immune system, epithelial–mesenchymal transition, proliferation, invasion, migration, angiogenesis, and metastasis), and the high diagnostic potential of exosomal microRNAs for detecting early BC.

Background: Primary bone sarcomas (BS) are aggressive malignant tumors characterized by a high prevalence in children and adolescents and frequent lung metastasis, which significantly worsens the prognosis. Early diagnosis of recurrence and metastases after treatment is crucial for improving patient survival. Methods: This study examines exosomal microRNA as potential biomarkers for BS, which is particularly relevant in light of the development of liquid biopsy methods, which offer a noninvasive and dynamic approach to disease monitoring, early diagnosis, and evaluation of the effectiveness of anticancer therapy. Plasma exosomal microRNA levels in the plasma of patients with BS were analyzed before and after radical intraoperative thermal ablation in combination with perioperative chemotherapy using quantitative PCR. Results: The results showed that miR-92a levels were significantly different between clinically healthy donors and sarcoma patients, and after therapy, there was a tendency for the level of this microRNA to decrease in exosomes, while for miR-101, there was a tendency for the level to increase in exosomes after therapy. Conclusions: The observed relationship between microRNA levels and treatment efficacy suggests that they hold promise as noninvasive markers for the early diagnosis of relapses using liquid biopsy. Further studies involving larger patient cohorts are needed to validate the findings and determine the clinical significance of the proposed markers.