Why Is Cancer So Hard to Treat? Discussing Cancer Metastasis and Early Detection from the "Seed-Blood Dissemination" Mechanism
CTC testing reveals the mechanisms of cancer metastasis, providing Hong Kong patients with new possibilities for early detection, recurrence monitoring, and multi-faceted intervention.
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The core reason cancer is so difficult to completely cure lies in metastasis. Even if the primary tumor is surgically removed or shrunk, residual cancer cells that disseminate outward and take root in distant organs can trigger new tumors, becoming the leading cause of death. From the patient’s perspective, we’ll use straightforward analogies and scientific mechanisms here to explain why this is so challenging, as well as the possibilities and limitations of detecting circulating tumor cells (CTCs) in the blood.
From Local Tumor to Systemic Disease: The Seed and Soil Theory
What sets cancer cells apart from benign growths is their invasiveness and migratory potential. When a tumor reaches a certain size (around 2 mm), its central area—due to insufficient vascular density, oxygen, and nutrients—triggers angiogenesis to boost blood supply. These new blood vessels act as “pipelines,” allowing cancer cells to enter the vascular system. Blood vessels serve like highways, providing escape routes for cancer cells from the primary tumor.
Once cancer cells enter the bloodstream or lymphatic system, they become “seeds” drifting around, seeking suitable “soil” (distant tissue environments) to regrow. This is the application of the seed and soil hypothesis in metastasis mechanisms. Experimental studies indicate that a 1 mm tumor may release millions of cancer cells into the blood daily; although most die, just a tiny fraction surviving and successfully implanting is enough to form metastatic tumors.
Biological Challenges and Survival Bottlenecks of Circulating Tumor Cells (CTCs)
The cancer cells drifting in the blood—known as CTCs—are key intermediaries in the metastatic process. However, after entering the bloodstream, CTCs must overcome immense hurdles to successfully form new tumors. The vast majority ultimately succumb to physical shear stress, oxidative stress, immune clearance, or apoptosis due to loss of basement membrane attachment.
Yet, some CTCs exhibit high heterogeneity, enabling them to withstand blood flow impacts, evade immunity, and even interact with blood components for protection. These specially adapted CTCs become potential metastatic seeds.
Platelets + CTCs: Evading Immunity and Forming a Shield
Platelets play a crucial role in CTC survival. When cancer cells enter the blood, platelets quickly adhere, forming a micro-barrier. This helps CTCs shield themselves from immune recognition, resist shear forces in the blood flow, and even release growth factors to enhance cancer cell invasiveness.
Moreover, platelets promote CTC adhesion to vessel walls and extravasation, making it easier for them to reach distant organs. This “platelet + CTC” alliance significantly boosts metastasis success rates.
CTC Clusters Are More Dangerous Than Single Cells
Beyond solitary CTCs, research shows that CTC clusters pose an even greater metastatic threat. Cells within clusters support each other, exhibiting stronger anti-apoptotic abilities and collective resistance to blood flow pressure. Larger clusters are more likely to arrest on vessel walls and extravasate, and they’re harder for the immune system to clear.
Thus, detecting CTC clusters often signals higher metastasis risk for patients and is clinically viewed as a poor prognostic indicator.
Clinical Detection of CTCs: Opportunities and Challenges
Liquid biopsy technology offers a chance to hunt for CTCs in the blood. For patients, it’s safer than traditional biopsies, allows repeated testing, and can be used for early recurrence detection, treatment response tracking, genetic mutation analysis, and precision medicine support.
Challenges persist, however. First, CTCs are extremely rare in blood, making isolation and purification difficult. Second, high heterogeneity among cancer cells means some may lack detection markers and go unnoticed. Additionally, even captured CTCs require breakthroughs to validate clinical significance, culture them, and test drug responses.
Countering CTCs and Metastasis with Multi-Angle Strategies?
For patients and clinical teams, suppressing cancer metastasis can’t rely on a single approach but requires multifaceted interventions.
First, inhibiting angiogenesis can reduce cancer cell entry into the blood. Second, moderate use of antiplatelet or anticoagulant drugs may weaken CTC protection. Third, immunotherapy can enhance immune surveillance of CTCs. Fourth, disrupting CTC cluster structures to break them into single cells could lower survival rates.
Finally, combining liquid biopsy for ongoing patient monitoring to establish dynamic risk alerts allows intervention before metastatic disasters strike. Such a “multi-angle, continuous monitoring” strategy truly meets patient needs.
Summary: The Patient's Viewpoint and Our Challenges
From the patient’s perspective, understanding why cancer is hard to treat isn’t just about accepting reality—it’s about grasping why doctors and scientists must attack from multiple angles. CTCs are the bridge in cancer metastasis; though most die, a minuscule few succeeding can have massive clinical impact.
Today’s science already allows us to capture and analyze these cells, and the future holds promise for blocking their dissemination chain. Early detection, precise intervention, and dynamic monitoring will be the shared direction for patients and healthcare teams.
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References
- Dai, X., et al. (2025). Circulating tumor cells: Blood-based detection, molecular biology, and clinical applications. Cancer Cell.
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- Jiang, et al. (2024). Circulating tumor cells: Origin, role, current applications, and future. Biomedicines, 12(9), 2137.
- Nasr, M. M., & Lynch, C. C. (2023). How circulating tumor cluster biology contributes to the metastatic cascade: from invasion to dissemination and dormancy. Cancer and Metastasis Reviews.
- Pereira-Veiga, T., et al. (2022). Circulating tumor cell–blood cell crosstalk: Biology and clinical consequences. Cell Reports.
- Yang, C., Liu, C., Xia, C., & Fu, L. (2025). Clinical applications of circulating tumor cells in metastasis and therapy. Journal of Hematology & Oncology, 18, 80.