Introduction

Cancer’s hallmark is metastasis, which is responsible for the majority of cancer-related deaths. However, it is still misunderstood. Some of the molecular underpinnings of this dispersion process have been uncovered thanks to the ongoing evolution of cancer biology research and the creation of new paradigms in the study of metastasis.
Metastasis involves the spread of cancer cells from the primary tumor to surrounding tissues and to distant organs and is the primary cause of cancer morbidity and mortality. Metastasis can be described as the process by which cancer cells migrate throughout the body.
Definition according to cancer.gov Metastasis is a process where cancer cells break away from where they first formed (primary cancer), travel through the blood or lymph system, and from new tumors(metastatic tumors) in other parts of the body. In patients with cancer, large numbers of cancer cells are released in circulation daily; however, melanoma studies in animal models suggest that <0.1% of tumor cells metastasize. The development of metastases requires cancer cells to leave their primary site, circulate in the bloodstream, endure pressure in blood vessels, acclimate to new cellular surroundings in a secondary site, and escape deadly combat with immune cells

How metastasis develop and where

Any type of cancer can spread, but it depends on a variety of circumstances. For example,
Some malignancies have a higher proclivity for spreading than others. This is also dependent on how quickly the cancer is spreading. The behavior of the malignancy could potentially be a result of the tumour’s spread.

There is no specific area where metastasis occurs; it can occur everywhere on the body; nonetheless, certain types of cancer are more prone to spread in specific areas than others. Breast cancer, for example, has a proclivity for spreading to the bones, liver, chest wall, and brain. Lung cancer tends to travel to the brain, bones, liver, and adrenal glands, while prostate cancer tends to spread to the bones.

Common sites where metastasis occur with their cancer type is given below:

S.No.	Cancer type	Main sites of Metastasis
1	Lung	Liver, other lung, brain adrenal gland, bone
2	Ovary	Lung, liver, peritoneum
3	Bladder	Bone, lung, liver
4	Kidney	Adrenal gland, bone, liver, brain, lung,
5	Breast	Bone, brain, liver, lung
6	Pancreas	Lung, liver, peritoneum
7	Stomach	Lier, lung, peritoneum
8	Prostate	Bone, liver, lung, adrenal gland
9	Uterus	Bone, lung, peritoneum, vagina, lung
10	Thyroid	Bone, lung, liver

How can metastatic cancer occur?

Cancerous cells spread through the body in a series of steps they are:

• growing  into, or invading, nearby normal tissue
• Moving through the walls of nearby lymph nodes or blood vessels
• Traveling through the lymphatic system and bloodstream to other parts of the body
• Stopping in small blood vessels at a distant location, invading the blood vessel walls, and moving into the surrounding tissue.
• Growing in this tissue until a tiny tumor forms
• Causing new blood vessels to grow, which creates a blood supply that allows the metastatic tumor to continue growing

References

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7067809/ 
2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3597235/
3. https://www.cancerquest.org/cancer-biology/metastasis  
4. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0261527 
5. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/metastasis 
6. Multistep nature of metastatic inefficiency: dormancy of solitary cells after successful extravasation and limited survival of early micrometastases.
7. https://www.nature.com/articles/s41698-019-0092-3 
8. Luzzi KJ, MacDonald IC, Schmidt EE, Kerkvliet N, Morris VL, Chambers AF, Groom AC
9. Am J Pathol. 1998 Sep; 153(3):865-73
10. https://pubmed.ncbi.nlm.nih.gov/30850740/  
11. https://www.cancer.gov/types/metastatic-cancer
12. https://www.cancer.net/navigating-cancer-care/cancer-basics/what-metastasis