Cancer biology or molecular basis of cancer include

 Cancer metastasis

To explain how cancer tumour becomes spreading to surrounding structures or nearby organ (local spread) or

 Direct extension eg direct invasion of bladder from adenocarcinoma of the sigmoid colon

Blood or haematogenous spread eg bone metastasis from follicular cell carcinoma of the thyroid
Lymphatic spread eg axillary nodes from carcinoma of the breast  or by
 Transcoelomic spread eg ovary or implantation spread
 Spillage of tumour cells during surgery

Metastases arise from the spread of cancer cells from the primary site and the formation of new tumors in distant sites

 The metastatic process consists of a series of steps that need to be successfully completed

First, the primary cancer must develop access to the circulation through either the blood circulatory system or the lymphatic system

After the cancer cells are shed into the circulation. they must survive
Next, the circulating cells lodge in a new organ and extravasate into the new tissue
Next the cells need to initiate growth in the new tissue and eventually establish vascularization to sustain the new tumour
Overall metastasis is an inefficeient process although the initial steps of hemoatogenous metastasis are believed to be performed efficiently metastasis can sometimes arise several years after the treatment of primary tumours
 For example although most breast cancer recurrences occur within the first 10 years after the initial treatment breast cancer recurrences has been reported as late as 50 years after the original tumour this phenomenon is referred to as dormancy and it is remains one of the biggest challenges  in cancer biology
Persistence of solitary cancer cells in a secondary site such as liver or bone marrow is one possible contributor to dormancy when these small metastasis acquire the ability to be vascularized substantial tumour growth can be achieved at the metastasic site leading to clinical detection

Several types of tumors metastasize in an organ-specific pattern

One explanation for this is mechanical and is based on the different circulatory drainage patterns of the tumors
 When different tumor types and their preferred metastasis sites were compared, 66% of organ-specific metastases were explained on the basis of blood flow alone
 The other explanation for preferential metastasis is what is referred to as the seed and soil theory, the dependence of the seed (the cancer cell) on the soil (the secondary organ
 According to this theory, once cells have reached a secondary organ, their growth efficiency in that organ is based on the compatibility of the cancer cell's biology with its new microenvironment
For example, breast cancer cells may grow more efficiently in bone than in some other organs because of favorable molecular interactions that occur in the bone microenvironment
The ability of cancer cells to grow in  specific site likely depends on features inherent to the cancer cell, features inherent to the organ, and the interplay between the cancer cell and its microenvironment.
Many of the oncogenes such as HER2|neu, ras, and myc, are thought to potentiate not only malignant transfor­mation but also one or more of the steps required in the metastatic process

 Metastasis also may involve the loss of metastasis suppres­sor genes

 Metastasis suppressor genes can decrease metastatic potential when reintroduced into a metastatically competent cell line without altering primary tumor growth

Laboratory work,involving cancer cell lines that have been selected to have a higher metastatic potential have led to the realization that these more highly metastatic cells have a different gene expression profile than their less metastatic parental counter­ parts

This in turn has led to the currently held belief that the ability of a primary tumor to metastasize may be predictable by analysis of its gene expression profile

It has been shown that such a gene expression profile can be used to predict the probability of remaining free of distant metastasis

Notably, this hypothesis differs from the multistep tumorigenesis theory in that the ability to metastasize is considered an inherent quality of the tumor from the beginning
 It is assumed that metastasis develops not from a few rare cells in the primary tumour that develop the ability to metastasis but that all cells in tumour with such molecular signatures develop the ability to metastasize
Haematogenous metastasis
Comprise entry of tumour cells to the circulation by tumour cells squeeze through gaps between endothelial cells to enter circulation,in a manner similar to that employed by cells of the immune system in inflammation
Basement membrane of lymphatic do not contain collagen or laminin and so are easier for the tumour to invade , this is a common method of metastasis of carcinoma , cells may become trapped in the filtering lymph nodes draining the site of the primary tumour where they are either destroyed or form deposite and start to grow

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