Grades and types of bowel cancer

Bowel cancer can start in the large bowel (colon) or the back passage (rectum). It is also known as colorectal cancer.

Bowel cancer is divided into different types depending on where it starts in the bowel, and the type of cell that it starts in. Knowing this helps your doctor decide which treatment you need. 

The grade means how abnormal the cells look under the microscope.

Grades

The grade of a cancer tells you how much the cancer cells look like normal cells. This gives your doctor an idea of how your cancer might behave and what treatment you need.

The grades of bowel cancer cells are from 1 to 4:

Low grade – slow growing

•   grade 1-  well differentiated, the cells look most like normal cells
•   grade 2 – moderately differentiated – the cells look a bit like normal cells

High grade – fast growing

•   grade 3 – poorly differentiated, the cells look very abnormal
•   grade 4 – undifferentiated, the cells look completely different from normal cells

The bowel

The bowel makes up a part of the digestive system. It’s split into the small and large bowel:

• cancer of the small bowel is called small bowel cancer

• cancer of the large bowel is called colon cancer

• cancer of the back passage is called rectal cancer

• cancer of the bowel opening is called anal cancer

This section is about colon and rectal cancer. We have separate sections about anal cancer and small bowel cancer.

Types of bowel cancer

Your type of cancer depends on what type of cell it starts in.

Adenocarcinoma 

Adenocarcinomas start in the gland cells in the lining of the bowel wall. The gland cells normally produce mucus. This is a slimy substance that makes it easier for the poo (also called faeces or stool) to pass through the bowel. Adenocarcinoma is the most common type of bowel cancer. 

There are 2 rare types of adenocarcinoma:

• mucinous tumours 
• signet ring tumours 

They are treated in the same way as the most common types of adenocarcinoma of the bowel.

Rare types of bowel cancer

Squamous cell tumours 

Squamous cells are the skin cells that make up the bowel lining, together with the gland cells. They are usually treated in the same way as cancer of the anus.

Neuroendocrine cancers

Large bowel and rectal neuroendocrine cancers are rare cancers. They start in the neuroendocrine cells of the large bowel (colon) or back passage (rectum). 

Neuroendocrine cells are part of the neuroendocrine system .

The umbrella term for this group of cancers is ‘neuroendocrine cancer’. Your healthcare team might call them neuroendocrine neoplasms (NENs). But they mean the same thing. There are 2 key groups of large bowel and rectal NENs:

• large bowel and rectal neuroendocrine tumours (NETs)
• large bowel and rectal neuroendocrine carcinomas (NECs)

NETs and NECs are very different. So, it is important to know which one you have. Talk to your doctor or specialist nurse if you are not sure.

Sarcomas 

Sarcomas are cancers of the supporting cells of the body, such as bone or muscle. Most sarcomas in the bowel are called leiomyosarcomas. This means they are sarcomas that started in smooth muscle.

A gastrointestinal stromal tumour (GIST) is a rare type of sarcoma found in the digestive system. GISTs can grow in any part of the digestive system, but the most common sites are the stomach and small bowel.

Sarcomas are treated differently to adenocarcinomas of the bowel. 

Lymphomas 

Lymphomas are cancers of the lymphatic system. They are treated very differently to other bowel cancers. 

Melanoma 

Melanoma is a type of skin cancer. Melanoma can begin in the back passage (rectum).