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Anal cancer MRI

Anal cancer is a type of cancer that develops in the tissues of the anus, which is the opening at the end of the gastrointestinal tract through which stool exits the body. It typically begins in the outer lining of the anus.

Causes

The exact cause of anal cancer is not fully understood, but several risk factors increase the likelihood of developing this disease:

  • Human Papillomavirus (HPV): The most significant risk factor for anal cancer is infection with HPV, particularly HPV-16. This virus is also linked to other cancers like cervical, vaginal, and penile cancers.
  • Smoking: Tobacco use significantly increases the risk.
  • Immunosuppression: Individuals with weakened immune systems, such as those with HIV or those who have undergone organ transplantation, are at higher risk.
  • Age: Risk increases with age, particularly for individuals over 50.
  • Sexual behavior: Having multiple sexual partners or engaging in anal intercourse may increase the risk of HPV infection.
  • Chronic local inflammation: Conditions like anal fistulas or chronic anal fissures may contribute to the development of cancer.

Symptoms

Symptoms of anal cancer might not be apparent in the early stages. When symptoms do appear, they may include:

  • Bleeding from the anus or rectum
  • Pain or pressure in the anal area
  • Itching or discharge from the anus
  • A lump near the anus, which may be painful
  • Changes in bowel habits or the shape of the stool

Diagnosis

Anal cancer is diagnosed through several steps:

  • Physical Examination: Includes checking the anal region for abnormalities.
  • Anoscopy: A small tube called an anoscope is used to see inside the anal canal and rectum.
  • Biopsy: A small sample of tissue is taken from a suspicious area during anoscopy and examined under a microscope.
  • Imaging Tests: MRI, CT scans, and PET scans may be used to determine the extent of the cancer.

Treatment

Treatment options depend on the stage of the cancer, the specific characteristics of the cancer, and the overall health of the patient:

  • Surgery: Early-stage tumors may be treated with surgical removal. For more advanced cancer, more extensive surgery may be required.
  • Radiation Therapy: Often used in combination with chemotherapy, especially for cancers that have not spread beyond the anal area.
  • Chemotherapy: Used to shrink the tumor before surgery or radiation, or to treat cancer that has spread.
  • Targeted Therapy and Immunotherapy: For some patients, these newer treatments might be options depending on the genetic characteristics of the cancer.

Staging of Anal Cancer

The TNM staging system is a commonly used method to describe the extent of cancer spread in the body. “TNM” stands for Tumor, Node, and Metastasis:

  • T refers to the size and extent of the main tumor.
  • N describes the involvement of nearby lymph nodes.
  • M indicates whether the cancer has metastasized to other parts of the body.

T (Tumor) Stages

  • TX: The primary tumor cannot be assessed.
  • T0: No evidence of a primary tumor.
  • Tis: Carcinoma in situ, which is a pre-cancerous condition where the tumor cells are confined to the surface layer of the anal canal and have not invaded deeper tissues.
  • T1: The tumor is 2 centimeters (cm) or smaller in its greatest dimension.
  • T2: The tumor is larger than 2 cm but not larger than 5 cm.
  • T3: The tumor is larger than 5 cm.
  • T4: The tumor of any size invades adjacent organs, such as the vagina, urethra, or bladder.

N (Node) Stages

  • NX: Regional lymph nodes cannot be assessed.
  • N0: No regional lymph node involvement.
  • N1: The cancer has spread to regional lymph nodes. This is further subdivided into:
    • N1a: Cancer has spread to the inguinal, mesorectal, or internal iliac lymph nodes.
    • N1b: Cancer has spread to both the inguinal and internal iliac lymph nodes on one side.
    • N1c: Cancer has spread to the lymph nodes in the pelvis and the groin on both sides.

M (Metastasis) Stages

  • M0: No distant metastasis (the cancer has not spread to other parts of the body).
  • M1: Distant metastasis is present (the cancer has spread to distant parts of the body).

Overall Staging Based on TNM

Combining the T, N, and M categories gives the overall stage of anal cancer, ranging from Stage 0 to Stage IV.

Stage 0 (Carcinoma in situ)

  • Tis, N0, M0: Abnormal cells are found only in the innermost lining of the anus and have not spread to nearby tissues.

Stage I

  • T1, N0, M0: The tumor is 2 cm or smaller and has not spread to lymph nodes or distant sites.

Stage II

  • T2, N0, M0: The tumor is larger than 2 cm but not larger than 5 cm and has not spread to lymph nodes or distant sites.
  • T3, N0, M0: The tumor is larger than 5 cm and has not spread to lymph nodes or distant sites.

Stage IIIA

  • T1-T2, N1, M0: The tumor is 5 cm or smaller, with regional lymph node involvement, but no distant metastasis.
  • T4, N0, M0: The tumor has invaded nearby organs but has not spread to lymph nodes or distant sites.

Stage IIIB

  • T3, N1, M0: The tumor is larger than 5 cm, with regional lymph node involvement, but no distant metastasis.
  • T4, N1, M0: The tumor has invaded nearby organs and has regional lymph node involvement but no distant metastasis.

Stage IIIC

  • Any T, N1, M0: The tumor can be any size, with regional lymph node involvement but no distant metastasis.

Stage IV

  • Any T, Any N, M1: The cancer has spread to distant parts of the body, regardless of the size of the primary tumor or lymph node involvement.

MRI Appearance of Anal cancer

MRI T2 Appearance of Anal cancer

  • T2-weighted MRI images are valuable for assessing the anatomical detail and extent of anal cancer. On T2-weighted images, anal cancer generally appears as a heterogeneous, hyperintense mass relative to the surrounding musculature and normal soft tissues. This high signal intensity is due to the higher water content in the tumor. T2-weighted imaging is also useful for evaluating the involvement of adjacent structures, including the sphincter complex, perianal fat, and possibly the pelvic organs, aiding in staging and treatment planning.

MRI T1 Appearance of Anal cancer

  • Although T1-weighted imaging is not commonly used for the primary staging of anal cancer, it can still provide useful anatomical details. On T1-weighted MRI images, anal cancer usually appears as a mass with intermediate to low signal intensity compared to the high signal intensity of surrounding fatty tissues. The tumor can sometimes be challenging to distinguish on T1-weighted images alone due to the less conspicuous contrast between the lesion and surrounding structures. However, post-contrast T1-weighted images (following gadolinium administration) typically show enhancement of the tumor, which helps to better delineate the cancer from normal tissues and highlights its vascularity, providing additional diagnostic information.

MRI DWI (b0, b1000, ADC) of Anal cancer

  • In the context of anal cancer, Diffusion-Weighted Imaging (DWI) on MRI plays a crucial role in lesion characterization. At b0 (no diffusion weighting), the images resemble conventional T2-weighted images. With b1000, higher diffusion sensitivity, malignant lesions such as anal cancer typically show high signal intensity due to restricted diffusion of water molecules within the densely packed tumor cells. The Apparent Diffusion Coefficient (ADC) map, which quantifies diffusion, will show corresponding low ADC values, indicating restricted diffusion, which helps in differentiating malignant from benign tissues

T2 axial oblique image shows Anal cancer at T2 stage

T2 axial image shows image shows of Anal cancer T2 stage

T2 TSE sagittal image shows Anal cancer at T2 stage

T2 TSE sagittal image shows of Anal cancer T2 stage

T2 coronal oblique image shows Anal cancer at T2 stage

T2 coronal oblique image shows image shows of Anal cancer T2 stage

References

  • Tonolini, M., & Bianco, R. (2013). MRI and CT of anal carcinoma: a pictorial review. Insights Imaging, 4(1), 53-62. https://doi.org/10.1007/s13244-012-0199-3
  • Kochhar, R., Plumb, A. A., Carrington, B. M., & Saunders, M. (2012). Imaging of anal carcinoma. American Journal of Roentgenology, 199(3). https://doi.org/10.2214/AJR.11.8027
  • Horvat, N., Rocha, C. C. T., Oliveira, B. C., Petkovska, I., & Gollub, M. J. (2019). MRI of Rectal Cancer: Tumor Staging, Imaging Techniques, and Management. RadioGraphics, 39(2). 
  • Jhaveri, K. S., & Hosseini-Nik, H. (2015). MRI of rectal cancer: An overview and update on recent advances. American Journal of Roentgenology, 205(1). https://doi.org/10.2214/AJR.14.14201
  • Gondal, T. A., Chaudhary, N., Bajwa, H., Rauf, A., Le, D., & Ahmed, S. (2023). Anal Cancer: The Past, Present, and Future. Current Oncology, 30(3), 3232–3250. https://doi.org/10.3390/curroncol30030246