Atlas of Colposcopy: Principles and Practice

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Transformation zone  

The area of the cervix covered by the metaplastic epithelium is the transformation zone. The inner (proximal) boundary of the transformation zone is the SCJ, and the outer (distal) boundary is the junction between the original and the metaplastic squamous epithelium. The proximal boundary of the SCJ is a well-defined entity and is easily identified. The distal boundary cannot be identified so easily. There is no definite line to demarcate the junction between the original and the metaplastic squamous epithelium. The outer extent of the transformation zone can be identified approximately based on the detection of the crypt opening or nabothian cyst farthest away from the SCJ. It sometimes corresponds to the borderline between yellow or patchy and dark mahogany brown iodine uptake.

To identify the outer extent of the transformation zone, look for the crypt opening (or nabothian cyst) that is situated farthest away from the SCJ. Considering that point as the farthest extent of the transformation zone, draw an imaginary circle all around. This circle is the outer margin of the transformation zone (also known as the “original” SCJ).

The transformation zone may have any of the features of metaplasia: crypt openings, nabothian cysts, islands of columnar epithelium, fine mosaics or punctations, and acetowhite epithelium.

Mature metaplastic epithelium is often acetowhite and is often difficult to distinguish from low-grade premalignant lesions.

Acetowhite patches due to metaplasia are usually centripetal (going towards the external os) but can sometimes be centrifugal (spreading away from the os). The margin of the metaplastic white area is invariably very irregular and “fluffy”. Fine mosaic is common in metaplasia.

In mature squamous epithelium, none of the features of metaplasia are visible and the distal extent of the transformation zone cannot be delineated.

The transformation zone is the most crucial zone of the cervix to be assessed during colposcopy. Human papillomavirus (HPV) infections may induce malignant transformation of the cells over this area. As a result, cervical neoplasias begin in the transformation zone. Adequate evaluation of the zone is essential to identify neoplastic lesions. Delineation of the transformation zone is also important to treat precancerous lesions.

The upper limit (proximal extent) of the transformation zone (the SCJ) is the most crucial feature to identify. The lower limit (distal extent) is defined by the outer border of the lesion, if there is one, or of metaplasia (where the lesion or the metaplastic epithelium meets the mature squamous epithelium). The active transformation zone is where the process of metaplasia is continuing or where cervical intraepithelial neoplasia (CIN) has developed.

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