This is best seen on T2-weighted scans as an intermediate signal intensity mass or thickening of the vulval skin. The relationship of the tumour to the clitoris, urethra, vagina and anus are important prognostically, and if deep invasion is suspected, T2-weighted images in the sagittal or coronal planes can be helpful to assess the cranial extent of tumour. Invasion is indicated by replacement of the relatively hypointense muscular coats of the urethra, vagina or anorectum by intermediate signal intensity tumour, contiguous with the vulval primary tumour. The axial T2-weighted sequence is also helpful to assess the other genital tract organs, since occasionally, a vulvo-vaginal tumour is metastatic from endometrial or ovarian carcinoma.
Lymph nodes can be assessed on all sequences, the T1 -weighted sequences providing morphological information (size, shape, margin, presence of a fatty hilum) and the T2-weighted sequences a degree of tissue characterisation (nodal signal intensity, presence of cystic areas) which together can improve the discrimination between reactive and metastatic nodes over that achieved using lymph node diameters alone. Inguinal lymph nodes having a long axis diameter >21 mm, short axis diameter>10mm, long: short axis ratio<1.3, irregularity of contour (including clumping of nodes), or cystic changes are abnormal. Contour irregularity and cystic change are the most reliable predictive signs of metastasis, indicating respectively extranodal spread and squamous cell carcinoma deposits.
Assessment of the vulva for residual tumour is difficult in the early post-operative period due to inflammatory changes and anatomical distortion. Reactive changes may cause false-positive assessment of inguinal lymph nodes if imaging is carried out soon after significant vulval resection. Recurrent tumour usually arises in the residual vulval tissue, or in the inguinofemoral lymph nodes because deep inguinal and femoral nodes frequently remain after inguinal lymph node resection.
Pitfalls of MRI
Staging the primary vulval tumour
The superficial extent of vulval tumour is assessed clinically; MRI has advantages in identifying deep tumour extension.
• Stage I carcinoma may be too small to visualise on MRI.
• Larger stage I or II tumours that are en plaque may be difficult to identify, or distinguish from chronic inflammatory changes on the vulva.
• Superficial invasion can be difficult to exclude in tumours confined to the labia, but lying adjacent to the urethral orifice, introitus or anal margin.
• In obese patients, with a large fatty apron, surface coils may be a significant distance from the inguinal lymph node chains, reducing signal: noise ratio.
• In obese patients, it may be difficult to judge the depth of lymph nodes from the surface, requiring careful positioning of the
FOV to ensure complete coverage of superficial and deep inguinal lymph nodes.
• Hip replacements or other orthopaedic hardware in the pelvic region, may interfere with signal: noise in the inguinal regions, reducing image quality.
• The field of view should be centred over the symphysis pubis and should extend to the lateral end of the inguinal ligament, to cover the lateral group of superficial inguinal nodes.
• Lymph node enlargement can be secondary to inflammatory changes in the perineum or lower limb.
• Microscopic metastases are not visible by MRI.
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