What are the contouring guidelines for carcinoma (ca) of the cervix?

Contouring Guidelines for Cervical Cancer

For cervical cancer radiation therapy, contour the Clinical Target Volume (CTV) to include the gross tumor volume, entire uterine cervix, uterine corpus, parametria, upper third of vagina, and uterosacral ligaments for the primary target, plus common iliac, external iliac, internal iliac, presacral, and obturator lymph node regions for nodal CTV, using MRI as the reference imaging modality when available. 1

Primary Target Volume (CTV Primary) Components

The CTV primary must encompass all structures at risk for microscopic disease spread:

• Gross Tumor Volume (GTV): The visible tumor on imaging, best delineated on T2-weighted MRI obtained at diagnosis and immediately before brachytherapy 2
• Entire uterine cervix: Include the full cervical anatomy regardless of visible tumor extent 1
• Uterine corpus: The entire uterine body should be included in the target volume 3, 1
• Parametria: Bilateral parametrial tissues extending to the pelvic sidewall 1
• Upper third of vagina: Approximately 3-4 cm of the vaginal cuff 3, 1
• Uterosacral ligaments: These structures are at high risk for microscopic involvement 1

Nodal Target Volume (CTV Nodal) Components

The nodal CTV encompasses all pelvic lymph node regions at risk:

• Common iliac nodes: Extend superiorly to the L4-L5 interspace or bifurcation of the aorta 1
• External iliac nodes: Follow the external iliac vessels from the common iliac bifurcation to the femoral canal 1
• Internal iliac nodes: Include the full extent along internal iliac vessels 1
• Obturator nodes: The obturator fossa and nodes surrounding the obturator vessels 1
• Presacral nodes: Nodes anterior to the sacrum 1
• Para-aortic nodes: Consider extending the superior border when documented nodal metastasis is present 3

Imaging Recommendations for Contouring

MRI is the reference standard for target delineation, providing superior soft tissue contrast compared to CT for tumor extension assessment. 3

• T2-weighted MRI: Obtain at diagnosis and immediately before brachytherapy to accurately estimate tumor size and topography 2
• MRI superiority: MRI is superior to CT for assessing tumor extension and equal to CT for nodal involvement 3
• CT-based contouring: When MRI is unavailable, use diagnostic MRI as reference while contouring on planning CT, incorporating clinical examination findings from both diagnosis and time of treatment 2
• PET/CT role: Consider for detecting lymph node involvement with sensitivity of 53-73% and specificity of 90-97% in early-stage disease 3

Brachytherapy High-Risk CTV (HR-CTV) Guidelines

For image-guided adaptive brachytherapy, the HR-CTV demonstrates the lowest delineation uncertainty and is the most robust volume for dose prescription:

• HR-CTV definition: The HR-CTV showed significantly lower inter-delineation distances (3.6±3.5 mm) compared to GTV (4.8±4.2 mm) and intermediate-risk CTV (4.7±5.2 mm) 4
• Greatest variation: Delineation uncertainty is most prominent in the cranio-caudal direction and posterolaterally 4, 5
• MRI-guided approach: MRI allows superior delineation of HR-CTV compared to CT-based approaches, translating to improved treatment outcomes 6
• Dose prescription: Use HR-CTV as the primary volume for dose optimization in brachytherapy planning 4

Stage-Specific Contouring Considerations

Adjust target volumes based on FIGO stage and extent of disease:

• Early stage (IA1-IB1): Imaging is optional for stage IB1 or smaller tumors, though MRI helps rule out disease high in the endocervix 3
• Locally advanced (IIB-IIIB): Ensure parametrial extension is fully encompassed; for stage IIIB, include regions extending to pelvic sidewall 3
• Nodal involvement: When documented nodal metastasis exists, extend the superior border appropriately and consider boosting grossly involved unresected nodes with additional 10-15 Gy of highly conformal EBRT 3

Dose Specifications and Normal Tissue Constraints

Target dose delivery must balance tumor coverage with normal tissue sparing:

• Total dose: Aim for 80 Gy for small volume tumors to ≥85 Gy for larger volume tumors 3
• Normal tissue protection: With higher doses, especially EBRT, severely limit the volume of normal tissue in high-dose regions 3
• Posthysterectomy: Cover upper 3-4 cm of vaginal cuff, parametria, and adjacent nodal basins with 45-50 Gy in standard fractionation 3

Critical Pitfalls to Avoid

• Inadequate cranio-caudal coverage: This is the most common area of delineation variation; ensure complete coverage of the uterine corpus superiorly and adequate vaginal margin inferiorly 4, 5
• Underestimating parametrial extent: In stage IIB and beyond, parametrial involvement extends toward the pelvic sidewall and must be fully encompassed 3
• Ignoring uterine motion: The uterus demonstrates significant inter-fractional motion; MRI-guided adaptive approaches allow daily target delineation to account for this 6
• Relying solely on CT: CT significantly underestimates tumor extent compared to MRI; always reference diagnostic MRI when available 3, 2
• Inadequate nodal coverage: When using traditional bony landmarks, ensure all at-risk nodal regions are covered, particularly the internal iliac and presacral nodes which are frequently undertreated 1