What are the guidelines for para-aortic contouring in radiation therapy?

Para-Aortic Contouring Guidelines for Radiation Therapy

For testicular cancer, the NCCN guidelines recommend contouring the aorta and inferior vena cava with a 1.2-1.9 cm margin to create the clinical target volume (CTV), followed by a uniform 0.5 cm expansion to create the planning target volume (PTV), with superior border at T11 and inferior border at L5. 1

Testicular Cancer: Standard Para-Aortic Strip Fields

Cranio-Caudal Borders

• Superior border: Bottom of vertebral body T11 1, 2
• Inferior border: Inferior border of vertebral body L5 1

Lateral Border Definition: Two Approaches

Conventional Approach:

• Para-aortic strip fields approximately 10 cm wide, encompassing the tips of the transverse processes of the para-aortic vertebrae 1
• Small renal blocks may be added at T12 level for patients with medially positioned kidneys 1

3D-CRT Vascular-Based Approach (Preferred):

• Contour the aorta and inferior vena cava on non-contrast CT planning scan 1
• Apply 1.2-1.9 cm margin on these vascular structures to create the CTV, which includes para-aortic, paracaval, interaortocaval, and preaortic nodes 1, 2
• Expand CTV uniformly by 0.5 cm in all directions to create the PTV, accounting for setup errors 1, 2
• Add 0.7 cm margin from PTV to block edge to account for beam penumbra 1

Renal Dose Constraints

• Two kidneys: Right and left kidney D50% should be ≤8 Gy (no more than 50% of each kidney receives ≥8 Gy) 1
• Single kidney: Kidney D15% should be ≤20 Gy (no more than 15% of kidney volume receives ≥20 Gy) 1

Cervical Cancer: Extended-Field Para-Aortic Contouring

Indications and Superior Extent

• Extended-field radiation indicated for documented common iliac and/or para-aortic nodal involvement 1
• Superior border: At least to the level of renal vessels, or more cephalad as directed by involved nodal distribution 1
• For microscopic disease, contouring should extend up to renal vessels rather than fixed bony landmarks 3
• Consider retrocrural lymph nodes if nodal involvement extends to renal vessels (38% of such patients have retrocrural involvement) 3

Lateral Margins: Asymmetric Expansion

Based on validated cervical cancer data, the recommended CTV expansion is: 4

• From aorta: 10 mm circumferentially, except 15 mm laterally 4
• From IVC: 8 mm anteromedially and 6 mm posterolaterally 4
• This approach covers 97% of pathological para-aortic nodes in validation studies 4

Anatomic Distribution Considerations

• Left lateral para-aortic region: Contains 59% of positive nodes 3
• Aortocaval region: Contains 35% of positive nodes 3
• Right paracaval region: Contains only 8% of positive nodes 3
• Nearly all patients with right paracaval involvement also have left para-aortic involvement 3
• Critical pitfall: Fixed circumferential margins around vessels are inadequate; asymmetric expansion is necessary 3, 4

Dose Recommendations

• Microscopic disease: 45 Gy in conventional fractionation 1
• Gross unresected adenopathy: Additional 10-15 Gy highly conformal boost may be considered 1
• Treatment must not exceed bowel, spinal cord, or renal tolerances 1

General Technical Principles

Treatment Planning Requirements

• CT-based planning with conformal blocking is standard of care 1
• Non-contrast CT simulation should be performed 1
• Slice thickness of 2-3 mm recommended for accurate delineation 2
• PET imaging useful to define nodal volume coverage in patients not surgically staged 1

IMRT Considerations

• IMRT may be helpful for minimizing dose to bowel and critical structures, particularly in para-aortic treatments 1
• Requires very careful attention to target definitions, organ motion, soft tissue deformation, and rigorous quality assurance 1
• Not recommended for testicular cancer due to concerns about secondary malignancies in kidneys, liver, or bowel 1

Motion Management

• For thoracic and upper abdominal targets, 4D-CT scanning strongly preferred to account for respiratory motion 2
• Internal target volume (ITV) approach can be used when accounting for respiratory motion 2

Common Pitfalls to Avoid

• Do not use fixed circumferential margins around vessels for gynecologic malignancies—this misses common nodal locations 3, 4
• Do not rely solely on bony landmarks for superior extent—use renal vessels as anatomic reference 3
• Do not ignore interclinician variation—contouring variation can result in dose differences ranging from -289% to 56% for mean OAR dose 5
• Do not manually adjust PTV—it accounts for setup errors and breathing motion 2, 6
• Ensure adequate coverage of left para-aortic and aortocaval spaces, as these are most common locations 3