CTV to ITV Margin for Cervical Cancer Radiotherapy Contouring
For cervical cancer radiotherapy, the CTV to ITV margin should be anisotropic: 10 mm in anterior-posterior and lateral directions, and 12-20 mm in the superior-inferior direction to account for uterine fundus motion, with smaller margins of <10 mm acceptable for the mid-cervical region.
Understanding the Distinction Between ITV and PTV
The Internal Target Volume (ITV) accounts for internal organ motion (primarily uterine movement from bladder and rectal filling), while the Planning Target Volume (PTV) accounts for setup uncertainties 1. These are distinct concepts that require separate margin considerations in cervical cancer radiotherapy.
Evidence-Based Margin Recommendations
Mid-Cervical Region
- The mid-cervical CTV requires <10 mm internal margins (CTV-ITV) based on prospective data showing mean shifts of 4.5-5.5 mm with systematic and random errors of 2-4 mm 2
- This region demonstrates relatively stable positioning throughout treatment 2
Uterine Fundus Region
- The uterine fundus requires significantly larger anisotropic margins:
- 10 mm in anterior-posterior and lateral directions
- 12-20 mm in superior-inferior directions 2
- This is based on prospective data showing mean superior shifts of 12.1 mm (compared to 4.0-7.5 mm in other directions) with systematic and random errors of 2-7 mm 2
- Conventional uniform 1 cm margins around the uterine fundus can result in approximately 5 Gy dose reduction to the fundus due to inadequate motion coverage 3
Impact of Adaptive Approaches
Online Adaptive Radiotherapy
- With daily cone-beam adaptive radiotherapy (OnC-ART), a uniform 5 mm CTV-to-PTV margin can cover 98.39% ± 3.0% of the end-treatment CTV 4
- This reduced margin is only appropriate when interfractional movement is accounted for through daily adaptation 4
- Time between CBCT scans >30 minutes and bladder volume changes significantly correlate with CTV coverage adequacy 4
MRI-Guided Adaptive Radiotherapy
- MRI-guided approaches demonstrate steady GTV reduction and increased uterine displacement as tumor size decreases, supporting the need for adaptive strategies 5
- Daily adaptive protocols with 3 mm CTV-to-PTV margins achieved complete CTV coverage in 94.6% of fractions while significantly reducing OAR doses 6
Critical Technical Considerations
Motion Modeling
- Uterine motion is progressively magnified moving superiorly from cervix to fundus, requiring tapered margin approaches 3
- Normal tissues can move into the void left by the mobile uterus, further complicating dosimetry 3
- A tapered CTV-to-PTV margin restores fundus and CTV doses but increases normal tissue volumes receiving 30-50 Gy by approximately 5% 3
Standard Non-Adaptive Approach
- For conventional non-adaptive IMRT, consensus guidelines recommend including the whole uterus with large PTV margins (1.5-2 cm) to account for interfractional motion 4
- Standard-of-care margins of 5-15 mm only completely encapsulated CTVs in 72% of fractions in retrospective analysis 6
Common Pitfalls to Avoid
- Do not use uniform margins throughout the entire uterocervical volume - the fundus requires substantially larger superior-inferior margins than the cervix 2
- Do not assume rigid body motion - uterine motion is non-rigid with progressive magnification superiorly 3
- Do not reduce margins without daily image guidance and adaptation - conventional approaches require larger margins to ensure adequate coverage 4, 6
- Account for bladder filling variations - changes in bladder volume significantly affect CTV coverage 4
Practical Algorithm for Margin Selection
For Non-Adaptive Conventional IMRT:
- Mid-cervix: 10 mm uniform expansion
- Uterine fundus: 10 mm anterior-posterior/lateral, 15-20 mm superior-inferior 2
For Daily Adaptive Radiotherapy (CBCT or MRI-guided):