What are the guidelines for brachytherapy based on High-Risk Clinical Target Volume (HRCTV) and Intermediate-Risk Clinical Target Volume (IRCTV) assessments?

Guidelines for Brachytherapy Based on HRCTV and IRCTV Assessments

For optimal outcomes in brachytherapy treatment, target volume delineation using High-Risk Clinical Target Volume (HRCTV) and Intermediate-Risk Clinical Target Volume (IRCTV) should guide dose prescription, with HRCTV requiring doses of at least 85-87 Gy EQD2 for optimal local control. 1

Target Volume Definition and Concepts

• HRCTV represents the gross tumor volume plus areas at highest risk of microscopic disease, requiring the highest radiation dose to achieve local control 1
• IRCTV encompasses a larger volume with intermediate risk of microscopic disease, typically receiving lower doses than HRCTV 1
• For cervical cancer, HRCTV includes the entire cervix and any visible tumor on MRI, while IRCTV extends to include potential microscopic spread areas 2
• For endometrial cancer, HRCTV includes the gross tumor volume plus the endometrial cavity 3

Dose Recommendations for Cervical Cancer

Definitive Treatment (Intact Cervix):

• External beam radiation therapy (EBRT) should deliver approximately 45 Gy (40-50 Gy) to the primary tumor and regional lymphatics 4
• Brachytherapy boost should deliver an additional 30-40 Gy to point A (in LDR equivalent dose) 4
• Total point A dose should reach 80 Gy for small-volume cervical tumors and ≥85 Gy for larger-volume tumors 4
• For HRCTV-based planning, D90 (dose covering 90% of HRCTV) should be at least 85-87 Gy EQD2 for optimal local control 1
• Higher doses may be needed for adenocarcinoma histology, with HRCTV D90 EQD2 ≥84 Gy showing improved local control 5

Post-Hysterectomy Treatment:

• Adjuvant radiotherapy should cover the upper 3-4 cm of the vaginal cuff, parametria, and adjacent nodal basins 4
• Standard dose is 45-50 Gy in conventional fractionation 4
• For positive vaginal margins, vaginal cylinder brachytherapy may be used as a boost to EBRT 4

Brachytherapy Modality Selection Based on Tumor Size

• For HRCTV size ≤4×3×3 cm: Image-guided intracavitary brachytherapy (IGICBT) is optimal 6
• For HRCTV size >4×3×3 cm but <5×4×4 cm: Combined intracavitary/interstitial brachytherapy (ICISBT) is recommended 6
• For HRCTV size ≥5×4×4 cm: Interstitial brachytherapy (ISBT) with perineal approach provides the best dosimetric coverage 6

Dose Fractionation Schemes

High-Dose Rate (HDR) Brachytherapy for Cervical Cancer:

• Common approach: 5 insertions with tandem and colpostats, each delivering 6 Gy to point A 4
• This results in a nominal HDR point A dose of 30 Gy in 5 fractions, equivalent to 40 Gy using LDR brachytherapy 4

Vaginal Brachytherapy for Endometrial Cancer:

• For HDR vaginal brachytherapy alone: 7 Gy × 3 fractions prescribed at 0.5 cm depth from vaginal surface, or 6 Gy × 5 fractions prescribed to vaginal surface 7
• For HDR vaginal brachytherapy as boost after EBRT: 4-6 Gy × 2-3 fractions or 5-6 Gy × 2 fractions prescribed to vaginal mucosa 7

Technical Considerations

• Target for vaginal brachytherapy after hysterectomy should be limited to the upper vagina 7
• Dose should be prescribed to either the vaginal surface or at 0.5 cm depth from vaginal surface, depending on the chosen regimen 7
• For rare cases where intracavitary brachytherapy is not feasible due to tumor geometry, interstitial approach should be considered, but only at institutions with appropriate expertise 4

Prognostic Factors and Dose Escalation

• Poor prognostic factors requiring consideration of dose escalation include:
  • Adenocarcinoma histology (associated with worse local control) 5, 2
  • HRCTV >40 cm³ at time of brachytherapy 2
  • Poor response to external beam radiation 2
• For patients with HRCTV >40 cm³, 2-year progression-free survival is significantly lower (66.2% vs 84.1%) compared to those with HRCTV ≤40 cm³ 2

Toxicity Prevention

• Patients receiving vaginal brachytherapy are prone to vaginal stenosis, which can impair sexual function 7
• Vaginal dilators can be used starting 2-4 weeks after treatment completion and continued indefinitely 7
• Modern image-guided brachytherapy techniques aim to optimize dose coverage of tumor while reducing dose to adjacent bladder, rectum, and bowel structures 4

Special Considerations

• For medically inoperable endometrial cancer, MRI-based brachytherapy with HRCTV defined as GTV plus endometrial cavity and a planning goal of summed EQD2 D90 of ≥85 Gy shows favorable local control 3
• Treatment time should be minimized when possible, as overall treatment time >51 days is associated with worse outcomes 2