What are the guidelines for dose and volume specification in intracavitary brachytherapy for gynecological cancers, such as cervical cancer, according to International Commission on Radiation Units and Measurements (ICRU) 38?

ICRU 38 Guidelines for Dose and Volume Specification in Intracavitary Brachytherapy for Gynecological Cancers

According to the International Commission on Radiation Units and Measurements (ICRU) Report 38, brachytherapy for gynecological cancers should specify dose at point A, with additional dose calculations at standardized point B and bladder and rectal points, while incorporating specific guidelines for radioactive source loading and distribution within the uterus and vagina. 1

Key Dosimetric Parameters

Reference Points for Dose Specification

• Point A is the primary reference point for dose specification, located 2 cm lateral to the central canal of the uterus and 2 cm superior to the lateral fornix 1
• Point B is used for reporting dose to the pelvic wall/lymphatic trapezoid, located 5 cm from the midline at the same level as point A 1
• Bladder and rectal reference points are defined to monitor doses to these critical organs 1

Dose Recommendations

• For small-volume cervical tumors, the total point A dose should reach 80 Gy (LDR equivalent) 1, 2
• For larger-volume cervical tumors, the total point A dose should be ≥85 Gy (LDR equivalent) 1, 2
• External beam radiation therapy (EBRT) typically delivers approximately 45 Gy (40-50 Gy) to the primary tumor and regional lymphatics 1, 2
• Brachytherapy boost should deliver an additional 30-40 Gy to point A (in LDR equivalent dose) 1, 2

Brachytherapy Application Techniques

Applicator Types and Placement

• Intracavitary approach is standard, using an intrauterine tandem and vaginal colpostats 1
• Vaginal component may be delivered using ovoids, ring, or cylinder (combined with the intrauterine tandem) 1
• Interstitial approach should be considered only when intracavitary brachytherapy is infeasible due to tumor geometry 1

Dose Rate Considerations

• Traditional LDR brachytherapy delivers 40-70 cGy/h to point A 1
• For HDR brachytherapy, a common approach is 5 insertions with tandem and colpostats, each delivering 6 Gy to point A 1, 2
• This results in a nominal HDR point A dose of 30 Gy in 5 fractions, equivalent to 40 Gy using LDR brachytherapy 1, 2

Organ at Risk (OAR) Dose Limitations

Bladder and Rectal Dose Constraints

• Every attempt should be made to keep the bladder dose below 80 Gy LDR equivalent 3
• Rectal dose should be kept below 75 Gy LDR equivalent 3
• ICRU bladder and rectal points often underestimate the actual maximum doses to these organs 4
• The mean D2 (dose to 2% volume) for rectum can be 1.66 times higher than the corresponding ICRU point dose 4
• The mean D2 for bladder can be 1.51 times higher than the ICRU point dose 4

Modern Image-Guided Brachytherapy Considerations

3D Planning vs. Conventional Planning

• 3D image-guided brachytherapy seeks to optimize dose coverage of tumor while reducing dose to adjacent organs 1, 2
• CT-based planning is superior to conventional planning in target volume coverage and appropriate evaluation of OARs 4
• Conventional planning based on orthogonal radiographs tends to overestimate tumor doses and underestimate OAR doses 4
• When implementing image-guided brachytherapy, care must be taken not to underdose tumors relative to the point A system 1

Target Volume Coverage

• Target volume coverage decreases with increasing tumor size and stage 4
• For smaller tumors (GTV mean 8.1 cc, CTV mean 20.6 cc), the 95% isodose line of the point-A prescription dose typically encompasses the CTV 4
• For larger tumors (GTV mean 24.7 cc, CTV mean 48.4 cc), the 95% isodose line often fails to encompass the CTV 4

Clinical Pitfalls and Considerations

Application Technique Pitfalls

• Brachytherapy should be initiated toward the latter part of EBRT treatment, when sufficient primary tumor regression has occurred to permit satisfactory applicator geometry 1
• Combining rectal retractor with radio-opaque vaginal packing can significantly reduce the ICRU rectal point dose compared to using rectal retractor alone 5
• Patient transportation from simulator room to treatment room can introduce small uncertainties in delivered dose 6

Treatment Planning Considerations

• Reference volumes from 70-160 Gy have not shown consistent correlation to primary cervical tumor control 7
• The 60 Gy reference volume has shown statistical significance for tumor control in stage Ib2 and stage 2 disease 7
• Interstitial brachytherapy should only be performed by individuals and at institutions with appropriate experience and expertise 1