Residency · Residency · Radiation Oncology
HDR Brachytherapy for Cervical Cancer: Technique and Applicator Selection
Introduction
Brachytherapy is a crucial and indispensable part of curative treatment for locally advanced cervical cancer. Over time, there has been a widespread shift from low-dose-rate (LDR) to high-dose-rate (HDR) brachytherapy, which is now the standard approach in most treatment centers globally. The implementation of 3D image-guided adaptive brachytherapy (IGABT), strongly supported by the GEC-ESTRO guidelines and validated through the EMBRACE studies, has led to significant improvements in patient outcomes while reducing treatment-related toxicity.
Rationale for HDR Brachytherapy
HDR brachytherapy offers several advantages. It can be delivered on an outpatient basis, eliminating the need for prolonged hospitalization. The technique allows for optimization of dose distribution through modulation of dwell times, enhancing treatment precision. Because the radioactive source is afterloaded, there is no radiation exposure risk to nursing staff. The use of rigid applicator fixation ensures reproducible geometry across treatment sessions. Radiobiologic equivalence to LDR brachytherapy has been well established through extensive clinical experience. Furthermore, delivering multiple fractions permits dose adaptation based on the tumor’s response over time, allowing for personalized treatment adjustments.
Applicator Systems
Intracavitary Applicators
The most commonly used intracavitary applicators include the tandem and ovoids (Fletcher-Suit-Delclos), which serve as the standard for most cases. Another option is the tandem and ring applicator (Nucletron/Elekta), where the ring provides more stable geometry and reproducible dosimetry. The length of the tandem is selected based on the uterine sounding depth, typically ranging from 4 to 8 cm. The diameter of the ovoids is chosen according to the capacity of the vaginal fornices, with sizes commonly available in 2.0, 2.5, and 3.0 cm.
Interstitial Needles
When the tumor extends beyond the reach of standard intracavitary geometry, interstitial needles are added to the applicator system. The Vienna applicator combines a ring applicator with channels for interstitial needles, while the Utrecht applicator is based on ovoids with interstitial capability. For extensive parametrial disease, freehand interstitial needle placement guided by a perineal template may be employed. Needle placement is guided by clinical examination and imaging to ensure accurate targeting.
Applicator Selection Criteria
For small cervical tumors with adequate vaginal fornices, tandem and ovoids or tandem and ring applicators are generally sufficient. When residual parametrial disease extends more than 5 mm beyond the applicator surface, interstitial needles should be added to achieve adequate coverage. In cases of a narrow vagina or distorted anatomy, the tandem and ring applicator may be preferred due to its more stable geometry. For extensive vaginal involvement, additional vaginal cylinders or molds may be considered to ensure comprehensive dose coverage.
Image-Guided Adaptive Brachytherapy (IGABT)
GEC-ESTRO Recommendations
MRI-based planning is considered the gold standard for target delineation in cervical cancer brachytherapy. When MRI is unavailable, CT-based planning is an acceptable alternative. Imaging should be performed with the applicator in situ at each fraction to allow for accurate assessment. Target volumes are adapted to the tumor’s response throughout the course of treatment, enabling personalized dose delivery.
Target Volume Definitions
The gross tumor volume at the time of brachytherapy (GTV-Tres) is defined as the residual visible tumor on MRI. The high-risk clinical target volume (HR-CTV) includes the GTV-Tres plus the entire cervix and any areas of residual disease seen as gray zones on imaging. The intermediate-risk clinical target volume (IR-CTV) encompasses the HR-CTV plus the initial tumor extent with an added margin. Dose reporting focuses on parameters such as D90 and D98 for target volumes, and D2cc for organs at risk.
Dose Prescription and Constraints
Typical HDR fractionation schedules include 7 Gy delivered in 4 fractions or 5.5 Gy delivered in 5 fractions, with doses prescribed to the HR-CTV D90. When combined with external beam radiation therapy (EBRT) delivering 45 Gy, the total HR-CTV D90 target dose should exceed 85-90 Gy equivalent dose in 2 Gy fractions (EQD2) to optimize local control. Dose constraints for organs at risk are critical: the bladder D2cc should be kept below 90 Gy EQD2, while the rectum, sigmoid colon, and bowel D2cc should each remain below 75 Gy EQD2.
Treatment Procedure
Pre-Procedure
Before the procedure, a clinical examination under anesthesia is performed to assess the residual tumor. Uterine sounding is conducted to determine the appropriate tandem length and uterine position. A Foley catheter with a 7 cc balloon is placed to aid bladder delineation during imaging. A rectal marker, such as a rectal tube with contrast, may also be inserted to assist in defining the rectum.
Applicator Insertion
Applicator insertion is performed under conscious sedation or general anesthesia. The cervical os is dilated to allow insertion and secure placement of the tandem. Ovoids or the ring are positioned within the vaginal fornices. If indicated, interstitial needles are inserted through the applicator template. Vaginal packing is then placed to displace the rectum and bladder away from the radioactive sources, optimizing dosimetry and reducing toxicity.
Imaging and Planning
With the applicator in place, the patient is transferred to an MRI or CT scanner for imaging. Target volumes and organs at risk are contoured on these images. Dose optimization is performed using inverse planning or graphical optimization techniques. The treatment plan is evaluated using dose-volume histogram (DVH) parameters and visual inspection of isodose lines. Final approval of the plan is given by the radiation oncologist prior to treatment delivery.
Treatment Delivery
The patient is then moved to the HDR treatment room where applicator connections and treatment channels are verified. Source position and dwell times are confirmed before initiating treatment, which typically lasts 10 to 20 minutes per fraction. Depending on the treatment protocol, the applicator may be removed after each fraction or left in place for multiple fractions.
EMBRACE Studies
EMBRACE I
The EMBRACE I study was a prospective multicenter trial that validated MRI-guided IGABT in locally advanced cervical cancer. It enrolled 1,416 patients and reported an unprecedented 3-year local control rate of 92% in the MRI era of brachytherapy. Grade 3 or higher late toxicity rates were relatively low, with bladder toxicity at 5%, rectal toxicity at 4%, and bowel toxicity at 5%. The study demonstrated that achieving a D90 HR-CTV dose greater than 87 Gy EQD2 optimizes local control.
EMBRACE II
EMBRACE II is an ongoing prospective study that incorporates stricter dose constraints for organs at risk with the goal of further reducing late toxicity while maintaining excellent local control rates. It employs standardized contouring and dose reporting protocols. The target dose constraints for organs at risk include rectum D2cc less than 65 Gy EQD2 and bladder D2cc less than 80 Gy EQD2.
Interstitial Technique Details
When to Add Needles
Interstitial needles are added when the HR-CTV extends more than 5 mm beyond the surface of the ovoid or ring applicator, in cases of asymmetric residual parametrial disease, lower vaginal involvement, or when adequate D90 coverage cannot be achieved with intracavitary applicators alone.
Needle Placement
Typically, 4 to 8 needles are inserted through applicator channels such as those in the Vienna or Utrecht applicators. The depth of needle insertion is guided by clinical examination and imaging findings. Care is taken to ensure that needles do not extend beyond the uterine serosa. Verification imaging is performed after insertion to confirm correct needle positioning.
Key Clinical Pearls
Brachytherapy is an essential component of curative treatment for cervical cancer, and omitting it reduces survival by 10-15%. MRI-guided IGABT represents the gold standard, with EMBRACE I demonstrating a 92% local control rate alongside acceptable toxicity profiles. Applicator selection depends on tumor geometry: tandem-ovoid or tandem-ring applicators are suitable for standard cases, while interstitial needles should be added when the tumor extends beyond the reach of intracavitary applicators. Achieving a target HR-CTV D90 dose greater than 85-90 Gy EQD2, combining EBRT and brachytherapy, is critical for optimal local control. Vaginal packing plays a vital role in displacing organs at risk and optimizing dose distribution.
References
- Potter R, Tanderup K, Schmid MP, et al. MRI-guided adaptive brachytherapy in locally advanced cervical cancer (EMBRACE-I): a multicentre prospective cohort study. Lancet Oncol. 2021;22(4):538-547.
- Haie-Meder C, Potter R, Van Limbergen E, et al. Recommendations from Gynaecological (GYN) GEC-ESTRO Working Group (I): concepts and terms in 3D image based 3D treatment planning in cervix cancer brachytherapy. Radiother Oncol. 2005;74(3):235-245.
- Dimopoulos JCA, Petrow P, Tanderup K, et al. Recommendations from Gynaecological (GYN) GEC-ESTRO Working Group (IV): basic principles and parameters for MR imaging within the frame of image based adaptive cervix cancer brachytherapy. Radiother Oncol. 2012;103(1):113-122.
- Fokdal L, Sturdza A, Mazeron R, et al. Image guided adaptive brachytherapy with combined intracavitary and interstitial technique improves the therapeutic ratio in locally advanced cervical cancer: analysis from the retroEMBRACE study. Radiother Oncol. 2016;120(3):434-440.