16. Cervix cancer - The GEC-ESTRO Handbook of Brachytherapy

Cervix cancer

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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 01/09/2023

Prior to applicator insertion, a urinary catheter is inserted into the bladder for patient comfort throughout the duration of the implant and to allow calculation and reporting of the dose to the bladder neck according to the ICRU definition of the bladder point [12, 26]. The catheter balloon is inflated with radiopaque solution (7 cm 3 ) and should be pulled down towards the base of the bladder until it is fixed at the bladder neck and fixed outside. After applicator insertion, the vagina is packed tightly with gauze to keep the applicators in position and to displace the bladder and rectum; in some applicators, a separate rectal retractor can also be attached. For larger residual disease and cases of unfavourable anatomy, combined IC/IS applicators with additional lateral (straight +/- oblique) needles are advantageous. These applicators have holes in the ovoids or rings to guide needle placement. The use of blunt needles is preferred to minimise complications from needle insertion. The planned depth of needle insertion in the parametrium is determined by the cranial extent of CTV-T_HR, usually 4–5 cm above the upper surface of the vaginal applicator. Pre-BT imaging can be used to determine the optimal length and location of the needles to be inserted while intra-operative transrectal US can be used to guide real-time needle placement. The goal of treatment planning is to obtain the best possible chance for an uncomplicated cure of the individual patient. Treatment planning is a multi-step process predicated on pre-defined planning aims and dose constraints for the combined dose distributions of EBRT and BT based on information available at diagnosis. BT in particular offers many opportunities to achieve the defined dose planning aims. Variations of the implant geometry, loading pattern and time–dose pattern (dose rate, fractionation) allow modification of the therapeutic window to deliver an optimal absorbed dose for the targets while sparing the OAR. With the introduction of BT afterloading machines, the historical radium systems were converted into a pattern of dwell positions and times for a single stepping source. The so-called “standard loading patterns” of modern BT were introduced to reproduce isodose distributions of the historical Manchester/Fletcher or Stockholm BT schools. 9.1 Combining EBRT and BT: dose and fractionation strategies The 3D dose distribution in definitive radiotherapy for cervical cancer is characterised by exposure of relatively large tissue volumes to intermediate dose levels from EBRT, and exposure of smaller volumes to high doses in the region of the BT boost and EBRT boosts to involved lymph nodes. The EBRT and BT doses are converted into equi-effective dose according to the linear quadratic (LQ) model to facilitate dose accumulation across the two modalities. The current standard for reporting equi-effective dose in cervix BT is equivalent dose in 2Gy fractions (EQD2) using α/β ratios of 10 Gy for tumour and 3 Gy for OAR. For PDR BT, a repair half time of 1.5 h is used. It is important to note that the characteristics of the combined EBRT and BT dose distribution are highly dependent on the weighting between EBRT and BT. The radiobiological aspects of BT are discussed in more detail in chapter 7 of ICRU Report 89 for cervix cancer and in the radiobiology chapter of this handbook (Chapter 5). 9. TREATMENT PLANNING

may result in the entire implant lying lower in the vagina which in turn may make it impossible to load the vaginal applicator. The addition of IS needles can compensate for a smaller sized vaginal applicator in patients with unfavourable anatomy. Serban et al. [20] compared the dosimetry of tandem-ring vs. tandem-ovoid treatments within the EMBRACE-I study. For similar point A doses with IC implants, the mean CTV-T_HR D90% was 3.3 Gy higher and V85Gy was 23% lower for ring compared with ovoid centres (at median target volumes of 30 cm 3 ). The mean bladder/rectum doses (D2 cm 3 and ICRU-point) were also 3.2-7.7 Gy lower with the ring although the dose to the vaginal 5-mm lateral point was 19.6 Gy higher. The addition of IS needles resulted in increased CTV-T_HR D90% in large target volumes of 60 cm 3 for both ring and ovoid centres (mean 8.9 Gy and 5.4 Gy respectively) while V85Gy and OAR doses were generally unchanged (ring) or reduced (ovoid). IC/IS implants therefore improve target dose as well as dose conformity compared to IC implants. 8.2 Applicator insertion Modern BT applicators are made of MR-compatible plastic or CT-compatible titanium or tungsten. The IS needles are mostly disposable plastic. Increasingly, the manufacturers have tended to reduce the number of options available in terms of length and angle of the IU tandem, and size and form of the vaginal component to minimise costs. Insertion of applicators is usually performed under spinal or general anesthesia. Less common strategies include oral or intravenous sedation with a local paracervical block [21-23] and hypnosedation [24]. The patient is positioned in the lithotomy position. At each insertion, the radiation oncologist begins with a thorough gynaecologic examination to assess the patient anatomy and extent of tumour. Vaginal specula are introduced to visualise the cervix, and one or two cervical forceps are placed on the anterior lip of the cervix. The length of the uterine cavity is measured using a semi-flexible or metal probe. The cervical os may be difficult to locate if there is extensive tumour destruction. Uterine perforation and false passages are recognised complications of applicator insertion, especially if the cervical os is difficult to identify due to tumour destruction or if the uterus is not in the standard anteflexed anteverted position. Transabdominal or transrectal US are useful to guide correct placement of the tandem within the uterine canal; a recent randomised controlled trial [25] showed that use of transabdominal US reduced the incidence of perforation from 12.5% to 1.25%. There are no standard recommendations for the management of uterine perforation - the clinician has to decide whether to proceed or abandon treatment; intravenous or oral antibiotics are usually prescribed. After the uterine canal is measured, the cervical canal is dilated to accommodate the tandem. With modern applicators, the diameter of the tandem is often small (~3 mm) and significant dilatation is not required. An IU tandem of the appropriate length and curvature is inserted through the cervical os into the uterine cavity. The intravaginal component of the applicator is chosen depending on tumour diameter and topography and patient anatomy. If the anatomy is very narrow, an IU applicator extending into the vagina may be used alone or in combination with a vaginal cylinder.