18. Primary vaginal cancer and vaginal recurrences - The GEC-ESTRO Handbook of Brachytherapy

Primary vaginal cancer and vaginal recurrences

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

of IS needles is done via the perineum or intravaginally. Perineal needles are inserted in a parallel configuration, with preferably equal spacing (10-15 mm depending on the target and anatomy), covering at least the residual tumour or CTV-T HR . In the case of upper tumours this can be challenging because of possible divergence or convergence of the needles. However, with 3D imaging, deviations from an ideal implant geometry can be partly compensated by dwell time optimisation during treatment planning. Nowadays, plastic needles are predominantly used, either with round or sharp tips. The advantage of sharp tips is a better steering during insertion, however there is an increased risk of puncturing organ walls. To aid precision during needle placement, it is recommended to use real-time ultrasound guidance in the operating theatre, via a trans-rectal or an endovaginal probe. In-room-CT or –MR suite solutions are also possible for fast implant quality verification in the operating room. Intracavitary BT alone is performed for target lesions less than 5-7 mm CTV-T HR thickness at the time of brachytherapy. In this case, the reference isodose follows the round curvature of the cylinder surface. It is prescribed to the vaginal wall for a certain length and thickness (in general 3-5 mm), either to cover the whole vagina or part of the vaginal length and circumference, respectively. Target delineation recommendations should be followed. The dose is reported to the CTV-T HR , as well as to a reference point at 5 mm radial into the tissue from the IC applicator surface, on the involved side (independent of dose prescription at a certain tissue depth). Using a multichannel cylinder, it is possible to spare OAR dose and the vagina wall on the opposite side of the CTV. To do so, either only the dwell positions in the peripheral needles near the CTV are activated, or the loading is distributed over the central catheter and the peripheral needles (Figure 7). The latter loading pattern will result in higher doses to the uninvolved vaginal tissue, but it may result in lower surface dose on the CTV side. The magnitude of the difference between these loading strategies, assuming the same target coverage, depends on the weighting of central vs. peripheral needles, and the cylinder diameter. There are currently no evidence-based recommendations for one or the other loading strategy. In interstitial implants, Paris system rules are applied for the implantation geometry, aiming for parallel and equidistant needle placement. If combined intracavitary and interstitial brachytherapy is performed ICRU 89 recommendations can be applied for reporting. In case of target volumes thicker than 5-7 mm at time of BT, the application of interstitial needles is advised, in addition to a multichannel intracavitary applicator. The reason for this is that with thicker tumours the vaginal mucosa will receive an excessive dose, or the CTV-T HR will not be adequately covered (Figure 8). The dose is prescribed to the GTV-T res , the CTV-T HR and CTV T IR . Depending on the complexity of the application, initial dose prescription can be based on dose points placed along the cylinder axis (e.g., at a tissue depth of 3 or 5 mm), followed by subsequent adjustment of the loading to cover the delineated target volumes. Depending on the anatomical configuration, a combination of loading of the peripheral catheters in the IC applicator +/- central catheter, and interstitial needles is performed (Figure 9). The choice of the IC loading is made as in an IC only implant, taking 9. TREATMENT PLANNING

In tumours limited to the upper third of the vagina and with an intact cervix, tandem-ovoids, tandem-ring applicators or tandem split rings with or without vaginal caps can be used (Figure 6, panel 3 and 4). In difficult cases, these plastic applicators are used as they allow the use of a central source (e.g., through a short tandem) and specific holes near to the applicator surface for introducing tubes or needles according to the geometry and extension of the specific tumour. Like vaginal cylinders, these applicators could be combined with a perineal template for placement of paravaginal needles or combined with free needles. 8.1.2 Interstitial implants The technique and the principles of interstitial (IS) brachytherapy, including the Paris system, are described in chapter 7. Although an IS only technique could be considered for treatment of extended vaginal disease, a combination of IC/IS is generally used. Implantation Figure 9. Example of a complex implant with intracavitary multichannel cylinder plus interstitial needles. Different loading patterns are shown. Top panel: loading only in peripheral IC and IS needles; middle panel: equal loading of central IC catheter, peripheral and interstitial needles; bottom panel: main IC loading in central catheter, low loading of peripheral needles, plus IS needles. All dose distributions were normalized to equal D90 of CTV-T HR . Green isodose line corresponds to 100% of planning aim for D90 CTV-THR. Other isodoses relate to this planning aim dose as follows: dark blue: 30%, light blue: 50%, cyan: 70%, yellow: 150%, red: 200%. Contours for rectum, CTV-T HR , CTV-T IR , and vagina outside of CTV-T HR are shown. This figure illustrates the effect of the loading pattern on contralateral dose to vagina, dose to rectum, and high dose volumes around needles, inside CTV. The planner‘s choice of the loading pattern will depend strongly on the given planning aims for all CTVs, including dose to contralateral vagina