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

TABLE 4 STRATEGIES FOR PRE-PLANNING Adapted from Table 12.1, ICRU Report 89

When contouring and reconstruction are performed in the same image series, the dose to targets and organs can be directly calculated without any image fusion. However, there are several circumstances when contouring is done in one image series or modality and reconstruction in another. When different image sets are used for contouring and reconstruction, they have to be co-registered according to the applicator [34]. Fusion uncertainties will translate into dose calculation uncertainties and can result in a miscalculation of dose-volume histogram (DVH) dose parameters by 4-6% per mm of fusion error [35]. Matching to bony structures must be avoided as the applicator can move more than 5 mm in relation to bone [36]. For X-ray based treatment planning, correct definition of reference points for target and OAR dose reporting is key (see 10.1.1). With volumetric MRI or CT-based BT, although dose prescription is mostly linked to target volumes and no longer to Point A, the reporting of point doses is still recommended and relevant. The definitions of these reference points on 3D images are described in chapter 9.2 of ICRU Report 89. 9.6 Loading patterns and optimisation of the dose distribution Remote afterloading equipment with stepping source technology typically has active dwell positions in step sizes of 1 mm to 10 mm. The width, height, and thickness of the isodose surface volumes (ISV) for the same BT applicator can differ substantially based on the loading pattern. In one study, seven different European and American institutions provided their standard loading patterns for a tandem-ring applicator and tandem-ovoid applicator [37]. The width of the normalised Point A ISV reported in these studies ranged from 4.8 cm to 6.0 cm at the level of the ring applicators. Standard sets of active dwell positions which simulate standardised radium-tube configurations can and have been used. These applicator-based loading patterns often result in unfavourable dose distributions in non-ideal applicator placements with asymmetrical tumour or OAR positions. In general, when using standardised Clinical examination will inform decisions on applicator type and implant geometry. This is used in the “mould technique,” where the applicator is modelled from a vaginal impression. An MRI just prior to brachytherapy allows assessment of the configuration and dimensions of the CTV_HR and the topography of OARs. Note that when the intracavitary applicator is inserted, the topography of the uterus and target volumes and their relation to the OAR will change substantially. The intracavitary applicator is inserted to serve as a template for optimal needle insertion which can be used to generate a full treatment pre-plan with regard to target coverage and OAR constraints. Before insertion of the brachytherapy applicator, a comprehensive clinical examination is essential to plan the implant. The insertion of the intrauterine tandem and parametrial needles can be guided by trans-abdominal or trans-rectal US Post-implant imaging allows for evaluating the quality of an implant with regard to dose–volume constraints for target and OAR.

Without applicator in place

Without volumetric imaging

With applicator in place

Pre-operative

Without applicator in place

With volumetric imaging

With applicator in place

Without volumetric imaging

During insertion of the applicator

Intra-operative

With volumetric imaging

With applicator in place

geometry. Direct digitization can be used when the source channels or marker wires are visible in the images. A library of applicators can be defined for fixed-geometry applicators and merged with the patient images based on anchor points or by direct positioning of the applicator shape according to visible structures of the applicator on the images (e.g. source path or outer surface of the applicator). With CT images, it is possible to directly visualise the source channel either by exploiting the contrast between the applicator material and the air-filled source channel or by inserting wires with radio-opaque markers separated by fixed gaps. Marker strings may have different flexibility and dimensions from the source wire and discrepancies in source position of 2–3 mm have been observed [10, 33]. Reconstruction of applicators with MRI is more challenging than with CT as the source channel is not visible due to lack of MR signal from air and the applicator materials. Furthermore, markers used for radiographs and CT cannot be used for MRI. Special MR markers, such as catheters containing a CuSO4 solution, water, glycerine, or US gel, can be inserted into the source channels in plastic applicators. Reference structures, such as needle holes or cavities filled with fluid, can also be used as long as the locations relative to the dwell positions are known. Fluid-filled marker catheters cannot be visualised inside the source channels of titanium applicators and there are also uncertainties due to distortions and artifacts that can change with MRI pulse sequence. MRI slice thickness has direct impact on the precision of reconstruction, and it is recommended that reconstruction be performed in an image series obtained with a slice thickness of ≤5 mm. Reconstruction of plastic needles on MR images can be particularly challenging as there are no commercially available MR needle markers and reconstruction is entirely reliant on image quality. Knowledge of needle insertion depth is crucial for correct needle reconstruction and can be obtained by measuring the outside needle length from the vaginal applicator or template.