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 2 STRATEGIES FOR CT-BASED CONTOURING Table showing four imaging strategies for CT-based contouring. Strategy 1 has the greatest contouring uncertainty while strategy 4 has the least. For each strategy, the addition of real-time transrectal ultrasound during brachytherapy may reduce contouring uncertainty. Careful clinical examination with precise documentation of findings is key for all strategies (see figure 5b) (Courtesy of Umesh Mahantshetty) At diagnosis Before brachytherapy

With applicator

1 2 3 4

CT

CT CT CT CT

MRI

CT

MRI MRI

MRI

The development and evolution of these concepts are described in detail in chapter 5 of ICRU report 89 [12]. Originally, these target concepts were developed for contouring on MRI. However, the limited availability of MRI in LMIC countries have given rise to adaptation of the concepts for contouring on CT [13] and US. A summary of key points regarding the contouring of these volumes using different imaging modalities is given below. Residual gross tumour volume (GTV-T res ) The GTV-T res is defined as residual high signal abnormality on T2 weighted MRI (Figure 9). Unlike the GTV-Tinit, the GTV-T res does not include palpable induration in the parametria as this cannot be easily distinguished from radiation effect. As a result, the GTV-T res cannot be reliably contoured if CT alone is used apart from visible disease on the cervix and/or vagina. Adaptive high-risk clinical target volume (CTV-T_HR) The CTV-T_HR consists of the GTV-T res , the whole cervix even if uninvolved, and any extra-cervical residual pathologic tissue as defined by clinical examination and on MRI. The residual (extra cervical) pathologic tissue is defined as one or more of the following: residual palpable mass, residual visible mucosal change, pathologic induration, residual grey zones, any other residual pathologic tissue on MRI or clinical examination. To be included as residual pathology, these regions must be located inside the GTV-Tinit and have characteristics that are consistent with residual disease and direct access to the diagnostic MRI during BT contouring is highly recommended. Such pathologic tissues may include parts of the cervix, parametria, uterine corpus, vagina, rectum, or bladder, according to the initial spread of disease. All suspect areas of residual disease should be included in delineation of the CTV-T_HR, with no margins added. CT-based contouring of the CTV-T_HR tends to result in overestimation of the width of the target [14] while the superior extent is not discernible. Various strategies have been suggested to minimise contouring uncertainties [13] based on the clinical remission pattern after EBRT and information available from supplementary imaging modalities (Table 2). Careful clinical examination with precise documentation of findings is key regardless of strategy – a revised diagram for recording clinical findings is provided in the IBS-GEC ESTRO-ABS recommendations for CT based contouring in IGABT for cervical cancer [13] (Figure 5b). Intermediate-risk clinical target volume (CTV-T_IR) The GTV-Tinit carries the highest density of tumour cells at the start of treatment. It can be assumed that residual tumour cells

Figure 10. Tumour and target concepts (MRI-based IGABT) MRI at diagnosis (left panels) and at time of brachytherapy with the applicator in situ (right panels). The brachytherapy targets (blue: GTVres, red: CTV_HR, green: CTV_IR) should be contoured on para-axial slices (upper right panel) and inspected for consistency in the sagittal sequence (lower right panel). The MRI at diagnosis (left panels) is used to identify grey zones and to ensure that the CTV_IR contour fully covers the primary tumour extension. Note that the CTV_IR has been edited to exclude areas where microscopic disease cannot extend directly (e.g. in peritoneal cavity outside the uterus). (From EMBRACE II protocol, www.embracestudy.dk)

will remain at various locations in this volume, even if the tissue has changed to a macroscopic normal appearance on clinical examination and imaging. The concept of the CTV-T_IR was developed to take the GTV-Tinit into account as superimposed on the topography at the time of BT. Special attention is particularly needed for cases with vaginal extension at diagnosis to ensure full coverage by the CTV-T_IR. In addition, the CTV-T_IR takes into account that there is likely to be significant microscopic tumour burden outside the CTV-T_HR by adding a margin surrounding the CTV-T_HR like a shell even where there was no initial GTV-T. The GEC ESTRO recommendations suggest a 10 mm margin in the lateral and cranio-caudal directions and 5 mm in the anterior–posterior direction. However, the CTV-T_IR should not include areas where microscopic disease