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

Cervix cancer

14

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

and rings, allow laterally asymmetric isodoses to be created in the regions close to the vaginal sources. In contrast, the shape of the isodoses around the tandem become more and more cylindrical superiorly, limiting the possibility of covering laterally situated target volumes. In practice, the planning-aim isodose cannot be placed more than 25 mm from the tandem at the level of Point A; the isodose can be pulled closer to the tandem if the dose to OAR is too high but it cannot be extended much further from the tandem if the target exceeds this distance. Acceptable dose coverage 35 mm from the IU tandem at the level of Point A can be achieved with combined IC/IS applicators with additional lateral needles; these typically cover the medial half of the parametrial space. Applicators with additional obliquely directed needles, or free-hand oblique needle placement, allows lateral dose coverage for distal parametrial and pelvic wall residual disease to be increased (Figure 15). It is important to note that the dose distributions used in published clinical series are still largely dependent on a contribution from the IC application [29, 30]. Needle loading should only be activated within or in immediate proximity to the CTV-T_HR and dwell times per dwell position in needles are normally limited to 10–20% of those used in standard loading patterns for IC implants [31]. 9.4 Pre-planning of the implant Pre-planning according to the definition of the CTV-T HR is necessary to determine the optimal applicator type and to assess the need for an IS approach. As a minimum, a gynaecological

examination should be performed, and the findings recorded on a clinical drawing and compared with the extent of disease at diagnosis. An MRI, with or without an IC applicator in situ, obtained a few days before the actual implant is a useful aid to pre-planning and provides additional information on the response to EBRT and the tumour and OAR topography at the time of BT. Table 4 presents an overview of current possibilities for pre- and intra-operative planning of cervical cancer BT to help make the best individual choice of applicator type, implant technique, and applicator placement in relation to the CTV-T HR and the anatomical situation at the time of BT. 9.5 Applicator reconstruction Recommendations regarding applicator reconstruction for IGABT for cervical cancer were published by GEC-ESTRO in 2010 [10]. Applicator reconstruction defines the relation between the radiation source and the anatomy of the patient in the treatment–planning system, so that the dose contribution from each source position can be calculated for each anatomical voxel. The accuracy of applicator reconstruction is of utmost importance because of the steep dose gradients inherent to BT. Incorrect reconstruction of the applicator will result in dose deviations in both target structures and OAR of up to 5–8% for each mm of applicator displacement [32]. The applicator and source path have to be defined on the individual patient images at the time of BT. This can be done by digitization directly on the acquired images (X-ray, CT or MRI) with the applicator in situ or by importing a library file of the applicator

Figure 15. Combined IC/IS implant with straight and oblique needles (Vienna II applicator) (a) MRI and clinical drawings at the time of diagnosis; (b) assembly of applicator; (c) MRI and clinical drawings at the time of brachytherapy with Vienna II applicator to encompass the residual disease in distal parametrium on right; (d) contouring the target and the organs at risk; (e) BT treatment planning showing 3D view of the applicator with straight and oblique needle reconstruction and final plan showing isodose distribution in different views (Reprinted from Radiother. Oncol., 141, Mahantshetty U, Sturdza A, Naga Ch P, et al, Vienna-II ring applicator for distal parametrial/pelvic wall disease in cervical cancer brachytherapy: An experience from two institutions: Clinical feasibility and outcome, 123-129, 2019, with permission from Elsevier).