12. Treatment Planning

Abstract

This section summarizes the practical aspects of clinical treatment planning for intracavitary cervical brachytherapy. Treatment planning is based on the overall planning aim for the combined dose distributions of external-beam radiotherapy (EBRT) and brachytherapy. Based on information available at diagnosis, a schedule for EBRT and brachytherapy, their relative contribution to the overall EQD2 for defined target volumes, fractionation, and timing is defined. Due to regression of the primary tumor, the target volume for brachytherapy can diminish significantly during treatment. Therefore, adaptive treatment planning is based on a reassessment of tumor and target volumes before and possibly at the time of brachytherapy. Adaptations of the brachytherapy implant itself to the anatomical situation after weeks of EBRT are also an integral part of the optimized adaptive-treatment-planning procedure. Adaptation is possible at different levels of complexity, ranging from the minimum requirement of a detailed clinical examination to image-guided approaches simulating the implantation technique and geometry (preplanning). During implantation, further optimization of the implant can be obtained by intraoperative image guidance. The final implantation geometry in relation to target volumes and organs at risks (OARs) is determined with volumetric imaging or radiographic approximation with the applicator in place. A set of dose–volume constraints for the individual brachytherapy fractions must be available prior to the optimization of dwell positions and dwell times, taking into account the pre-defined overall planning aims as well as spatial distributions of absorbed dose from previous brachytherapy and/or external-beam fractions. The method to achieve reproducible and controlled absorbed dose distributions is to start the optimization process with standardized loading patterns for the active dwell positions. In an iterative process, the dwell positions and dwell times are adjusted until an acceptable compromise between target coverage and OAR constraints is achieved. Inverse optimization and graphically assisted dosedistribution shaping should be performed with care as the spatial distribution of over-dosed and underdosed spots within the treated volume is often changed substantially compared with the manual iterative procedure. Clinical experience and quantitative radiobiology has shown that dose–effect curves for toxicity in the pelvis can be steep depending on the OAR and the chosen endpoint (Bentzen, 1993; Georg et al., 2012; Perez et al., 1998; Petereit et al., 1999; Pourquier et al., 1982; 1987). Figure 8.1 illustrates an example of dose–volume correlations for late rectal morbidity in cervical cancer patients treated with MRI-based brachytherapy, where a steep dose–effect is evident, especially when D2cm3 is used as a descriptor of the cumulative dose of EBRT and brachytherapy EQD2 delivered to the rectum (Georg et al., 2009). Considering the sharp absorbed-dose fall-offs inherent in brachytherapy, a close balance exists when attempting to deliver a curative absorbed dose to the tumor with minimal toxicity to neighboring structures (Dimopoulos et al., 2009c; 2009d). The goal of treatment planning is to obtain the best possible chance for an uncomplicated cure of the individual patient (Holthusen, 1936) by careful planning of absorbed-dose delivery by a brachytherapy application (Kirisits et al., 2005; 2006a; Pötter et al., 2006). However, in the broadest sense, brachytherapy treatment planning should also involve an effort to implement brachytherapy into the whole treatment chain, including EBRT and concomitant chemotherapy, focusing on items such as balance of absorbed dose between EBRT and brachytherapy, overall treatment time, and brachytherapy-implant strategy (Lindegaard et al., 2011). Radiographic-based gynecological brachytherapy has provided the basis for treatment planning and led to very impressive clinical results (Eifel et al., 1994b; Gerbaulet et al., 1995; Horiot et al., 1988; ICRU, 1985; Perez et al., 1998; Pernot et al., 1995). The introduction of volumetric-image-based brachytherapy has added new information in terms of both volume for the target and OAR and how those volumes change with time, providing improved understanding (Barillot et al., 1994; Haie-Meder et al., 2010b; Kirisits et al., 2005; 2006a; Lindegaard et al., 2008; 2013; Pelloski et al., 2005; Pötter et al., 2007; 2011; Viswanathan et al., 2006b). Journal of the ICRU Vol 13 No 1–2 (2013) Report 89 doi:10.1093/jicru/ndw016 Oxford University Press