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

Cervix cancer

17

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

10. DOSE, DOSE RATE AND FRACTIONATION

Forward planning modifications can also be made using graphical tools. However, extreme caution is necessary if modifying a single isodose curve on a single cross-sectional image as this can result in unexpected dose changes at other locations and other dose levels [42]. Inverse planning may speed up the dose optimisation process and decrease the time to prepare a satisfactory treatment plan. However, current inverse optimisation algorithms only take into account objectives and constraints that are specified. Several important OAR (e.g. nerves, vessels, ureter, urethra) are not routinely contoured and the dose constraints for these structures are currently unknown. Moreover, there are no constraints to maintain inhomogeneity within the target with high doses in the centre of the CTV-T_HR. For large tumours and combined IC-IS implants, in particular, the DVH constraints on contoured organs will not restrict the inverse planning algorithm from escalating the dose in regions without contours and additional constraints on dwell-time gradients or maximum dwell times are required. Published clinical results are largely based on forward planning and major deviations from the standard pear-shaped loading pattern should be carefully evaluated before clinically implemented [43]. New automated approaches including knowledge-based planning approaches and artificial intelligence may eventually provide fast and high-quality treatment planning for cervix cancer BT which is based on similar principles as is currently achieved with manual planning.

10.1 Dose prescription, recording and reporting Defining the dose in BT is challenging because of the very high dose gradients around the BT sources. Historically, the classical methods of Paris and Stockholm used the radium (226Ra) source-mass duration (mgh Ra) to specify the dose in a radium implant. With these systems, it was considered that, “for a given geometric arrangement of Ra sources, the treated volume is proportional to the amount of mgh Ra” while clinical experience was used to determine normal tissue tolerance. The Manchester system represented an improvement over the mgh-Ra systems, as it standardised treatments by specifying dose and dose rate to predefined points/distances relative to anatomy and applicator, e.g. point A was used to evaluate dose in the paracervical region and point B to assess the dose to the obturator lymph nodes. In the 1970s, the concept of the “60 Gy reference volume”, based on height, width and thickness, was developed by GEC and formed the basis for the recommendations on reporting gynaecological BT in ICRU report 38 [26]. Additionally, well-defined reference points, to record and report dose to bladder and rectum were introduced (ICRU bladder and rectal points). With the advent of 3D IGABT, a new common language was developed by GEC-ESTRO based on DVH parameters to targets and OAR. 10.1.1 TRAK and point doses Traditionally, dose assessment for BT was mainly based on the amount of radiation incident on the patient and/or the absorbed dose at specified points (e.g. point A or ICRU bladder and rectal points). A large clinical experience has been, and is still being, accumulated with these metrics worldwide. The continuation of their use makes it possible to build upon previous experience and

Figure 17. Point A definition Point A is defined with respect to the superior surface of the vaginal applicator (not the source plane) (From ICRU 89).