9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 207

8.5.2.2 Rectum reference point The reference point for the rectal dose is related to the applicator and located 5 mm behind the posterior vagina wall on an AP line drawn from the middle of the vaginal sources (Fig 6.29). On the lateral radiograph, an anterior-posterior line is drawn from the lower end of the intrauterine source (or from the middle of the intravaginal sources). The point is located on this line, 5 mm behind the posterior vaginal wall (not in the contrast filling tube). The posterior vaginal wall is visualised, depending upon the technique, by means of an intravaginal mould or by opacification of the vaginal cavity with radio-opaque gauze used for packing. On the frontal radiograph, this reference point is taken at the intersection of (the lower end of) the intrauterine source through the plane of the vaginal sources. 8.5.3 Clinical significance of the bladder and rectum reference points The clinical significance of the bladder and rectum reference points has been questioned and is still a matter for debate (7,28,39,51,57,67,97,99,100). A significant number of observations support a correlation between rectal complications and the dose to the rectum reference point (19,77,93,104). In contrast, the bladder reference point was found to be reproducible, but does not correlate well with bladder complications. All investigators found that the actual maximum dose to the bladder was, in most cases, significantly higher than the dose at the ICRU reference points, usually located more cranially. Indeed, some centres calculate the dose 1.5 - 2 cm cranial (Point ALG) relative to the ICRU Reference Point (8,27). However, because of variation in individual anatomical conditions, extent of disease, applicator design and technique, it has not been possible so far to agree on new reference points to replace the points recommended in ICRU Report 38 (43) . Different vaginal reference points are used in different traditions, e.g. at 5 mm lateral from the surface of the ovoids at the level of the vaginal sources (see Fig 6.24C) or at the vaginal surface (25). No international agreement has been reached on a definitive vaginal reference point. 8.5.4 Volume approach to evaluate doses in Organs At Risk The development of 3-D image based treatment planning has significantly changed the situation. Dose distribution can now be evaluated in different volumes such as the GTV, CTV, PTV, Treated Volume and Organs at Risk, such as bladder, rectum, sigmoid, bowel (24,27,29,89,95,97,104). Dose-volume histograms can be derived and analysed but there are not yet any generally agreed methods (see Chapter 14). In the present situation, following values are recommended for reporting doses to the Organs at Risk : 1°) The maximum bladder and rectum doses. The maximum dose to consider is the dose received in a volume of at least 2 and 5 cm 3 ; 2°) The volume of the Organ At Risk that receives a dose close to or higher than the dose considered to be significant in relation to tolerance. Because of lack of definitive data, volumes corresponding to dose levels of 60- 90 Gy should be considered. The volumes should be reported in cm 3 (absolute values) and as a percentage of the organ volume.