16 Cervix Carcinoma

340 Cervix Cancer

In case of standard applications the dimensions and the volume are known from precalculated dose distributions, in case of individualised geometry, the dimensions and the volume are to be calculated case by case. For reporting brachytherapy alone (with or without external beam therapy) the dimensions of the 60 Gy Reference Volume have to be indicated encompassed by the 60 Gy isodose by brachytherapy only (h,w,t). For reporting brachytherapy combined with external beam therapy, the dose for external beam therapy has to be subtracted from 60 Gy and the reference volume for the resulting isodose has to be indicated: e.g if 45 Gy are given with open field external beam therapy, this is 15 Gy, and the reference volume encompassed by the 15 Gy isodose of brachytherapy is to be reported. In addition, the dimensions are to be recorded as Height, Width, and Thickness (H,W,T).

Fig 14.15: ICRU 38 Recommendations: A: Dimensions of the Reference Volume AP and lateral (compare for details also Fig 6.27) The dose level should be selected in such a way that the Reference Volume should not be too different from the Treated Volume (i.e. the volume receiving the dose considered as appropriate to achieve the goal of the treatment). If the selected dose level is too low, the Reference Volume becomes spherical; if the selected dose level is too high, the Reference Volume is closely surrounding the individual sources. In both cases, it contains little useful information. The concept of Referenve volume has been introduced by the ICRU for comparisom purposes only. Specification of the 60 Gy Reference Volume does not imply that the ICRU recommeds 60 Gy as prescription dose. However, without an agreement on a dose level it is hardly possible to compare, in a relevant way, applications performed in different centres using different techniques. If this 60 Gy volume is also used for prescription then - from a practical point of view- it looks easier to start with the determination of the isodose enclosing 60 Gy taking into account the clinical situation and other information and particular in which area, in which points, in which volume this dose of 60 Gy must be prescribed. Afterwards, the different data will be reported according to the ICRU recommendations. Absorbed dose at reference points ICRU 38 defines several reference points for reporting: Related to rectum and bladder two reference points are defined which are located relatively close to the sources: the “ICRU-bladder reference point” and the “ICRU-rectum reference point” (Fig 14.15B). They are reproducible and reliable, but they do not necessarily represent the maximum dose to these organs at risk. Therefore, in addition reporting of more points indicating different doses to the organ at risk is encouraged, also mean and maximum doses.