9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 205

The dose rate of the brachytherapy application may be different from the conventional low dose rate (0.5 Gy/hour): e.g. 1.5 Gy/hour (LDR), 20-30 Gy/hour (HDR). The dose level selected to define the reference volume is the dose which is considered to be “equivalent” to or “isoeffective” with the dose delivered at the conventional low dose rate, in the same clinical condition (Fig 6.28). A biological weighting factor, W rate , thus has to be applied to compensate/normalize for differences in dose rate. The difficult radiobiological problem raised by the selection of this weighting factor is discussed in Chapter 4. The value of W rate actually applied should always be based on widely accepted models and clearly reported. It should be kept in mind that the biological weighting factor W rate is valid only for a given tissue, effect and dose/dose rate. Following the same example as above, if 85 Gy is considered appropriate to define the reference volume for conventional low dose rate, and if 45 Gy external beam therapy has been delivered, a dose of 85 Gy – 45 Gy = 40 Gy defines the reference volume at LDR brachytherapy. For HDR brachytherapy, 4 fractions of 7 Gy are considered to correspond to 40 Gy LDR brachytherapy according to biological models and clinical experience (79,88). The reference volume is thus defined as the volume encompassed by the 4 x 7 Gy isodose (assuming, α/β = 10 for the effects on the tumour, see chapter 4). The three orthogonal dimensions and the volume of that reference volume (7 Gy x 4) must be reported as recommended above, and can be compared with the dimensions of other brachytherapy applications performed with HDR, MDR, or LDR (88). An analogue procedure must be followed when using the 60 Gy reference volume. NB : Additional recommendations for reporting the 3 dimensions of the volumes (1) As an additional recommendation, the width (d wA ) and the thickness (t wA ) of the volumes measured in the planes passing through Point A may also be reported. Indeed these dimensions are closely related to the GTV/PTV2. (2) The right and left dimensions of the width and the anterior and posterior dimensions of the thickness of the volumes should be reported separately if they differ significantly with regard to the intrauterine source (e.g., if different loadings of vaginal sources are used). (3) The 3 orthogonal dimensions of the volumes should be reported individually and not their product. Firstly, for cervix carcinoma, because of the „pear shape“, the actual computed volumes are much smaller (about 50%) compared with the product of their 3 dimensions. Secondly, the lateral and AP-PA dimensions are clinically meaningful for tumour control with regard to parametrial extension and complications, respectively. 8.4.4 Volume approach based on 3-D imaging and computer assisted 3-D dose calculation Development of 3-D image based treatment planning in brachytherapy has significantly improved the 3 D assessment of dose-volume relations. Dose distribution can now be evaluated in different volumes such as the GTV, CTV and PTV (24,27,29,89,95,97,104,105). Dose-volume histograms can be derived and analysed but the methods for deriving the relevant information still need to be defined and no agreement has been reached so far (compare Fig 5.15,14.14,16.7,17.10). In order to link the information gained so far from extensive clinical experience before these new tools were available, correlations of coverage of GTV and PTV with doses at point A and with reference volumes, such as 60, 75, and 85 Gy volumes should be analysed (24).