9 Reporting in Brachytherapy: Dose and Volume Specification

Reporting in Brachytherapy 189

Fig 6.19C (continued) reported are at 5 mm (100%) and at the applicator/lumen surface 290%. This surface represents mainly the macroscopic tumour and only partly the mucosa. The lowest dose within the oesophageal wall and within the GTV is < 50%.

7.5 Dimensions of volumes for reporting (Fig 6.20) 7.5.1 The reference volume

Definition

The reference volume, in intraluminal brachytherapy, is defined as the tissue volume encompassed by the isodose corresponding to 90% of the dose at the reference point (see section 6.7.4.4).

Length of the reference volume

The length of the reference volume is defined parallel to the (main) lumen axis or source axis, at the reference depth, i.e. 5 mm from the mucosa (1 and 2 mm for intravascular brachytherapy) as defined in section 7.4.2. (Fig 6.20) The lengths are defined along the direction of the (main) axis of the source (or source line). However, the lengths measured on the axis or the lumen themselves are obviously irrelevant. NB : The length of the reference volume was called reference isodose length in the EVA GEC ESTRO Report (86) (Fig 6.20A). 7.5.2 Treated Volume The Treated Volume is encompassed by an isodose surface corresponding to the Minimum Target Dose. (compare 2.4) The length of the Treated Volume is measured along the direction of the main axis at the depth of the CTV. The diameter of the Treated Volume may vary along the axis dependent on the shape of the CTV. 7.5.3 Active source length and length of the Treated- and Reference Volume A distinction has to be made between the active source length, as defined in section 6.3.1, which is a technical characteristic of the source itself and the lengths of the resulting Treated Volume and reference volume, which are clinical and dosimetric concepts (Fig 6.20). For the same active source length and total activity, the lengths of the Treated Volume and reference volume, and the dose distribution may be different for a (solid) linear source and a set of point sources. These differences are even greater when the point sources have different activities and/or different spacings to improve dose distribution (20). The same is true for a moving source with different dwell times. The ratio between the active source length and the length of the PTV or Treated Volume length (measured at a given depth) may be different, sometimes significantly, depending on the dose fall-off around the extremities of the source. This in turn depends on the source characteristics and the lumen diameter.