29 Skin Cancer

Skin Cancer

10

THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 30/04/2017

Fig. 31.13: a) Dosimetry of the Valencia applicator™, and b) comparison of isodoses with the Leipzig applicator™ (in red the prescription isodoses at 3 mm depth for both applicators).

Table 31.1: Geometric relations between active source length, intersource spacing, and treated length, thickness and width in single plane, double plane squares and triangles for a stepping source implant with parallel and equidistant source configurations [28].

Treated length / Radioactive length

Treated thickness / spacing

Lateral margin/ spacing

Safety margin/ spacing

Implant type

2 lines

0.8

0.5

0.37

_

n lines in one plane

0.8

0.6

0.33

_

n lines in «square»

0.8

1.55 to 1.60

_

0.27

n lines in triangle

0.8

1.3

_

0.20

and Xoft™), the gradient is 10% more dose per mm, and 7% with Esteya™ (Fig. 31.13). Withmoulds or masks, prescription doses may lie between 3-6 mm depth. In those cases, some points of the skin can receive even 150% of the prescribed dose due to the steep dose gradient. Because of this reason, it is advisable to keep plastic tubes at a distance at least of 2 -5mm from the skin, by using wax, bolus or masks, in order to get a more homogeneous distribution on the skin and a lower gradient of dose in depth (Fig. 31.14 and 15). If we need to treat a deeper target, an interstitial procedure is advisable. Unlike interstitial implants, where no variations can happen between fractions and then CTV is equal to PTV, contact brachytherapy requires several fractions and generates some uncertainties, which should be compensated with a small margin in the lateral direction, therefore, a PTV is required. Interstitial implants should be planned with forecast dosimetry, using the rules of the Paris System. Based on the dimensions of the CTV: thickness, width and length, the number of planes, the number of source lines and the spacing between them are determined. A CTV smaller than 12 mm can be treated with 1 plane. The treated thickness is 0.5 times the spacing for 2 lines, and 0.6 times for more than 2 lines. The treated width is n times the spacing plus 2 times the lateral margin (see table 1). The length is 0.8 times the Stepping Source Length (compare chapter 7).

When geometrical optimization is applied in interstitial brachytherapy with stepping source afterloaders, the Paris System rules can be modified to the Stepping Source Dosimetry System (SSDS) of van der Laarse [24]. The treated length improves to 85% of the Stepping Source Length and the prescription isodose may be changed is to the 90% isodose instead of the 85% of the Mean Central Dose. 9.2 3D-dose planning 3 D dose planning is not possible with surface contact applicators (not CT-compatible metallic applicators), but is feasible with plastic tubes applied in individual shaped contact moulds, wax, bolus or masks. With the Stepping Source technology the depth of treatment and individual adaptation on individual target contours become possible.The GTV and CTV can be delineated on CT scans (MRI) with the applicator in place. Using 3D treatment planning an individual 3D treatment plan can be generated. As at present there is no clinical evidence available for 3D prescription and reporting, the 3D treatment planning starts with the traditional prescription of depth and surface dose according to 2D clinical practice and then improves on D90 and D98 values for optimal target coverage. All values from 2D and 3D should be recorded and reported.