22 Penis Cancer

486 Penis Cancer

The dose to the testis should be as low as possible. Some authors use lead shielding, but it is poorly tolerated. Therefore, we recommend to use a sponge to keep the radioactive sources away from the penis. This maintains the organ in a semierect position and maximizes the distance between the testis and the sources (Fig 21.8) At the end of the implant, the hypodermic needles are loaded with Iridium wires (Fig 21.9) and radiographs are taken (Fig 21.10).

8

Dosimetry

8.1 For plesiobrachytherapy the reference isodose must be chosen according to the tumour volume (GTV). Two parameters are considered : thickness and external boundaries of the rumour. The CTV includes the thickness of the tumour plus a safety margin of 5mm, and its boundaries plus a safety margin of 10mm, taking into account the position of the urethra. 8.2 For interstitial brachytherapy the classical rules of the Paris System can be used without difficulty, since the two templates are perfectly parallel and the position of the needles going through the selected perforations are perfectly parallel and equidistant (see Fig 21.10). (27) In the Paris System, the recommended spacing between the radioactive lines is 8 to 22mm. For carcinoma of the penis, we recommend a distance between 15 and 18mm, allowing 2 - 8 needles to be implanted (mean ≈ 5 (11,30)). Sometimes, to encompass the target volume (PTV) adequately, some peripheral needles will be in air (Fig 21.11). These needles are implanted through the templates and the dose distribution in the air is comparable to the dose distribution in the glans tissue. (12)

Fig 21.11: Interstitial brachytherapy combined with plesiobrachytherapy (two needles are in the air) to adapt PTV to GTV.

9 Dose, Dose Rate, Fractionation • For LDR plesiobrachytherapy as well as for LDR interstitial brachytherapy the minimal target dose (PTV) is usually 65 Gy delivered in 6 - 7 days at a dose-rate of 40cGy per hour. The mean central dose is 75Gy. The treated volume is dependent on the target volume, mean 50 cm 3 . 18,19 Some