Dose distributions in the eye from Ru applicator

Abstract

Beta irradiation with ¹⁰⁶Ru applicator is one of the methods of treatment of the tumours located in eye.

Method

In this method we give a dose of 60 Gy to the tumour top. The tumaur top is located between 2 and 10mm from eye surface and it is defined as the border of cancer infiltration. We use echogram to border localisation. From the source certificate we know that the dose rate is charged with a 30% determination error. We increased the calculated dose considering the error value sa that at least 60 Gy dose to the tumour top was delivered.

The critical dose for the sclera is 1500 Gy and for the lens 15 Gy respectively. We calculated the maximal dose by the same procedure as the minimal dose through modifying dose rate by certificate error. We calculated doses in the sclera in three places: on the sclera's surface, in the middle of it (about 0.6 mm depth) and on the sclera's bottom edge which is the base of the tumaur (about 1.2 mm depth).

Results

Nineteen patients were treated. In all cases the tumour top received at least 60 Gy. In 4 cases it was impossible to deliver a sufficient dose (minimum 60 Gy) at the tumour top without exceeding 1500 Gy in the sclera. It was because of the tumour size was too large (more than 8 mm). In these cases, there was a probability of exceeding 1500 Gy on the sclera's surface when we took the maximal dose rate option. In 4 cases the dose in the lens exceeded a critical dose but we have got the medical agreement to do that.

The important thing is to check the tumour size because the applicator should cover the cancer.

This method of treatment time calculation is fast and easy. We have written the computer program for that procedure which reduced the time of calculation and preparation of the protocol up to a few minutes.