Dose measurement verification in solid state phantom in place of field connection for non-standard radiotherapy conditions

Abstract

Background

This work describes chosen combinations of therapeutic fields during the patient's total body irradiation (TBI). The TBI technique requires as large a radiation field as one can achieve [1][2]. That is why the source-skin distance (SSD) is greater than 100 cm (SSD for standard treatment conditions). In such non-standard radiotherapy conditions all the measurements described in this paper were done.

Materials/Methods

All beam profiles were obtained by irradiating films with 20 MV (nominal energy) photon rays in non-standard conditions of radiotherapy. The method of measurement (film dosimetry) and used materials (self-developing GAFCHROMIC EBT films) are presented in the Material and Method section [3].

Aim

Because in the therapy some areas must receive a greater dose than others, the size of the therapeutic fields must be adjusted. That is why the areas where two fields overlap inside the patient's body differ. The diversity of absorbed dose in these areas was measured and presented in the schemas in the Results section.

Results

The profiles presenting dose distribution in the areas where therapeutic fields overlap in most cases show increase of the delivered dose. For the most often used therapeutic fields the increase exceeds 180% of the planned dose in the sector about 2.5 – 10 cm. There were also two cases where the delivered dose was lower than the planned one (about 29 – 86% of the planned dose). Chosen measurements and combinations of the therapeutic fields are discussed in detail and all the results are collected in a table at the end of the section.

Conclusions

The profiles obtained from the scans of the irradiated GAFCHROMIC EBT films and their digital processing include 0.05 Gy noise. This means that the described method requires a high quality scanner, dedicated to RTG films. The spectrophotometer measurements showed high film dependency on light wavelength. It seems that using a monochromatic source of light may give better results.