Three‐dimensional dose distribution for partial irradiation of rat parotid glands with 200 kV X‐rays

Abstract

Purpose: To investigate dose distributions in partial‐volume irradiation experiments in small experimental animals, in particular the parotid gland of rat.

Materials and methods: High‐resolution magnetic resonance imaging images were made that provided the outlines of the parotid glands, which were used to design collimators with conformal radiation ports for 100 and 50% cranial/caudal partial‐volume irradiation. A protocol for absolute dosimetry was designed and relative dose measurements were performed. From the three‐dimensional topographical data and the three‐dimensional dose distribution, dose–volume histograms were determined.

Results: The standard uncertainty of absorbed entrance dose was within 3%. Radiochromic film, thermoluminescence dosemeters and ionization chamber dose measurements revealed that the relative doses measured were in good agreement. The 20–80% penumbra of the beam across the 50% field edge was only 0.4 mm at a 6 mm depth. The gradient of the percentage depth dose from the skin of the rat to a depth of 12 mm was 1.5% mm⁻¹. The absorbed doses in the cranial 50% and the caudal 50% partial volumes were comparable. This finding was reflected in the calculated dose–volume histograms of the different regions, which were similar. The dose in the shielded area between the left and right ports was about 14% of the dose near the centres of the beams.

Conclusion: The designed set‐up showed that irradiation of small volumes could be performed with high accuracy allowing the study of differences in radiation damage. Similar doses were given to the 50% cranial and 50% caudal gland volumes and, therefore, a possible difference in radiosensitivity in these volumes was not a dose effect. The approach used was also applicable for the irradiation of small volumes of other tissues.