Session Item

Total Body Irradiation (TBI) of mice is used to generate models of diseases such as leukaemia, by suppressing the haematopoietic system prior to systemic tumour cell engraftment. Such models can be used to evaluate the outcomes of different therapeutic methods.

Dosimetry of TBI in image guided preclinical irradiators is challenging. The main problem arises from the variety of pre-clinical irradiation conditions and the difficulties in relating those to the recommendations from well-established clinical dosimetry protocols in the range of medium energy x-rays.

In this study, a methodology for accurate and standardized deliveries of dose to mice during TBI was experimentally determined. Dose across different regions of the mouse and the effect of the mouse positioning system were assessed. As a consequence, a reproducible irradiation protocol, for the use of the Small Animal Radiation Research Platform (SARRP), was established.

The SARRP’s reference dosimetry was performed with the local secondary standard system with a calibration traceable to the National Physical Laboratory’s Air Kerma primary standard. Measurements were performed in a solid water slab phantom following manufacturer’s recommendations. Alanine verification of the output was determined, in the same conditions of irradiation of the ionization chamber measurements. [1]. 

EBT3 Gafchromic film and alanine pellets were subsequently used to measure dose delivered to three mice phantoms, mimicking positioning of real mice during TBI irradiations. The mice phantoms were different in their material composition [2] and the type of detectors that they were able to hold: film, alanine or both (Figure 1). Irradiation times to film and alanine were different, considering that alanine uncertainties are lower for dose over 10 Gy. 

Dose to film in the abdominal region of the Gray and White mice phantoms compared within 3.2%. Uncertainty based on the standard deviation of the mean (SDOM) of three independent repetitions was 2.3% Dose to alanine in the abdominal region of the Brown and White phantoms agreed within 2.1% with 0.6% uncertainty. Film and alanine dose at approximately the same position in the abdominal region of the White phantom, agreed within 2%. On average (both detectors and three phantoms), the dose rate (DR) (measured dose/irradiation time) to a point in the mid plane of the mouse in the abdominal region was 2.7 Gy/min. The DR currently used to calculate TBI irradiation times within our institute (based on 2018 measurements with an inadequate dosimetric system) was 2.37 Gy/min.

Previous to this work, the dose delivered during TBI irradiations was 13% larger than required. The DR previously used to calculate irradiation times did not consider the differences between reference output measurement conditions and real setup of mice during TBI experiments. A standardized protocol with a detailed description of the irradiation conditions and table for time calculation is now routinely used.