The development of technology for effective respiratory-gated irradiation using an image-guided small animal irradiator

The development of image-guided small animal irradiators represents a significant improvement over standard irradiators enabling preclinical treatments to mimic radiotherapy in humans. The ability to deliver tightly collimated targeted beams, in conjunction with gantry or animal couch rotation, has the potential to maximise tumour dose while sparing normal tissues. However the current commercial platforms do not incorporate respiratory gating as required for accurate and precise targeting in organs subject to breath-related motions, which may be up to the order of 5 mm in mice. Therefore a new treatment head assembly for the Xstrahl Small Animal Radiation Research Platform (SARRP) has been designed. This includes a fast x-ray shutter subsystem, a motorised beam hardening filter assembly, an integrated transmission ionisation chamber to monitor beam delivery, a kinematically positioned removable beam collimator and a targeting laser exiting the centre of the beam collimator. The x-ray shutter not only minimises timing errors but also allows beam gating during imaging and treatment, with irradiation only taking place during the breathing cycle when tissue movement is minimal. The breathing-related movement is monitored by measuring, using a synchronous detector/lock-in amplifier that processes diffuse reflectance light from a modulated light source. Following thresholding of the resulting signal, delays are added around the inhalation/exhalation phases, enabling the ‘no movement’ period to be isolated and to open the x-ray shutter. Irradiation can either be performed for a predetermined time of x-ray exposure, or through integration of current from the transmission monitor ionisation chamber (corrected locally for air density variations). The ability to successfully deliver breathing gated x-ray irradiations has been demonstrated by comparing movies obtained using planar x-ray imaging with and without breathing gating, in addition to comparing dose profiles observed from a collimated beam on EBT3 radiochromic film mounted on the animals’ chest. Altogether, the development of breathing gated irradiation facilitates improved dose delivery during animal movement and constitutes an important new tool for preclinical radiotherapy studies. This arrangement is particularly well suited for treatments of orthotopic tumours or other targets within the chest and abdomen where breathing related movement is significant