Microbeam-Radiation-Therapy (MRT): Characterizing a Novel MRT Device Using High Resolution 3D Dosimetry

Purpose:

The feasibility of MRT has recently been demonstrated utilizing a new technology of Carbon-Nano-Tube (CNT) field emission x-ray sources. This approach can deliver very high dose (10's of Gy) in narrow stripes (sub-mm) of radiation, which enables the study of novel radiation treatment approaches. Here we investigate the application of high- resolution (50𝑢𝑚 isotropic) PRESAGE®/Optical-CT 3D dosimetry techniques to characterize the radiation delivered in this extremely dosimetrically challenging scenario.

Methods:

The CNT field emission x-ray source irradiator comprises of a linear cathode array and a novel collimating system. The device delivers small `stripe' beams of approximately X long and Y wide, at an energy of 160 kVp. To characterize the MRT beams, an ultra-high-resolution prototype 3D dosimetry system was constructed and optimized, consisting of two parts: a radiochromic 3D dosimetry material PRESAGE, and a high resolution small field-of-view optical-CT imaging system for dose-readout (DMicrOS - Duke Micro Optical-CT Scanner). Small PRESAGE cylindrical dosimeters (~2.2cm in height and ~2.5cm in diameter) were irradiated by CNT MRT delivering 3 stripes of radiation with a nominal entrance dose of 32 Gy (16Gy for the second batch). PRESGAE dosimeters (with same dimensions) were also irradiated with at 32 Gy entrance dose, with a regular x-ray irradiator collimated to microscopical stripe- beams using a customized cerrobend material collimator. 50𝑢𝑚 (isotropic) 3D dosimetry was performed on all dosimeters using an in-house optical-CT system designed and optimized for high-resolution imaging (including a stray light deconvolution correction). The Percentage Depth Dose (PDD), Peak-to-Valley Dose Ratio (PVR) and beam width (FWHM) data were obtained and analyzed in both cases. Independent verification against EBT2 radiochromic film is ongoing.

Results:

Basic testing of the DMicrOS system indicated the following performance: Modulated-Transfer-Function (MTF), dynamic range, resolution, largest Field-Of-View (FOV), Point-Spread-Function (PSF) were performed. When applied to the PRESAGE dosimeters irradiated with MRT stripe beams, high-resolution 3D images were successfully achieved with the prototype system, enabling extraction of dose profiles. The PDDs for the CNT irradiation showed pronounced attenuation in UNC_A and UNC_C (little attenuation in UNC_F), but less build-up effect than that from the multibeam irradiation. The beam spacing between the three strips has an average value of 0.9mm while that for the 13 strips is 1.5 mm at a depth of 16.5 mm. The spacing between the three strips' barely varies with depth, while the 13 beams exhibit clear divergence. The three stripes show consistent PVR values (the average value is 18 at all depth). The stray light corrected image shows line profiles with reduced noise and consistent PVR values.

Conclusion:

MRT dosimetry is extremely challenging mostly due to little beam divergence tolerance and high dose rate required associated with the ultra small geometry. As a result, various artifacts (ring, donut, "dirty" fluid, imperfection removing the were observed and cannot be removed easily) present in the data. This preliminary application of a novel, ultra-high resolution, optical-CT 3D dosimetry system showed promise (reduced dose diverging, more accurate dose delivery), but suggested extremely careful techniques (flood matching, mounting, rotation stability). Further work is required to further validate the accuracy of dose distribution and investigate the causes of the artifacts as well as their removal methods. Especially, the stray light correction is believed to have a substantial impact in this extreme geometry, further optimizing the correcting methods is necessary to be explored.