Evaluation of a pixelated large format CMOS sensor for x‐ray microbeam radiotherapy

Abstract

Purpose Current techniques and procedures for dosimetry in microbeams typically rely on radiochromic film or small volume ionization chambers for validation and quality assurance in 2D and 1D, respectively. Whilst well characterized for clinical and preclinical radiotherapy, these methods are noninstantaneous and do not provide real time profile information. The objective of this work is to determine the suitability of the newly developed vM1212 detector, a pixelated CMOS (complementary metal‐oxide‐semiconductor) imaging sensor, for in situ and in vivo verification of x‐ray microbeams. Methods Experiments were carried out on the vM1212 detector using a 220 kVp small animal radiation research platform (SARRP) at the Helmholtz Centre Munich. A 3 x 3 cm2 square piece of EBT3 film was placed on top of a marked nonfibrous card overlaying the sensitive silicon of the sensor. One centimeter of water equivalent bolus material was placed on top of the film for build‐up. The response of the detector was compared to an Epson Expression 10000XL flatbed scanner using FilmQA Pro with triple channel dosimetry. This was also compared to a separate exposure using 450 µm of silicon as a surrogate for the detector and a Zeiss Axio Imager 2 microscope using an optical microscopy method of dosimetry. Microbeam collimator slits with range of nominal widths of 25, 50, 75, and 100 µm were used to compare beam profiles and determine sensitivity of the detector and both film measurements to different microbeams. Results The detector was able to measure peak and valley profiles in real‐time, a significant reduction from the 24 hr self‐development required by the EBT3 film. Observed full width at half maximum (FWHM) values were larger than the nominal slit widths, ranging from 130 to 190 µm due to divergence. Agreement between the methods was found for peak‐to‐valley dose ratio (PVDR), peak to peak separation and FWHM, but a difference in relative intensity of the microbeams was observed between the detectors. Conclusions The investigation demonstrated that pixelated CMOS sensors could be applied to microbeam radiotherapy for real‐time dosimetry in the future, however the relatively large pixel pitch of the vM1212 detector limit the immediate application of the results.

Investigating Slit-Collimator-Produced Carbon Ion Minibeams with High-Resolution CMOS Sensors

“…Due to their high spatial resolution, the MIMOSA-28 sensors are well suited for precision particle fluence measurements [21], similar to those required for characterizing the fluence profile at the MBC slits. This corroborates previous works on X-ray minibeam therapy [18] and pMBRT [19], which already indicated promising potential of CMOS technology for providing high-resolution dosimetry in minibeam experiments. The CMOS sensor high resolution revealed a shoulder at the base of the slit lateral fluence profiles at small distance to the collimator exit (Figure 4d), stemming from edge scattering effects at the corners of the MBC slits.…”

“…Experimental characterization of the brass MBC was performed with clinical carbon ion beams at the Marburg Ion-Beam Therapy Center (MIT), employing a high-resolution CMOS monolithic active pixel sensor, the MIMOSA-28 [17] sensor originally developed for the STAR experiment at RHIC. CMOS sensors have been presented in the literature to be useful tools for X-ray minibeam experiments [18] and recently also for pMBRT [19]. The MIMOSA-28 sensor employed in our study has already been characterized for various incident ion beams [20], demonstrating its capability of discerning secondaries and primaries based on their energy loss in the sensor.…”

Development of CMOS dosimetry in proton minibeams for enhanced QA and primary standard absorbed dose calorimetry

“…Relative Dose (log) manufacturer [43], and the film response would be especially sensitive to any slight miscalibration in this region. This again highlights the versatility of sensitive CMOS detectors for measurement of low dose valleys as previously demonstrated for x-ray microbeams [25].…”

“…The vM2428 detector, also referred to as Athena or LASSENA [23], is a large-format Complementary Metal-Oxide-Semiconductor (CMOS) sensor designed for scientific and medical x-ray imaging. Although it was not designed for pMBRT, previous investigations at University College Hospitals London NHS Foundation Trust (UCLH) [24] have demonstrated that the detector was a viable candidate for use with proton beams, whilst the precursor to the vM2428 detector, the vM1212 detector, was found to be able to measure static and dynamic x-ray microbeams in a Small Animal Radiation Research Platform (SARRP) [25,26]. The vM2428 detector's specifications are described further in table 1.…”

Clinical microbeam radiation therapy with a compact source: specifications of the line-focus X-ray tube

“…The generated x-rays traveled through a 0.8 mm beryllium window, aluminum or copper filters of different thickness, and a microbeam collimator (field size , divergent slits of [23] , [24] ), which was positioned 50 cm from the focal spot. The absorber thickness of the collimator was adjusted to the maximum photon energy for less than 0.25 % leakage radiation.…”