MagicPlate-512: A 2D silicon detector array for quality assurance of stereotactic motion adaptive radiotherapy

Abstract

Purpose: Spatial and temporal resolutions are two of the most important features for quality assurance instrumentation of motion adaptive radiotherapy modalities. The goal of this work is to characterise the performance of the two-dimensional high spatial resolution monolithic silicon diode 15 array named "MagicPlate-512" for Quality Assurance of Stereotactic Body Radiation Therapy (SBRT) and Stereotactic Radiosurgery (SRS) combined with a dynamic multi leaf collimator (MLC) tracking technique for motion compensation. Methods:MagicPlate-512 is used in combination with the movable platform HexaMotion and a research version of radiofrequency tracking system Calypso driving MLC tracking software. We 20 reconstruct 2D dose distributions of small field square beams in three modalities: in static conditions, mimicking the temporal movement pattern of a lung tumor, and tracking the moving target while the MLC compensates almost instantaneously for the tumor displacement. Use of Calypso in combination with MagicPlate-512 requires a proper radiofrequency interference shielding. Impact of the shielding on dosimetry has been simulated by Geant 4 and verified experimentally. Temporal and spatial 25 resolution of the dosimetry system allows also for accurate verification of segments of complex stereotactic radiotherapy plans with identification of the instant and location where a certain dose is delivered. This feature allows for retrospective temporal reconstruction of the delivery process and easy identification of error in the tracking or the multileaf collimator driving systems. A sliding MLC wedge combined with the lung motion pattern has been measured. The ability of the MP512 in 2D 30 dose mapping in all three modes of operation was benchmarked by EBT3 film. Results:Full width at half maximum and penumbra of the moving and stationary dose profiles measured by EBT3 film and MagicPlate-512 confirm that motion has a significant impact on the dose distribution. Motion, no motion and motion with MLC tracking profiles agreed within 1 mm and 0.4 mm, respectively, for all field sizes tested. Use of electromagnetic tracking system generates a 35 fluctuation of the detector baseline up to 10% of the full scale signal requiring a proper shielding strategy. MagicPlate-512 is also able to reconstruct the dose variation pulse-by-pulse in each pixel of the detector. An analysis of the dose transients with motion and motion with tracking shows that the tracking feedback algorithm used for this experiment can compensate effectively only the effect of the slower transient components. The fast changing components of the organ motion can contribute only 40 to discrepancy of the order of 15% in penumbral region while the slower components can change the dose profile up to 75% of the expected dose. Conclusions:MagicPlate-512 is shown to be, potentially, a valid alternative to film or 2D ionizing chambers for Quality Assurance dosimetry in SRS or SBRT. Its high spatial and temporal resolution allows for 45 accurate reconstruction of...

On Monolithic Silicon Array Detectors for Small-Field Photon Beam Dosimetry

“…The Centre for Medical Radiation Physics (CMRP), University of Wollongong, has designed and characterized two generations of 2D monolithic silicon array devices. The 1 st generation (MP512 and Duo [4], [6], [20]) was fabricated on a bulk p-type silicon substrate and featured 512 SVs. In the case of the MP512 the SVs were uniformly distributed on the silicon wafer surface with a 2 mm pitch, whereas for the Duo they were arranged with 200 μm pitch along 2 linear orthogonal arrays.…”

“…Whilst the MP512 and the Duo performed excellently under flattened beam (FB) irradiation in terms of OFs, OARs, PDD distribution, with a small dose per pulse (DPP) dependence [4], [6], [20], their applicability for small-field dosimetry was impaired by the coarse spatial resolution of the MP512 and by the limited spatial characterization of the 2D dose map offered by the Duo.…”

Initial testing of a pixelated silicon detector prototype in proton therapy

“…The addition of either a second orthogonal array of SV's or a complete 2D array of SV's would also allow for concurrent measurement of point dose and orthogonal beam profiles (or the entire beam profile in the case of a 2D array), which would provide further QA information in pencil beam scanning applications where the size and shape of the proton beam also need to be evaluated. The feasibility of this approach was recently demonstrated by the CMRP in development, characterization, and application of a 2D monolithic silicon pixel array (with submillimeter SVs and a pitch of 2 mm) in dose mapping on a medical linac 26, 27…”

CyberKnife® fixed cone and Iris™ defined small radiation fields: Assessment with a high‐resolution solid‐state detector array

“…2) has a submillimeter resolution with diodes having a 0.3 mm pitch along the vertical and horizontal arrays and 0.43 mm pitch along the two diagonal arrays. The diodes are operated in passive mode and are connected to a multichannel readout electronics data acquisition (DAQ) system based on a commercially available analogue front end (AFE0064, Texas Instruments), which was previously described in detail 22, 23. An equalization procedure24 is used to correct for small differences in each channel response.…”