Matthew Newall

A 2D silicon detector array for quality assurance in small field dosimetry: DUO

Purpose: Nowadays, there are many different applications that use small fields in radiotherapy treatments. The dosimetry of small radiation fields is not trivial due to the problems associated with lateral disequilibrium and source occlusion and requires reliable quality assurance (QA). Ideally such a QA tool should provide high spatial resolution, minimal beam perturbation and real time fast measurements. Many different types of silicon diode arrays are used for QA in radiotherapy; however, their application in small filed dosimetry is limited, in part, due to a lack of spatial resolution. The Center of Medical Radiation Physics (CMRP) has developed a new generation of a monolithic silicon diode array detector that will be useful for small field dosimetry in SRS/SRT. The objective of this study is to characterize a monolithic silicon diode array designed for dosimetry QA in SRS/SRT named DUO that is arranged as two orthogonal 1D arrays with 0.2 mm pitch. Methods: DUO is two orthogonal 1D silicon detector arrays in a monolithic crystal. Each orthogonal array contains 253 small pixels with size 0.04 × 0.8 mm2 and three central pixels are with a size of 0.18 × 0.18 mm2 each. The detector pitch is 0.2 mm and total active area is 52 × 52 mm2. The response of the DUO silicon detector was characterized in terms of dose per pulse, percentage depth dose, and spatial resolution in a radiation field incorporating high gradients. Beam profile of small fields and output factors measured on a Varian 2100EX LINAC in a 6 MV radiation fields of square dimensions and sized from 0.5 × 0.5 cm2 to 5 × 5 cm2. The DUO response was compared under the same conditions with EBT3 films and an ionization chamber. Results: The DUO detector shows a dose per pulse dependence of 5% for a range of dose rates from 2.7 × 10−4 to 1.2 × 10−4 Gy/pulse and 23% when the rate is further reduced to 2.8 × 10−5 Gy/pulse. The percentage depth dose measured to 25 cm depth in solid water phantom beyond the surface and for a field size of 10 × 10 cm2 agrees with that measured using a Markus IC within 1.5%. The beam profiles in both X and Y orthogonal directions showed a good match with EBT3 film, where the FWHM agreed within 1% and penumbra widths within 0.5 mm. The effect of an air gap above the DUO detector has also been studied. The output factor for field sizes ranging from 0.5 × 0.5 cm2 to 5 × 5 cm2 measured by the DUO detector with a 0.5 mm air gap above silicon surface agrees with EBT3 film and MOSkin detectors within 1.8%. Conclusions: The CMRPs monolithic silicon detector array, DUO, is suitable for SRS/SRT dosimetry and QA because of its very high spatial resolution (0.2 mm) and real time operation.

Characterisation of Silicon Diode Arrays for Dosimetry in External Beam Radiation Therapy

Modern stereotactic radiation therapy modalities utilize small beams and large dose gradients to deliver radiation in few fractions, reducing the possibility to correct for mistakes during the treatment process. Therefore, in order to ensure best possible treatment for the patient, quality assurance for such treatments necessitates a stable, linear, and sensitive radiation detector with high spatial resolution and radiation hardness. In this work, two silicon detector arrays with high spatial resolution have been characterized by 6 MV and 18 MV medical LINAC irradiation, and 5.5 MeV He2+ heavy ion microprobe. A maximum discrepancy of 0.6 mm in field size has been found when comparing to two-dimensional radiochromic film dose profile, and charge collection efficiency obtained by means of ion beam induced charge collection (IBICC) is 66% when operating the array in photovoltaic mode. Radiation damage study by photons and photoneutrons is presented.

Real‐time high spatial resolution dose verification in stereotactic motion adaptive arc radiotherapy

Abstract Purpose Radiation treatments delivered with real‐time multileaf collimator (MLC) tracking currently lack fast pretreatment or real‐time quality assurance. The purpose of this study is to test a 2D silicon detector, MagicPlate‐512 (MP512), in a complex clinical environment involving real‐time reconfiguration of the MLC leaves during target tracking. Methods MP512 was placed in the center of a solid water phantom and mounted on a motion platform used to simulate three different patient motions. Electromagnetic target tracking was implemented using the Calypso system (Varian Medical Systems, Palo Alto, CA, USA) and an MLC tracking software. A two‐arc VMAT plan was delivered and 2D dose distributions were reconstructed by MP512, EBT3 film, and the Eclipse treatment planning system (TPS). Dose maps were compared using gamma analysis with 2%/2 mm and 3%/3 mm acceptance criteria. Dose profiles were generated in sup‐inf and lateral directions to show the agreement of MP512 to EBT3 and to highlight the efficacy of the MLC tracking system in mitigating the effect of the simulated patient motion. Results Using a 3%/3 mm acceptance criterion for 2D gamma analysis, MP512 to EBT3 film agreement was 99% and MP512 to TPS agreement was 100%. For a 2%/2 mm criterion, the agreement was 95% and 98%, respectively. Full width at half maximum and 80%/20% penumbral width of the MP512 and EBT3 dose profiles agreed within 1 mm and 0.5 mm, respectively. Patient motion increased the measured dose profile penumbral width by nearly 2 mm (with respect to the no‐motion case); however, the MLC tracking strategy was able to mitigate 80% of this effect. Conclusions MP512 is capable of high spatial resolution 2D dose reconstruction during adaptive MLC tracking, including arc deliveries. It shows potential as an effective tool for 2D small field dosimetry and pretreatment quality assurance for MLC tracking modalities. These results provide confidence that detector‐based pretreatment dosimetry is clinically feasible despite fast real‐time MLC reconfigurations.

The angular dependence of a two dimensional monolithic detector array for dosimetry in small radiation fields

The purpose of this study is to investigate the directional dependence of a two dimensional monolithic detector array (M512) under 6 MV photon irradiation and to evaluate the effect of field size on angular dependence. Square fields of sizes: 3x3 cm2 and 10x10 cm2 were measured at the iso-centre of a cylindrical phantom. Beam angles with incidences from 00- 1800 in increments of 150 were used to investigate the central pixel angular response of M512, normalized to the pixel response for normal (0°) beam incidence. The angular response of the detector was compared to the response of EBT3 radiochromic film in the identical geometric orientation. The maximum angular dependence was observed at the angle 90°±15° to be -18.62% and -17.70% for the field sizes 3x3 cm2 and 10x10 cm2, respectively. The angular dependence of M512 showed no significant difference between field sizes of 3x3 cm2 and 10x10 cm2 (p>0.05). The maximum dose difference measured by the central pixel of M512 and EBT3 for all angles are -20% for 3x3 cm2 field size and -18.58% for the 10x10 cm2 field. The diode arrays size and packaging effects the angular response of the detector. The angular correction factor is necessary to apply to increase accuracy in dosimetry for arc treatment delivery.

Introducing dynamic dosimaging: Potential applications for MRI-linac

The new era of intra-fraction dose tracking in radiation therapy delivery demands new dosimetry methods, whereby a moving frame of reference as a function of time may be required. This introduces a new paradigm into radiation therapy dose verification. The term we propose to describe this is dynamic dosimaging, which by our definition is tracking the location of a dosimeter array in real time during on-line radiation dose acquisition.

Innovative detectors for quality assurance dosimetry in SBRT of stationary and movable targets

The high spatial and temporal resolution 2D monolithic silicon detector arrays M512 and DUO for quality assurance (QA) in real time motion adaptive radiotherapy (ART) have been developed. The DUO array possesses a spatial resolution of 0.2 mm and has demonstrated agreement within 5% with EBT3 film measurements of 6MV linac beam profiles for field sizes 1 × 1 cm2 and SRS cone diameter 0.5 cm. Dynamic characterisation of the M512 for QA in real time ART evaluated the performance of M512 for small fields while the detector experiences periodic motion. It was demonstrated with M512 that MLC tracking with Calypso electromagnetic array compensates for the periodic motion and improves the agreement between static and dynamic beam profiles for field size 1 × 1 cm2 from within 75% in the penumbra to within 11% agreement. The dynamic profile is returned to a similar distribution as the static case.

OC-0551: High spatial and timing resolution silicon based dosimeter for quality assurance of real time adaptive radiotherapy

Conclusions: The potential of the contrast enhancement method in increasing the applicability of markerless lung tumour tracking based on kV imaging was demonstrated on clinical data. A time-resolved (4D) CT scan can be used instead of the breath-hold CT scan available for this study in order to improve the robustness in the background subtraction operation, especially in case of lower lobe tumours for the compensation of diaphragm motion due to breathing. [1] Yang Y et al, IJROBP 2012;82:e749-56. [2] Spadea MF et al, IJROBP 2014;90:628-36.