Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate

Towards customizable thin-panel low-Z detector arrays: electrode design for increased spatial resolution ion chamber arrays

The purpose of the present development is to employ 3D printing to prototype an ion chamber array with a scalable design potentially allowing increased spatial resolution and a larger active area. An additional goal is to design and fabricate a custom size thin-panel detector array with low-Z components. As a proof of principle demonstration, a medium size detector array with 30 × 30 air-vented ion chambers was 3D-printed using PLA as frame for the electrodes. The active-area is 122 mm × 120 mm with 4 × 4 mm² spatial resolution. External electrodes are cylindrical and made from conductive PLA. Internal electrodes are made from microwire. The array is symmetric with respect to the central plane and its thickness is 10 mm including build-up/-down plates of 2.5 mm thickness. Data acquisition is realized by biasing only selected chamber rows and reading only 30 chambers at a time. To test the device for potential clinical applications, 1D dose profiles and 2D dose maps with various square and irregular fields were measured. The overall agreement with the reference doses (film and treatment planning system) was satisfactory, but the measured dose differs in the penumbra region and in the field size dependence. Both of these features are related to the thin walls between neighboring ion chambers and different lateral phantom scatter in the detector panel vs homogeneous material. We demonstrated feasibility of radiation detector arrays with minimal number of readout channels and low-cost electronics. The acquisition scheme based on selected row or column 'activation' by bias voltage is not practical for 2D dosimetry but it allows for rapid turn-around when testing of custom arrays with the aid of multiple 1D dose profiles. Future progress in this area includes overcoming the limitations due high chamber packing ratio, which leads to the lateral scattering effects.

Two-dimensional solid-state array detectors: A technique for in vivo dose verification in a variable effective area

Purpose

We introduce a technique that employs a 2D detector in transmission mode (TM) to verify dose maps at a depth of d_max in Solid Water. TM measurements, when taken at a different surface‐to‐detector distance (SDD), allow for the area at d_max (in which the dose map is calculated) to be adjusted.

Methods

We considered the detector prototype “MP512” (an array of 512 diode‐sensitive volumes, 2 mm spatial resolution). Measurements in transmission mode were taken at SDDs in the range from 0.3 to 24 cm. Dose mode (DM) measurements were made at d_max in Solid Water. We considered radiation fields in the range from 2 × 2 cm² to 10 × 10 cm², produced by 6 MV flattened photon beams; we derived a relationship between DM and TM measurements as a function of SDD and field size. The relationship was used to calculate, from TM measurements at 4 and 24 cm SDD, dose maps at d_max in fields of 1 × 1 cm² and 4 × 4 cm², and in IMRT fields. Calculations were cross‐checked (gamma analysis) with the treatment planning system and with measurements (MP512, films, ionization chamber).

Results

In the square fields, calculations agreed with measurements to within ±2.36%. In the IMRT fields, using acceptance criteria of 3%/3 mm, 2%/2 mm, 1%/1 mm, calculations had respective gamma passing rates greater than 96.89%, 90.50%, 62.20% (for a 4 cm SSD); and greater than 97.22%, 93.80%, 59.00% (for a 24 cm SSD). Lower rates (1%/1 mm criterion) can be explained by submillimeter misalignments, dose averaging in calculations, noise artifacts in film dosimetry.

Conclusions

It is possible to perform TM measurements at the SSD which produces the best fit between the area at d_max in which the dose map is calculated and the size of the monitored target.

HDR brachytherapy in vivo source position verification using a 2D diode array: A Monte Carlo study

PURPOSE

This study aims to assess the accuracy of source position verification during high-dose rate (HDR) prostate brachytherapy using a novel, in-house developed two-dimensional (2D) diode array (the Magic Plate), embedded exactly below the patient within a carbon fiber couch. The effect of tissue inhomogeneities on source localization accuracy is examined.

METHOD

Monte Carlo (MC) simulations of 12 source positions from a HDR prostate brachytherapy treatment were performed using the Geant4 toolkit. An Ir-192 Flexisource (Isodose Control, Veenendaal, the Netherlands) was simulated inside a voxelized patient geometry, and the dose deposited in each detector of the Magic Plate evaluated. The dose deposited in each detector was then used to localize the source position using a proprietary reconstruction algorithm.

RESULTS

The accuracy of source position verification using the Magic Plate embedded in the patient couch was found to be affected by the tissue inhomogeneities within the patient, with an average difference of 2.1 ± 0.8 mm (k = 1) between the Magic Plate predicted and known source positions. Recalculation of the simulations with all voxels assigned a density of water improved this verification accuracy to within 1 mm.

CONCLUSION

Source position verification using the Magic Plate during a HDR prostate brachytherapy treatment was examined using MC simulations. In a homogenous geometry (water), the Magic Plate was able to localize the source to within 1 mm, however, the verification accuracy was negatively affected by inhomogeneities; this can be corrected for by using density information obtained from CT, making the proposed tool attractive for use as a real-time in vivo quality assurance (QA) device in HDR brachytherapy for prostate cancer.

Impact of a monolithic silicon detector operating in transmission mode on clinical photon beams

PURPOSE

To investigate the effect on surface dose, as a function of different field sizes and distances from the solid water phantom to transmission detector (D_sd), of using the monolithic silicon detector MP512T in transmission mode.

METHODS

The influence of operating the MP512T in transmission mode on the surface dose of a phantom for SSD 100cm was evaluated by using a Markus IC. The MP512T was fixed to an adjustable stand holder and was positioned at different D_sd, ranging from 0.3 to 24 cm. For each D_sd, measurements were carried out for irradiation field sizes of 5 × 5cm², 8 × 8 cm² and 10 × 10 cm². Measurements were obtained under two different operational setups, (i) with the MP512T face-up and (ii) with the MP512T face-down. In addition, the transmission factors for the MP512T and the printed circuit board were only evaluated using a Farmer IC.

RESULTS

For all D_sd and all field sizes, the MP512T led to the surface dose increasing by less than 25% when in the beam. For D_sd >18 cm the surface dose increase is less than 5%, and negligible for field size 5 × 5 cm². The difference in the surface dose perturbation for the MP512T operating face up or operating face down is negligible (<2%) for all field sizes. The transmission factor of the MP512T ranged from 1.020 to 0.9950 for all measured D_sd and field sizes.

CONCLUSION

The study demonstrated that positioning the MP512T in air between the Linac head and the phantom produced negligible perturbation of the surface dose for D_sd >18 cm, and was completely transparent for 6 MV photon beams.