Ongoing developments in medical imaging technology have resulted in both increasingly smaller treatment targets – often necessitating beams of less than one centimetre in diameter – and a need for high-precision beam delivery. In turn, this has generated a need for alternative dosimetric tools that can quantify these fields to a higher degree of spatial resolution, without exhibiting partial volume averaging.

Traditionally, radiochromic film has served as the standard tool for small-field and beam profile measurements, providing high spatial resolution. For proton therapy, however, such film can vary show varied responses to changing proton linear energy transfer (LET). Radiochromic films can also be difficult to implement in quality assurance programmes, as they require post-processing, frequently produce measurement artefacts, and can suffer from inconsistent dose mapping due to minor differences between different film lots and scanners.

With this in mind, researchers from the University of Wollongong's Centre for Medical Radiation Physics are working to create more efficient and real-time dosimetric methods for small-field applications. One such development is the dose magnifying glass – a prototype pixelated silicon detector intended to afford both high-spatial resolution point dose measurements and beam profile measurements in real time.

The dose magnifying glass consists of a funnel-shaped array of 128 n+ strips, each 20 µm wide and 2 mm long, placed atop a p-type silicon substrate. The device can provide real-time measurements and has a high spatial resolution – equivalent to that of dosimetric film – allowing for measurement of the smallest fields currently being used in clinical proton therapy. While the device has been tested in photon radiotherapy, intensity-modulated radiation therapy and stereotactic radiotherapy, its potential for use in proton therapy had not previously been assessed.

In this latest study, therefore, medical physicist Andrew Wroe of the Loma Linda University Medical Center and colleagues conducted tests to explore the potential of the dose magnifying glass for use with proton therapy. The researchers recorded depth dose and lateral beam profiles in a water tank using the prototype device, and compared the results with those recorded by a PTW parallel-plate ionization chamber, a PTW proton-specific dosimetry diode and an EBT3 Gafchromic film. They also compared their measurements with data simulated using an in-house developed Geant4-based Monte Carlo application.

The results revealed similar measurements of the depth dose profile between the dose magnifying glass and the diode, ionization chamber and Monte Carlo simulations. The prototype device accurately located the Bragg peak along the depth dose profile, and the recorded response of the dose magnifying glass at the centre of modulation lay within 2.5% of those taken by the PTW dosimetry diode for all energies and modulations.

Furthermore, real-time FWHM and FW90 (full-width at half and 90% of the maximum) beam profile measurements of a 5 mm-wide, 127 MeV proton beam were within one channel, or 0.1 mm, of both the film and Monte Carlo data for all depths tested.

"The dose magnifying glass tested here proved to be a useful device at measuring depth dose profiles in proton therapy, with a stable response across the entire spread-out Bragg Peak," Wroe and colleagues write in their paper. "In addition, the linear array of small sensitive volumes allowed for accurate point and high-spatial resolution one-dimensional profile measurements of small radiation fields in real-time to be completed with minimal impact from partial volume averaging."

"The application of two-dimensional silicon pixel transmission arrays based on this technology and already developed by the Centre for Medical Radiation Physics could be the next step in real-time quality assurance and commissioning in proton therapy," the researchers conclude.

With this study complete, the researchers are now looking to refine the dose magnifying glass design with a mind to future clinical proton therapy applications – such as by resolving manufacturing issues that led to unresponsive channels in the prototype, and improving the durability of the cabling that connects the magnifying glass to the data acquisition setup.

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