Dose ratio proton radiography using the proximal side of the Bragg peak

Purpose: In recent years there has been a movement towards single-detector proton radiography, due to its potential ease of implementation within the clinical environment. One such single-detector technique is the dose ratio method, in which the dose maps from two pristine Bragg peaks are recorded beyond the patient. To date, this has only been investigated on the distal side of the lower energy Bragg peak, due to the sharp fall-off. We investigate the limits and applicability of the dose ratio method on the proximal side of the lower energy Bragg peak, which has the potential to allow a much wider range of water-equivalent thicknesses (WET) to be imaged. Comparisons are made with the use of the distal side of the Bragg peak. Methods: Using the analytical approximation for the Bragg peak we generated theoretical dose ratio curves for a range of energy pairs, and then determined how an uncertainty in the dose ratio would translate to a spread in the WET estimate. By defining this spread as the accuracy one could achieve in the WET estimate, we were able to generate look-up graphs of the range on the proximal side of the Bragg peak that one could reliably use. These were dependent on the energy pair, noise level in the dose ratio image and the required accuracy in the WET. Using these look-up graphs we investigated the applicability of the technique for a range of patient treatment sites. We validated the theoretical approach with experimental measurements using a complementary metal oxide semiconductor active pixel sensor (CMOS APS), by imaging a small sapphire sphere in a high energy proton beam. Results: Provided the noise level in the dose ratio image was 1% or less, a larger spread of WETs could be imaged using the proximal side of the Bragg peak (max 5.31 cm) compared to the distal side (max 2.42 cm). In simulation it was found that, for a pediatric brain, it is possible to use the technique to image a region with a square field equivalent size of 7.6 cm2, for a required accuracy in the WET of 3 mm and a 1% noise level in the dose ratio image. The technique showed limited applicability for other patient sites. The CMOS APS demonstrated a good accuracy, with a root-mean-square-error of 1.6 mm WET. The noise in the measured images was found to be σ =1.2% (standard deviation) and theoretical predictions with a 1.96σ noise level showed good agreement with the measured errors. Conclusions: After validating the theoretical approach with measurements, we have shown that the use of the proximal side of the Bragg peak when performing dose ratio imaging is feasible, and allows for a wider dynamic range than when using the distal side. The dynamic range available increases as the demand on the accuracy of the WET decreases. The technique can only be applied to clinical sites with small maximum WETs such as for pediatric brains

The application of active pixel sensors to proton imaging

Proton therapy is one of the most precise modalities of cancer treatment, but some of its benefits cannot be fully exploited due to the uncertainties during treatment planning and dose delivery. The clinical implementation of imaging techniques such as proton radiography or proton computed tomography have been prevented by detector technology. The major challenges for the development of a device intended for proton detection are: high energy resolution, high speed data acquisition rates, single particle tracking and large area sensors with sufficient radiation hardness. Complementary metal oxide semiconductor (CMOS) active pixel sensors (APS) can be designed to meet all these demands. Preliminary experiments were performed to evaluate the feasibility of using CMOS APS technology for proton detection. This was done in an attempt to identify the parameters that will lead to the design of an ideal application specific CMOS APS. It was found that the detector used in this thesis has a dependence on proton intensity and it is not radiation hard. Proton transmission radiographs were successfully acquired at the Clatterbridge Centre for Oncology. A detailed description of the image formation process is given analysing the process of energy loss on the detector. Single proton detection was achieved using a region of interest of 1 mm2 and an acquisition rate of 1100 frames per second. Some limitations in the detector performance were identi ed when comparing these results with the Landau theory of particle identification. A series of modifications to the pixel architecture are listed to overcome issues such as pixel cross talk and to improve radiation hardness. The Most likely Path (MLP) method was applied on the data obtained by simulating the passage of protons through an arrangement of CMOS detectors. It was demonstrated that, CMOS APS in combination with the MLP method, can be used to correct for Multiple Coulomb scattering in proton radiography images. The performance of CMOS APS for beam monitoring and dosimetry was also studied. It was found that the detector tested can be used to perform beam monitoring since low doses were measured accurately and that its application could be escalated to 2D measurements for monoenergetic protons