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

Abstract

Purpose: In recent years, there has been a movement toward 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 falloff. The authors 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, the authors 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, the authors were able to generate lookup 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 lookup graphs, the authors investigated the applicability of the technique for a range of patient treatment sites. The authors 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 cm 2 , 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, the authors 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. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License...

Higher order analysis of time-resolved proton radiographs

“…An alternative approach is to use a single distal detector beyond the patient, in a similar fashion to portal imaging devices in radiotherapy. A variety of approaches have been tested (Martišíková et al 2013, Rinaldi et al 2014, Telsemeyer et al 2012b, 2014, Doolan et al 2015a, but in this work we investigate an extension to the time-resolved method. This approach relies on the use of a proton beam whose range is modulated by a rotating range modulator (RM) wheel, which in turn generates a time-varying dose at the detector (Lu 2008, Gottschalk et al 2011, Testa et al 2013.…”

“…As well as providing structural information not otherwise visible in the WET map, these images provide information about which WET values are reliable. Many works have argued the importance of the WET map for proton treatments and quality assurance (Schneider and Pedroni 1995, Telsemeyer et al 2012a, Doolan et al 2015a and the higher order images provide confidence on which WET values can be trusted and which are potentially subject to range mixing. In this work we have shown that it is possible to use the higher order images to create a mask of unreliable pixels that can be subsequently applied to the WET map.…”

PRaVDA: The first solid-state system for proton computed tomography

“…Range uncertainties appear to be a major reason that prevents proton therapy reaching its maximum potential in sparing healthy tissue [3] . A number of different approaches to mitigate the effects (robust treatment planning [5] ) or to reduce range uncertainties (proton radiography [6] , proton computed tomography (pCT) [7] , dual energy CT (DECT) [8] ) are being investigated.…”

Proof of principle of helium‐beam radiography using silicon pixel detectors for energy deposition measurement, identification, and tracking of single ions

“…Integrating systems measure the integrated signal of the particles downstream of the object being imaged (pRad: 14,15 ; iRad: 16 ; pCT: 17 ; iCT: 16,[18][19][20]. Most of these works focus on the usage of carbon ions due to the less pronounced MCS, which is especially important for nontracking systems.…”