Experimental realization of dynamic fluence field optimization for proton computed tomography

Abstract

Proton computed tomography (pCT) has high accuracy and dose efficiency in producing spatial maps of the relative stopping power (RSP) required for treatment planning in proton therapy. With fluence-modulated pCT (FMpCT), prescribed noise distributions can be achieved, which allows to decrease imaging dose by employing object-specific dynamically modulated fluence during the acquisition. For FMpCT acquisitions we divide the image into region-of-interest (ROI) and non-ROI volumes. In proton therapy, the ROI volume would encompass all treatment beams. An optimization algorithm then calculates dynamically modulated fluence that achieves low prescribed noise inside the ROI and high prescribed noise elsewhere. It also produces a planned noise distribution, which is the expected noise map for that fluence, as calculated with a Monte Carlo simulation. The optimized fluence can be achieved by acquiring pCT images with grids of intensity modulated pencil beams. In this work, we interfaced the control system of a clinical proton beam line to deliver the optimized fluence. Using three phantoms we acquired images with uniform fluence, with a constant noise prescription, and with an FMpCT task. Image noise distributions as well as fluence maps were compared to the corresponding planned distributions as well as to the prescription. Furthermore, we propose a correction method that removes image artifacts stemming from the acquisition with pencil beams having a spatially varying energy distribution that is not seen in clinical operation. RSP accuracy of FMpCT scans was compared to uniform scans and was found to be comparable to standard pCT scans. While we identified technical improvements for future experimental acquisitions, in particular related to an unexpected pencil beam size reduction and a misalignment of the fluence pattern, agreement with the planned noise was satisfactory and we conclude that FMpCT optimized for specific image noise prescriptions is experimentally feasible.

Image guidance for FLASH radiotherapy

“…In particular, proton transmission imaging could be used in radiographic mode to enable an optimization of the planning CT calibration to SPR, 51 potentially also compensating for anatomical changes when using a sufficient number of high‐quality radiographies, 52 or provide full tomographic capabilities for a patient model in treatment position to be used for daily replanning. The latter scenario could be also favored by the recent developments of fluence field modulation in combination with pencil‐beam delivery, which open up further perspective of dose saving for daily imaging 53 . But of particular relevance to FLASH‐RT is the possibility to exploit information from sparse radiographies, which could ideally be simultaneously acquired to the treatment in the recently proposed scenario of shoot‐through FLASH proton therapy 54 …”

“…The latter scenario could be also favored by the recent developments of fluence field modulation in combination with pencil-beam delivery, which open up further perspective of dose saving for daily imaging. 53 But of particular relevance to FLASH-RT is the possibility to exploit information from sparse radiographies, which could ideally be simultaneously acquired to the treatment in the recently proposed scenario of shoot-through FLASH proton therapy. 54 For patient setup, an alternative to volumetric imaging that has become standard of care in most radiotherapy centers is surface guidance, based upon optical scanners.…”

Comparative accuracy and resolution assessment of two prototype proton computed tomography scanners

“…The scanner has been shown to produce a near 100% detection efficiency for count rates up to 1 MHz with homogeneous proton fluence 35 . For operation of the scanner with scanned pencil beams, the local count rate increases and a pileup‐free operation was demonstrated for count rates up to 400 kHz 47,48 . For the purpose of this study, a 200 MeV broad proton beam was utilized with the phase‐II scanner.…”

“…35 For operation of the scanner with scanned pencil beams, the local count rate increases and a pileup-free operation was demonstrated for count rates up to 400 kHz. 47,48 For the purpose of this study, a 200 MeV broad proton beam was utilized with the phase-II scanner. More details about the beam characteristics are given in Section 2.6…”

Fluence-modulated proton CT optimized with patient-specific dose and variance objectives for proton dose calculation

“…With this, dose savings of up to 40% outside of the ROI could be achieved, outperforming the simple intersection-based approach used in Dedes et al (2017). Optimized fluence modulations were employed experimentally by modulating pencil beams and using a prototype pCT scanner resulting in good agreements between simulated and experimental scans in terms of image variance and RSP accuracy (Dickmann et al 2020a). The studies of Dickmann et al (2020Dickmann et al ( , 2020a were limited to phantoms with tissue-equivalent materials and the optimization algorithm only took into account image variance and not imaging dose.…”