Optimizing multi-stage CdZnTe Compton camera for real-time proton range determination in proton radiotherapy

Objectives Proton therapy is a promising method for external beam radiotherapy due to its potential to precisely deliver high radiation dose to a target tumor volume, while sparing healthy tissue. To achieve this goal, CT-based predictions of proton range in tissue are used. However, uncertainties caused by tumor motion, patient anatomy, and relating CT-numbers to proton range make this task difficult. To address this problem, a real-time monitoring of proton range would be required. This could be achieved by imaging prompt gammas (PG’s) created by interactions of protons with tissue. Due to the high energies of these photons, use of the Compton camera (CC) has been proposed. However, although preliminary studies have been performed, a practical implementation is yet to be achieved. The aim of this study is to investigate an optimized design of such a CC based on CdZnTe (CZT) detectors.

Methods A series of Monte-Carlo simulations with GATE V7.1 were performed. First, the production of PGs by a 150MeV proton pencil beam in Lucite, bone and muscle were simulated and corresponding spectra were created. Then, two and three-stage CZT-based CC’s (with stage one defined as the layer closest to the source) were modeled assuming parallel-plane geometry. The efficiency of detecting different sequences of photon-detector interactions as a function of detector thickness was investigated for both the two and three-stage CC’s over a range of relevant PG energies (1-10MeV). The angular uncertainty of event detection, defined by the thickness of the conical surface created by each pair of events recorded by the CC, was then investigated. In these tests, CZT detector energy and spatial resolution, thicknesses, and the distance between detector stages and PG source were varied. We investigated spatial and energy resolutions ranging from 1.0mm-5.0mm, and 1.0%-5.0%, respectively, and detector thicknesses ranging from 0.05cm to 10.00cm. The distance between detectors ranged from 0.5cm to 10.0cm.

Results Analysis of the PG spectra produced by protons in tissue indicates that imaging using a group of photons with energies equal to 4.4MeV, which are produced in ¹⁶O(p,x)¹²C[asterisk], ¹²C(p,p’)^12C[asterisk], ¹²C(p,2p)¹¹B[asterisk] and ¹⁴N(p,x)¹²C[asterisk] reactions, correlated best with proton range. CC efficiency of the two stage CZT detector was found to be 0.098% ± 0.002%. By adding a third CZT stage, CC efficiency increased by 0.156% ± 0.024% (which triples the total CC efficiency) (fig.2). PGs undergoing triple Compton scatter interactions (one in each stage) made the most significant contribution to CC efficiency for CZT detector thicknesses 蠅 2.0cm. PG’s undergoing photoelectric absorption directly following a single Compton scatter comprised the largest contribution to CC efficiency over the 0.05cm-2.00cm range of investigated thicknesses. The total camera efficiency showed a maximum for detectors 1.0cm-2.0cm thick. Theoretical calculations indicated (fig. 1) that PG photons that scatter by the angle of 10.1⁰-23.3⁰ in the CZT detector with a stage separation 蠅 3.0cm correspond to the best angular uncertainty of 6.3⁰ ± 0.3⁰ for 1.0% detector energy and 1.0mm spatial resolutions. Detectors with 5.0% energy and 5.0mm spatial resolutions resulted in 30.6⁰ ± 10.0⁰ angular uncertainty.

Conclusions The performance of a CZT Compton camera detector has been studied. A CC efficiency of 0.255% ± 0.034% can be achieved by assembling a triple stage parallel plane CZT detector with stage separations 蠅 3.0cm and detector thicknesses ~1.0cm. Restricting detection to PG’s scattered by 10.1⁰-23.3⁰ will minimize camera angular uncertainty and will consequently improve proton range determination. CC's with efficiency of this magnitude are an improvement over previously investigated triple stage CC's with HPGe detectors and constitutes a promising system for proton range determination in proton radiotherapy.

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