Precision of the PET activity range during irradiation with ¹⁰C, ¹¹C, and ¹²C beams

Abstract

\textit{Objective}. Beams of stable ions have been a well-established tool for radiotherapy for many decades. In the case of ion beam therapy with stable $^{12}$C ions, the positron emitters $^{10,11}$C are produced via projectile and target fragmentation, and their decays enable visualization of the beam via positron emission tomography (PET). However, the PET activity peak matches the Bragg peak only roughly and PET counting statistics is low. These issues can be mitigated by using a short-lived positron emitter as a therapeutic beam. \textit{Approach.} An experiment studying the precision of the measurement of ranges of positron emitting carbon isotopes by means of PET has been performed at the FRS fragment-separator facility of GSI Helmholtzzentrum f"ur Schwerionenforschung GmbH, Germany. The PET scanner used in the experiment is a dual-panel version of a Siemens Biograph mCT PET scanner. \textit{Main results.} High quality in-beam PET images and activity distributions have been measured from the in-flight produced positron emitting isotopes $^{11}$C and $^{10}$C implanted into homogeneous PMMA phantoms. Taking advantage of the high statistics obtained in this experiment, we investigated the time evolution of the uncertainty of the range determined by means of PET during the course of an irradiation, and show that the uncertainty improves with the inverse square root of the number of PET counts. The uncertainty is thus fully determined by the PET counting statistics. During the delivery of 1.6$\times$10$^7$ ions in 4 spills for a total duration of 19.2~s, the PET activity range uncertainty for $^{10}$C, $^{11}$C and $^{12}$C is 0.04, 0.7 and 1.3~mm, respectively. The gain in precision related to the PET counting statistics is thus much larger when going from $^{11}$C to $^{10}$C than when going from $^{12}$C to $^{11}$C. The much better precision for $^{10}$C is due to its much shorter half-life, which, contrary to the case of $^{11}$C, also enables to include the in-spill data in the image formation. \textit{Significance}. Our results can be used to estimate the contribution from PET counting statistics to the precision of range determination in a particular carbon therapy situation, taking into account the irradiation scenario, the required dose and the PET scanner characteristics.

Quasi-real-time range monitoring by in-beam PET: A case for 15O

“…Due to its simplicity and the small number of parameters necessary, the so-called GE function, which is a central Gaussian smoothly connected to exponential functions at both or one of the peak edges, was chosen 43 . The capability of the GE function to model the positron activity profiles produced by the implantation of carbon isotopes in a PMMA phantom was successfully demonstrated in the preceding work by Kostyleva et al 30 . The maximum of the GE peak coincides with the mean of the central Gaussian and the maximum of the positron activity profile is defined by this parameter.…”

“…Overall, 15 O comes out as a clear favorite among all considered isotopes for quasi-real-time range monitoring by in-beam PET during therapy. The values presented here are specific to the experimental setup's geometric configuration and reconstruction algorithms used in this experiment, as detailed in Kostyleva et al's work 30 . It should be noted that scaling the results for a different PET system should be straightforward.…”

“…With the method and parameters used in this work (including the intensity of the primary beam, energy of the primary and secondary beam, production target material and thickness, phase space acceptance of the separator, choice of degrader material and thickness), the beam intensity of 15 O exceeds that of 14 30 ). The average beam intensity used during the implantation is indicated in the legend.…”

“…Systematic offline measurements using a 22 Na source were performed to map the sensitivity and attenuation corrections for the two different phantom positions used during the irradiation. A detailed description of the procedure can be found in 30 . Figures 3 and 4 display the in-beam 2D PET images of 14,15,16 O ions implanted into the PMMA phantom during both the high-energy run and the low-energy run, following various implantation cycles.…”

“…The primary goal of the experiments was to establish a relationship between the minimum amount of detected decays required to achieve a range verification precision better than the typical systematic uncertainty of the patient positioning systems. The results from the carbon experiment, which provide a detailed description of the experimental setup and methodology, have already been published 30 . This article reports on an experiment with positron emitters of oxygen, 14 O and 15 O, and the results are compared to the data obtained for 10 C and 11 C 30 within the context of quasi-real-time in-beam range monitoring.…”