Optimizing dual energy cone beam CT protocols for preclinical imaging and radiation research

Objective: The aim of this work was to investigate whether quantitative dual-energy CT (DECT) imaging is feasible for small animal irradiators with an integrated cone-beam CT (CBCT) system. Methods: The optimal imaging protocols were determined by analyzing different energy combinations and dose levels. The influence of beam hardening effects and the performance of a beam hardening correction (BHC) were investigated. In addition, two systems from different manufacturers were compared in terms of errors in the extracted effective atomic numbers (Z(eff)) and relative electron densities (rho(e)) for phantom inserts with known elemental compositions and relative electron densities. Results: The optimal energy combination was determined to be 50 and 90kVp. For this combination, Z(eff) and r rho(e) can be extracted with a mean error of 0.11 and 0.010, respectively, at a dose level of 60cGy. Conclusion: Quantitative DECT imaging is feasible for small animal irradiators with an integrated CBCT system. To obtain the best results, optimizing the imaging protocols is required. Well-separated X-ray spectra and a sufficient dose level should be used to minimize the error and noise for Z(eff) and rho(e). When no BHC is applied in the image reconstruction, the size of the calibration phantom should match the size of the imaged object to limit the influence of beam hardening effects. No significant differences in Z(eff) and rho(e) errors are observed between the two systems from different manufacturers. Advances in knowledge: This is the first study that investigates quantitative DECT imaging for small animal irradiators with an integrated CBCT system

Evaluation of a novel triple-channel radiochromic film analysis procedure using EBT2

A novel approach to read out radiochromic film was introduced recently by the manufacturer of GafChromic film. In this study, the performance of this triple-channel film dosimetry method was compared against the conventional single-red-channel film dosimetry procedure, with and without inclusion of a pre-irradiation (pre-IR) film scan, using EBT2 film and kilo-and megavoltage photon beams up to 10 Gy. When considering regions of interest averaged doses, the triple-channel method and both single-channel methods produced equivalent results. Absolute dose discrepancies between the triple-channel method, both single-channel methods and the treatment planning system calculated dose values, were no larger than 5 cGy for dose levels up to 2.2 Gy. Signal to noise in triple-channel dose images was found to be similar to signal to noise in single-channel dose images. The accuracy of resulting dose images from the triple-and single-channel methods with inclusion of pre-IR film scan was found to be similar. Results of a comparison of EBT2 data from a kilovoltage depth dose experiment to corresponding Monte Carlo depth dose data produced dose discrepancies of 9.5 +/- 12 cGy and 7.6 +/- 6 cGy for the single-channel method with inclusion of a pre-IR film scan and the triple-channel method, respectively. EBT2 showed to be energy sensitive at low kilovoltage energies with response differences of 11.9% and 15.6% in the red channel at 2 Gy between 50-225 kVp and 80-225 kVp photon spectra, respectively. We observed that the triple-channel method resulted in non-uniformity corrections of +/- 1% and consistency values of 0-3 cGy for the batches and dose levels studied. Results of this study indicate that the triple-channel radiochromic film read-out method performs at least as well as the single-channel method with inclusion of a pre-IR film scan, reduces film non-uniformity and saves time with elimination of a pre-IR film scan

Evaluation of a novel triple-channel radiochromic film analysis procedure using EBT2

A novel approach to read out radiochromic film was introduced recently by the manufacturer of GafChromic film. In this study, the performance of this triple-channel film dosimetry method was compared against the conventional single-red-channel film dosimetry procedure, with and without inclusion of a pre-irradiation (pre-IR) film scan, using EBT2 film and kilo-and megavoltage photon beams up to 10 Gy. When considering regions of interest averaged doses, the triple-channel method and both single-channel methods produced equivalent results. Absolute dose discrepancies between the triple-channel method, both single-channel methods and the treatment planning system calculated dose values, were no larger than 5 cGy for dose levels up to 2.2 Gy. Signal to noise in triple-channel dose images was found to be similar to signal to noise in single-channel dose images. The accuracy of resulting dose images from the triple-and single-channel methods with inclusion of pre-IR film scan was found to be similar. Results of a comparison of EBT2 data from a kilovoltage depth dose experiment to corresponding Monte Carlo depth dose data produced dose discrepancies of 9.5 +/- 12 cGy and 7.6 +/- 6 cGy for the single-channel method with inclusion of a pre-IR film scan and the triple-channel method, respectively. EBT2 showed to be energy sensitive at low kilovoltage energies with response differences of 11.9% and 15.6% in the red channel at 2 Gy between 50-225 kVp and 80-225 kVp photon spectra, respectively. We observed that the triple-channel method resulted in non-uniformity corrections of +/- 1% and consistency values of 0-3 cGy for the batches and dose levels studied. Results of this study indicate that the triple-channel radiochromic film read-out method performs at least as well as the single-channel method with inclusion of a pre-IR film scan, reduces film non-uniformity and saves time with elimination of a pre-IR film scan

Comparison of Daily Online Plan Adaptation Strategies for a Cohort of Pancreatic Cancer Patients Treated with SBRT

Purpose: To study the trade-offs of three online strategies to adapt treatment plans of patients with locally advanced pancreatic carcinoma (LAPC) treated using the CyberKnife with tumor tracking. Methods and Materials: A total of 35 planning computed tomography scans and 98 daily in-room computed tomography scans were collected from 35 patients with LAPC. Planned dose distributions, optimized with VOLO, were evaluated on manually contoured daily anatomies to collect daily doses. Three strategies were tested to adapt treatment plans: (1) unrestricted full replanning using a patient-specific plan template, (2) time-restricted replanning on organs at risk (OARs) within 3 cm from the planning target volume (PTV) structure, and (3) dose realignment optimization to stay within OAR constraints. Dose distributions resulting from each plan adaptation strategy were dosimetrically compared by means of gross tumor volume (GTV), PTV coverage, and OAR tolerances. Results: Planned doses did not result in dose-constraint violations for 28 of 98 daily anatomies. None of the suggested plan adaptation strategies improved planned doses significantly for this subset. For 70 of the 98 reported violations, the median (interquartile range) PTV coverage of the planned dose was 84% (76% to 86%). After plan adaptation, unrestricted replanning achieved clinically acceptable plans in 93% of these fractions, time-restricted replanning in 90%, and dose realignment in 74%, at median computational times of 8.5, 3, and 0.5 minutes. Over all 98 fractions, PTV coverage was reduced: –1% (–3% to 1%), –2% (–5% to 0%), and –2% (–8% to 0%) after each strategy, respectively. In 3 of 70 fractions, none of the suggested strategies achieved clinically acceptable OAR dose volumes. Conclusions: Unrestricted replanning was the most time-consuming method but reached the highest number of successfully adapted plans. Time-restricted replanning and dose realignment resulted in a high number of plans within dose constraints. Depending on the resources available, an adaptive strategy can be selected for each patient to address the specific anatomic challenges on the treatment day. The increase in the complexity of the strategy corresponds with an increasing number of successfully adapted plans