Implementation of water calorimetry in a 180 MeV scanned pulsed proton beam including an experimental determination of k(Q) for a Farmer chamber

Water calorimetric measurements have been performed in a 180 MeV scanned pulsed proton beam and the absorbed dose determined has been compared with the results obtained using two NE2571 Farmer chambers and the IAEA TRS-398 code of practice. The depth of measurement in water corresponded to a residual range of R-res = 16.5 cm, corresponding to a mean energy of about 150 MeV. Ionization chambers were calibrated in terms of the absorbed dose to water in Co-60 at the Swedish Secondary Standard Dosimetry Laboratory, directly traceable to Bureau International des Poids et Mesures. The present experimental investigation has shown that water calorimetry is feasible in a high-energy scanned pulsed proton beam. When comparing the results obtained with water calorimetry and ionometry, the beam quality correction factor, k(Q), could be determined for the two NE2571 ionization chambers used. The k(Q)-factor was found to be 1.032 +/- 0.013, which is in good agreement with the factor tabulated in IAEA TRS-398 for this chamber type (1.039 +/- 0.018). The present result has also been compared with a previously obtained result in a passively scattered proton beam having similar energy. This comparison yielded a 1.1% deviation, which is not significant considering the combined uncertainties of the two experimental determinations of k(Q). The dominating contribution to the combined uncertainty stems from the correction factor for ion recombination in the scanned proton beam (1%), and further studies are required in order to reduce this uncertainty and reveal any possible differences in the k(Q)-factor between these two proton beam delivery techniques

Experimental determination of k Qfactors for two types of ionization chambers in scanned proton beams

Objective. Experimental determination of beam quality k Q factors for two types of Farmer ionization chambers, NE2571 and IBA FC65-G, in a scanned proton beam for three nominal energies (140 MeV, 180 MeV and 220 MeV) based on water calorimetry. Approach. Beam quality correction factors were determined comparing the results obtained with water calorimetry and ionometry. Water calorimetry was performed to determine the absorbed dose at a depth of measurement in water of 5 g cm-2, limited by the extension of the calorimeter glass vessel used. For the ionometry, two chambers of each type were included in the study. The ionization chambers were calibrated in terms of absorbed dose to water in 60Co at the Swedish Secondary Standard Dosimetry Laboratory, directly traceable to the BIPM, and were used according to the IAEA TRS-398 Code of Practice. Main results. The k Q values determined in the present work have been compared with the values tabulated in TRS-398 and its forthcoming update and also with those obtained in previous water calorimetric measurements and Monte Carlo calculations. All results were found to agree within the combined uncertainties of the different data. Significance. It is expected that the present work will serve as an experimental contribution to k Q -factors for the two chamber types and three scanned proton beam qualities used

Gradient corrections for reference dosimetry using Farmer‐type ionization chambers in single‐layer scanned proton fields

Purpose: The local depth dose gradient and the displacement correction factor for Farmer-type ionization chambers are quantified for reference dosimetry at shallow depth in single-layer scanned proton fields. Method: Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5 cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer-type ionization chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement. Results: Depth dose gradients are as low as 0.1–0.4% per mm at measurement depths from 1 to 5 cm in the highest clinical proton energies (with residual ranges higher than 15 cm) and increase to 1% per mm at a residual range of 4 cm and become larger than 3% per mm for residual ranges lower than 2 cm. The literature review shows that the effective point of measurement of Farmer-type ionization chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer-type ICs can be used at measurements depths from 1 to 5 cm for which the residual range is 4 cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2 cm and for residual ranges of 15 cm or higher. Conclusion: The literature review confirmed that for proton beams the effective point of measurement of Farmer-type ionization chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer-type ionization chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4 cm while the third option only for residual ranges of at least 15 cm

Current best estimates of beam quality correction factors for reference dosimetry of clinical proton beams

Objective. To review the currently available data on beam quality correction factors, kQ, for ionization chambers in clinical proton beams and derive their current best estimates for the updated recommendations of the IAEA TRS-398 Code of Practice. Approach. The reviewed data come from 20 publications from which kQ values can be derived either directly from calorimeter measurements, indirectly from comparison with other chambers or from Monte Carlo calculated overall chamber factors, fQ. For cylindrical ionization chambers, a distinction is made between data obtained in the centre of a spread-out Bragg peak and those obtained in the plateau region of single-energy fields. For the latter, the effect of depth dose gradients has to be considered. To this end an empirical model for previously published displacement correction factors for single-layer scanned beams was established, while for unmodulated scattered beams experimental data were used. From all the data, chamber factors, fQ, and chamber perturbation correction factors, pQ, were then derived and analysed. Main results. The analysis showed that except for the beam quality dependence of the water-to-air mass stopping power ratio and, for cylindrical ionization chambers in unmodulated beams, of the displacement correction factor, there is no remaining beam quality dependence of the chamber perturbation correction factors pQ. Based on this approach, average values of the beam quality independent part of the perturbation factors were derived to calculate kQ values consistent with the data in the literature. Significance. The resulting data from this analysis are current best estimates of kQ values for modulated scattered beams and single-layer scanned beams used in proton therapy. Based on this, a single set of harmonized values is derived to be recommended in the update of IAEA TRS-398

MRI-only based treatment with a commercial deep-learning generation method for synthetic CT of brain

ObjectivesTo show feasibility of synthetic computed tomography (sCT) images generated using a commerciallyavailable software, enabling MRI-only treatment planning for the brain in a clinical setting.Patients and Methods20 and 16 patients with brain malignancies, including post-surgical cases, were included for validationand treatment, respectively. Dixon MR images of the skull were exported to the MRI Planner software(Spectronic Medical AB), which utilizes convolutional neural network algorithms for sCT generation.In the validation study, sCT images were rigidly registered and resampled to CT geometry for eachpatient. Treatment plans were optimized on CT and retrospectively recalculated on sCT images forevaluation of dosimetric and geometric endpoints. Clinical robustness in patient setup verification wasassessed by rigidly registering cone beam CT (CBCT) to sCT and CT images, respectively.The treatment study was performed on sCT images, using CT solely for QA purposes.ResultsAll sCT images were successfully generated in the validation study. Mean absolute error of the sCTimages within the body contour for all patients was 62.2 ± 4.1 HU. Average absorbed dose differenceswere below 0.2%. Mean pass rate of global gamma (1%/1mm) for all patients was 100.0 ± 0.0 % withinPTV and 99.1 ± 0.6 % for the full dose distribution. No clinically relevant deviations were found in theCBCT-sCT vs CBCT-CT image registrations. Areas of bone resection due to surgery were accuratelydepicted in the sCT images. Finally, treatment success rate was 15/16. One patient was excluded due tosCT artifacts from a haemostatic substance injected during surgery.Conclusion15 patients have successfully received MRI-only RT for brain tumours using the validated commerciallyavailable sCT software. Validation showed comparable results between sCT and CT images for bothdosimetric and geometric endpoint

Prospective Clinical Feasibility Study for MRI-Only Brain Radiotherapy

Objectives: MRI-only radiotherapy (RT) provides a workflow to decrease the geometric uncertainty introduced by the image registration process between MRI and CT data and to streamline the RT planning. Despite the recent availability of validated synthetic CT (sCT) methods for the head region, there are no clinical implementations reported for brain tumors. Based on a preceding validation study of sCT, this study aims to investigate MRI-only brain RT through a prospective clinical feasibility study with endpoints for dosimetry and patient setup. Material and Methods: Twenty-one glioma patients were included. MRI Dixon images were used to generate sCT images using a CE-marked deep learning-based software. RT treatment plans were generated based on MRI delineated anatomical structures and sCT for absorbed dose calculations. CT scans were acquired but strictly used for sCT quality assurance (QA). Prospective QA was performed prior to MRI-only treatment approval, comparing sCT and CT image characteristics and calculated dose distributions. Additional retrospective analysis of patient positioning and dose distribution gamma evaluation was performed. Results: Twenty out of 21 patients were treated using the MRI-only workflow. A single patient was excluded due to an MRI artifact caused by a hemostatic substance injected near the target during surgery preceding radiotherapy. All other patients fulfilled the acceptance criteria. Dose deviations in target were within ±1% for all patients in the prospective analysis. Retrospective analysis yielded gamma pass rates (2%, 2 mm) above 99%. Patient positioning using CBCT images was within ± 1 mm for registrations with sCT compared to CT. Conclusion: We report a successful clinical study of MRI-only brain radiotherapy, conducted using both prospective and retrospective analysis. Synthetic CT images generated using the CE-marked deep learning-based software were clinically robust based on endpoints for dosimetry and patient positioning

MRI-only radiotherapy of gliomas – a prospective implementation study.

Minna Lerner, Joakim Medin, Christian Jamtheim Gustafsson, Sara Alkner and Lars E. Olsson