Accuracy assessment of the CNAO dose delivery system in the initial period of clinical activity and impact of later improvements on delivered dose distributions

Abstract

Purpose: A retrospective analysis of the dose delivery system (DDS) performances of the initial clinical operation at CNAO (Centro Nazionale di Adroterapia Oncologica) is reported, and compared with the dose delivery accuracy following the implementation of a position feedback control. Methods: Log files and raw data of the DDS were analyzed for every field of patients treated with protons and carbon ions between January 2012 and April 2013 (~3800 fields). To investigate the DDS accuracy, the spot positions and the number of particles per spot measured by the DDS and prescribed by the treatment planning system were compared for each field. The impact of deviations on dose distributions was studied by comparing, through the gamma-index method, 2 three-dimensional (3D) physical dose maps (one for prescribed, one for measured data), generated by a validated dose computation software. The maximum gamma and the percentage of points with gamma ≤ 1 (passing volume) were studied as a function of the treatment day, and correlated with the deviations from the prescription in the measured number of particles and spot positions. Finally, delivered dose distributions of same treatment plans were compared before and after the implementation of a feedback algorithm for the correction of small position deviations, to study the effect on the delivery quality. A double comparison of prescribed and measured 3D maps, before and after feedback implementation, is reported and studied for a representative treatment delivered in 2012, redelivered on a polymethyl methacrylate (PMMA) block in 2018. Results: Systematic deviations of spot positions, mainly due to beam lateral offsets, were always found within 1.5 mm, with the exception of the initial clinical period. The number of particles was very stable, as possible deviations are exclusively related to the quantization error in the conversion from monitor counts to particles. For the chosen representative patient treatment, the gamma-index evaluation of prescribed and measured dose maps, before and after feedback implementation, showed a higher variability of maximum gamma for the 2012 irradiation, with respect to the reirradiation of 2018. However, the 2012 passing volume is >99.8% for the sum of all fields, which is comparable to the value of 2018, with the exception of one day with 98.2% passing volume, probably related to an instability of the accelerating system. Conclusions: A detailed retrospective analysis of the DDS performances in the initial period of CNAO clinical activity is reported. The spot position deviations are referable to beam lateral offset fluctuations, while almost no deviation was found in the number of particles. The impact of deviations on dose distributions showed that the position feedback implementation and the increased beam control capability acquired after the first years of clinical experience led to an evident improvement in the DDS stability, evaluated in terms of gamma-index as a measure of the impact on dose distributions. However, the clinical effe...

Dosimetric validation of a GPU‐based dose engine for a fast in silico patient‐specific quality assurance program in light ion beam therapy

“…In addition, also the online algorithm for the beam position feedback adopts a self -correcting strategy, based on the same rationale. 9 Therefore, for a rigorous evaluation of the global performance of the DDS, an independent measurement of (at least) the number of particles should be performed. For the current version of the DDS, Box 2 is not yet configured to write-out this statistic; however, the beam monitor calibration is checked daily with absolute dose measurements as part of the QA program, as well as the beam position, as done also by 13 or.…”

“…Particularly, for scanned beam delivery systems, log-files can be, therefore, an easy solution to determine the inaccuracies of the delivered fraction, providing a thorough understanding of the performance of the system and its stability, validating the data transfer and confirming the deliverability of the plan. 9 The machine log-files fulfill the main requirements for an independent dose calculation engine to reconstruct the actually delivered dose and to verify the calculated planning dose distribution. The recalculated dose can then be fully 3D compared against the nominal planning dose by means of a voxel-based γ-passing rate (γ-PR) (percentage of voxels satisfying the distance-toagreement, DTA, and the dose-difference, DD, criteria).…”

“…In the last decade, most of the aforementioned limitations and issues (specifically, poor sensitivity to range errors or to absolute dose measurements in steep dose gradient regions, low spatial resolution, or few measurement points and failures outside of the measurement area) have been demonstrated to be overcome by analyzing machine log‐files , that is, files storing machine parameters as recorded by the beam monitors: delivered spot positions, beam energy, and number of particles (or monitor units, MU) per spot, for a given field of a treatment fraction. Particularly, for scanned beam delivery systems, log‐files can be, therefore, an easy solution to determine the inaccuracies of the delivered fraction, providing a thorough understanding of the performance of the system and its stability, validating the data transfer and confirming the deliverability of the plan 9 …”

Analysis of the Dose Drop at the Edge of the Target Area in Heavy Ion Radiotherapy

“…Uniform scanning is one of the passive beam delivery mode, when heavy ions are applied in clinical radiotherapy, which adopts triangular wave scanning mode to extend the beam laterally, ridge filter to lengthwise expand the Bragg peak of the beam, and combined with a lobed collimator to carry out lateral conformal irradiation [ 16 ]. In our literature, we analyze the beam mode, under the condition of multiple doses split on the edge of the target dose drop speed, for heavy ion radiotherapy, provide reference for clinicians treating patients with data, and can also be used for delineation, relatively safe distance between target and endanger organ to provide the reference data.…”

Current and Future Technologies of the CNAO Dose Delivery System

“…The main topic of this review is to describe the DDS of the Italian National Center of Oncological Hadrontherapy (CNAO) [14]- [16]. In the first section, the technologies used to treat more than 3200 patients since 2011 are reviewed.…”