A concept of a multimodule multifiducial phantom has been introduced. Analytical framework has been developed to extract geometric characteristics of radiotherapy devices from projection images of a phantom. The phantom design and the methodology developed have been tested in simulations.
The gamma index is a measure used routinely for the quality control of dose delivery in radiotherapy, implemented in commercial systems for the verification of treatment plans. It involves comparison of the difference between planned and delivered doses to a single reference. The same reference value is selected for all points in the plan that can potentially hide dose delivery errors, especially in medium and low dose areas. In this study, a receiver operating characteristic analysis is used to demonstrate the limits of the performance of the global gamma index as a method for detecting dose delivery errors. The performance of a global gamma index is compared with two approaches based on statistical tests for outlier detection. Two statistical approaches are considered: according to the first, the distribution of the delivered doses is estimated based on an appropriate calibration procedure. According to the second, the distribution of the delivered doses is estimated based on the detection of relatively homogeneous regions of a plan and analyzing the distributions of planned doses within these regions. The performance of the three approaches is compared based on analytical considerations and in simulations in which errors are intentionally introduced to the plan delivery and noise related to dose delivery is modeled. We have shown that a statistics-based approach to gamma analysis generally leads to better detection of true delivery errors. The results of analytical consideration coincide with the simulations. In simulations, we observe that both statistical approaches are better detectors of true delivery errors than the global method for the gamma-index passing rate in the range from 0.9–1.0. It is shown that the global gamma index is a weak detector of dose delivery errors, which in some circumstances behaves only slightly better than a purely random classifier.
Purpose The quality of a measured distribution of dose delivered against its corresponding radiotherapy plan is routinely assessed by gamma index (GI) and dose–volume histogram (DVH) metrics. Any correlation between error detection rates, as based on either of these approaches, while argued, has never been convincingly demonstrated. The dependence of the strength of correlation between the GI passing rate (γP) and DVH quality assurance (QA) metrics on various elements of the therapy plan has not been systematically investigated. Methods A formal analysis of the relation between γP and DVH metrics has been undertaken, leading to a relationship which may partly approximate γP with respect to the DVH. This relationship was further validated by studying examples of simulated clinical radiotherapy plans and by studying the correlation between γP and the derived relationship using a simple two‐dimensional representations of the planning target volume (PTV) and organs at risk (OAR), where penumbra regions, distance‐to‐agreement tolerances and dose delivery errors were systematically varied. Results It is shown formally that there cannot be any correlation between γP and other commonly applied DVH‐derived QA measures. However, γP may be partly approximated given the planned and measured DVH. The derived γP approximation (the “γ‐slope indicator”) may be clinically useful in some practical cases of radiotherapy plan QA. Conclusions In formal terms, there cannot be any correlation between γP and any common DVH‐calculated patient‐specific measures, with respect to PTV or OAR. However, as demonstrated analytically and further confirmed in our simulation studies, the γP approximation derived in this study (the “γ‐slope indicator”) may in some cases offer a degree of correlation between γP and the PTV and OAR DVH QA metrics in measured and planned patient‐specific dose distributions—which may be potentially useful in clinical practice.
The aim of this study is to investigate secondary mixed radiation field around linac, as the first part of an overall assessment of out-of-field contribution of neutron dose for new advanced radiation dose delivery techniques. All measurements were performed around Varian Clinic 2300 C/D accelerator at Maria Sklodowska-Curie Memorial, Cancer Center and Institute of Oncology, Krakow Branch. Recombination chambers REM-2 and GW2 were used for recombination index of radiation quality Q4 determination (as an estimate of quality factor Q), measurement of total tissue dose Dt and calculation of gamma and neutron components to Dt. Estimation of Dt and Q4 allowed for the ambient dose equivalent H*(10) per monitor unit (MU) calculations. Measurements around linac were performed on the height of the middle of the linac's head (three positions) and on the height of the linac's isocentre (five positions). Estimation of secondary radiation level was carried out for seven different configurations of upper and lower jaws position and multileaf collimator set open or closed in each position. Study includes the use of two photon beam modes: 6 and 18 MV. Spatial distribution of ambient dose equivalent H*(10) per MU on the height of the linac's head and on the standard couch height for patients during the routine treatment, as well as relative contribution of gamma and neutron secondary radiation inside treatment room were evaluated.
In the present paper a general setup for determination of imperfect geometry of radiotherapeutic devices has been proposed that base on geometric algebra framework. To account for this imperfect geometry, two methods of a calibration were presented, consisting of determining for each angular position of a gantry a correction shift which must be applied to the origin of a laboratory frame of reference to place it along a radiation axis for this angular position. Closed form solutions for these corrections are provided.
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