Search

The purpose of this work is to provide guidelines for the routine use of portal dosimetry and in vivo diode measurements to verify intensity-modulated radiotherapy (IMRT) treatments. To achieve tolerance levels that are sensitive enough to intercept problems, both the portal dosimetry and the in vivo procedure must be optimised. Portal dosimetry was improved by the introduction of an optimised twodimensional (2D) profile correction, which also accounted for the effect of backscatter from the R-arm. The scaled score, indicating the fraction of points not meeting the desired gamma evaluation criteria within the field opening, was determined as the parameter of interest. Using gamma criteria of a 3%dose difference and 3 mm distance to agreement, a ''scaled score'' threshold value of 1.5% was chosen to indicate excessive tongue and groove and other problems. The pre-treatment portal dosimetry quality assurance (QA) does not encompass verification of the patient dose calculation or position, and so it is complemented by in vivo diode measurements. Diode positioning is crucial in IMRT, and so we describe a method for diode positioning at any suitable point. We achieved 95% of IMRT field measurements within ±5% and 99% within ±8%, with improved accuracy being achieved over time owing to better positioning. Although the careful preparation and setup of the diode measurements can be time-consuming, this is compensated for by the time efficiency of the optimised procedure. Both methods are now easily absorbed into the routine work of the department. [ABSTRACT FROM AUTHOR]

Background and Purpose: This study presents a technical description of the solid multipurpose phantom (MPP) 'OPERA' (operational phantom for external radiotherapy audit) for mailed dosimetry checks of photon beams in reference and non-reference conditions. The aim is to test the functionality of this phantom by comparing the results provided by the local treatment planning system (TPS) with the measurements by film and thermoluminescence dosimeters (TLDs) in a number of clinical conditions., Material and Methods: The polystyrene MPP is constructed to check the following five irradiation conditions: square fields, asymmetrical fields, wedged beams, oblique incidence and influence of inhomogeneities in the field. The absorbed dose on the central beam axis is measured with TLDs for the first three irradiation conditions and the relative dose distributions are verified with film., Results: The 'OPERA' phantom and the corresponding instruction sheets were mailed to radiotherapy centres of 12 different countries, in order to verify its functionality. A good agreement between TPS and the film data is found: the mean deviations between relative dose profiles obtained from film and locally applied TPS data range between -1.2 and 0.4%, except for the short side of the asymmetrical field. The standard deviations (SDs) generally increase off-axis, with a large SD on the asymmetrical side. Considering the TLD results, a mean ratio of measured to stated dose of 0.995+/-0.014 (1 SD), 0.988+/-0.019 (1 SD) and 0.981+/-0.024 (1 SD) for set-up 1 (square field), 2 (asymmetrical field) and 3 (wedged field), respectively, has been found., Conclusion: The 'OPERA' phantom can be useful for on- and off-axis verification of the TPS, as well as for mailed dosimetrical checks of photon beams in reference and non-reference conditions.

Background and Purpose: Quality assurance (QA) in intensity modulated treatments is a complex and time-consuming process. In spite of intensive quality control procedures some types of errors still can go undetected through the course of a treatment. This study aims to develop an objective QA filter for fast, automatic detection of errors, based on the creation of a global platform monitoring treatment parameters by comparison with existing local standards. Since such a conceptually new type of QA has already proven successful for conventional treatments, we aim to extend it to intensity modulated radiotherapy (IMRT) treatments., Material and Methods: The feasibility of developing such a QA platform was evaluated on 12 prostate treatment plans. Apart from the classical treatment parameters, the optimised fluence distributions were compared and screened for repetitive patterns. This screening was performed by calculating specific parameters, defined to characterise the fluence maps., Results: Repetitive patterns were found in the beam shapes as well as in the intensity distributions, and useful parameters could be defined to quantify typical field patterns. By statistical analysis of these parameters, mean values and tolerance levels were derived, providing a means to automatically filter out unprobable or erroneous intensity modulated treatment fields during the treatment preparation stage., Conclusion: It is possible to identify parameters quantifying the characteristic patterns found in fluence distributions of intensity modulated fields of a specific treatment, allowing the development of a platform for automatic pre-treatment quality control.

Background and Purpose: Although intensity modulated radiation therapy is characterized by three-dimensional dose distributions which are often superior to those obtained with conventional treatment plans, its routine clinical implementation is partially held back by the complexity of the beam verification. This is even more so when a dynamic multileaf collimator (dMLC) is used instead of a segmented beam delivery. We have therefore investigated the possibility of using a commercially available, liquid-filled electronic portal imaging device (EPID) for the pre-treatment quality assurance of dynamically delivered dose distributions., Methods and Materials: A special acquisition mode was developed to optimize the image acquisition speed for dosimetry with the liquid-filled EPID. We investigated the accuracy of this mode for 6 and 18 MV photon beams through comparison with film and ion chamber measurements. The impact of leaf speed and pulse rate fluctuations was quantified by means of dMLC plans especially designed for this purpose. Other factors influencing the accuracy of the dosimetry (e.g. the need for build-up, remanence of the ion concentration in the liquid and bulging of the liquid at non-zero gantry angles) were studied as well. We finally compared dosimetric EPID images with the corresponding image prediction delivered without a patient in the beam., Results: The dosimetric accuracy of the measured dose distribution is approximately 2% with respect to film and ion chamber measurements. The accuracy declines when leaf speed is increased beyond 2 cm/s, but is fairly insensitive to accelerator pulse rate fluctuations. The memory effect is found to be of no clinical relevance. When comparing the acquired and expected distributions, an overall agreement of 3% can be obtained, except at areas of steep dose gradients where slight positional shifts are translated into large errors., Conclusions: Accurate dosimetric images of intensity modulated beam profiles delivered with a dMLC can be obtained with a commercially available, liquid-filled EPID. The developed acquisition mode is especially suited for fast and accurate pre-treatment verification of the intensity modulated fields.

Purpose: To present the results of the dummy run of the European Organization for Research and Treatment of Cancer (EORTC) trial investigating the role of adjuvant internal mammary and medial supraclavicular (IM-MS) irradiation in Stage I--III breast cancer., Methods and Materials: All participating institutions were asked to produce a treatment plan without (Arm 1) and with (Arm 2) simultaneous IM-MS irradiation of 1 patient after mastectomy and of 1 patient after lumpectomy. Thirty-two dummy runs have been evaluated for compliance to protocol guidelines, with respect to treatment technique and dose prescription., Results: A number of more or less important deviations in treatment setup and prescription have been found. The dose in the IM-MS region deviated significantly from the prescribed dose in 10% of the cases for Arm 1, and in 21% for Arm 2. Assuming a true 5% 10-year survival benefit from optimal IM-MS irradiation, an increase of only 3.8% will be found due to this suboptimal dose distribution., Conclusion: In the dummy run, a number of potential systematic protocol deviations that might lead to false-negative results were detected. By providing recommendations to the participating institutions, we expect to improve the interinstitutional consistency and to promote a high quality irradiation in all institutions participating in the trial.

Aim: A feasibility study has been performed to investigate the possibility of using mailed thermoluminescence dosimetry (TLD) for external audits of clinical electron beams in Europe., Methods: In the frame of the EC Network Project for Quality Assurance in Radiotherapy, instruction sheets and mailing procedures have been defined for mailed TLD dosimetry using the dedicated holder developed by a panel of experts of the International Atomic Energy Agency (IAEA). Three hundred and thirty electron beam set-ups have been checked in the reference centres and some local centres of the EC Network Project and in addition through the centres participating to the EORTC Radiotherapy Group trial 22922., Results: The mean ratio of measured dose to stated dose is 0.2% and the standard deviation of measured dose to stated dose is 3.2%. In seven beam set-ups, deviations greater than 10% were observed (max. 66%), showing the usefulness of these checks., Conclusion: The results of this feasibility study (instruction sheets, mailing procedures, holder) are presently endorsed by the EQUAL-ESTRO structure in order to offer in the future to all ESTRO members the possibility to request external audits of clinical electron beams.

This article deals with the potential benefits of portal imaging as a tool for the dosimetric control of a radiotherapy treatment. In the introduction the advantages of dosimetric control with portal imaging are described. Subsequently, the physical problem is considered between patient exit dose and dose at the level of the portal detector. Measurements with phantoms have shown that this relationship is complex: different algorithms have been developed by groups of researchers and some of them are summarised in this article. Finally, three examples of dosimetric control using portal imaging are presented: in vivo dosimetry on the beam axis, dosimetric profiles in two dimensions and portal dosimetry for intensity modulation.

Background and Purpose: Due to the complexity of the treatment preparation in radiotherapy, a number of errors go undetected until after the first treatment session. Some of these errors could easily have been noticed before treatment if an objective filter existed in addition to human supervision. With this in mind, a conceptually novel extension to conventional quality assurance procedures was explored to create a global platform monitoring treatment preparation by comparison with the existing local standards., Materials and Methods: The feasibility of developing such a platform was evaluated for a test case on a cohort of 202 patients having received breast irradiation. By statistical analysis of the treatment parameters, mean values and tolerance levels could be defined for most parameters based on the observed standard deviations. Useful correlations were traced providing us with a means to automatically track errors, the detection of which would otherwise solely depend upon the alertness of the supervisor., Results and Conclusions: Apart from its possibilities as a mere quality control tool, the platform, developed in the framework of EQUART (European Quality Assurance Program in Radiotherapy by Monitoring Treatment Preparation), can be incorporated in the treatment preparation chain, providing standard setup values for the simulation. A crucial achievement of EQUART lies in the fact that filtering out of errors occurs prior to treatment initiation.

The possibility of using a commercially available electronic portal imaging device for transit dosimetry was investigated. The detection unit of the device comprises a metal plate/fluorescence screen and a camera. Basic parameters of this system were investigated: stability, detector uniformity, dose-response curve, field-size dependence and phantom-thickness dependence. It was found that in terms of relative dosimetry, portal images corrected for detector non-uniformity are in good agreement (within 5%) with transit dose distributions measured by film dosimetry. For dose determination, it was found that the use of the device is hampered by an important field-size dependence and phantom-thickness dependence. Both correction factors should be applied if the device is to be used for this purpose.