Typical values of z-resolution for different Digital Breast Tomosynthesis systems evaluated in a multicenter study.

PURPOSE: The aim of the present study, conducted by a working group of the Italian Association of Medical Physics (AIFM), was to define typical z-resolution values for different digital breast tomosynthesis (DBT) models to be used as a reference for quality control (QC). Currently, there are no typical values published in internationally agreed QC protocols. METHODS: To characterize the z-resolution of the DBT models, the full width at half maximum (FWHM) of the artifact spread function (ASF), a technical parameter that quantifies the signal intensity of a detail along reconstructed planes, was analyzed. Five different commercial phantoms, CIRS Model 011, CIRS Model 015, Modular DBT phantom, Pixmam 3-D, and Tomophan, were evaluated on reconstructed DBT images and 82 DBT systems (6 vendors, 9 models) in use at 39 centers in Italy were involved. RESULTS: The ASF was found to be dependent on the detail size, the DBT angular acquisition range, the reconstruction algorithm and applied image processing. In particular, a progressively greater signal spread was observed as the detail size increased and the acquisition angle decreased. However, a clear correlation between signal spread and angular range width was not observed due to the different signal reconstruction and image processing strategies implemented in the algorithms developed by the vendors studied. CONCLUSIONS: The analysis led to the identification of typical z-resolution values for different DBT model-phantom configurations that could be used as a reference during a QC program.

Characterization of XR-RV3 GafChromic(®) films in standard laboratory and in clinical conditions and means to evaluate uncertainties and reduce errors.

PURPOSE: To investigate the optimal use of XR-RV3 GafChromic(®) films to assess patient skin dose in interventional radiology while addressing the means to reduce uncertainties in dose assessment. METHODS: XR-Type R GafChromic films have been shown to represent the most efficient and suitable solution to determine patient skin dose in interventional procedures. As film dosimetry can be associated with high uncertainty, this paper presents the EURADOS WG 12 initiative to carry out a comprehensive study of film characteristics with a multisite approach. The considered sources of uncertainties include scanner, film, and fitting-related errors. The work focused on studying film behavior with clinical high-dose-rate pulsed beams (previously unavailable in the literature) together with reference standard laboratory beams. RESULTS: First, the performance analysis of six different scanner models has shown that scan uniformity perpendicular to the lamp motion axis and that long term stability are the main sources of scanner-related uncertainties. These could induce errors of up to 7% on the film readings unless regularly checked and corrected. Typically, scan uniformity correction matrices and reading normalization to the scanner-specific and daily background reading should be done. In addition, the analysis on multiple film batches has shown that XR-RV3 films have generally good uniformity within one batch (<1.5%), require 24 h to stabilize after the irradiation and their response is roughly independent of dose rate (<5%). However, XR-RV3 films showed large variations (up to 15%) with radiation quality both in standard laboratory and in clinical conditions. As such, and prior to conducting patient skin dose measurements, it is mandatory to choose the appropriate calibration beam quality depending on the characteristics of the x-ray systems that will be used clinically. In addition, yellow side film irradiations should be preferentially used since they showed a lower dependence on beam parameters compared to white side film irradiations. Finally, among the six different fit equations tested in this work, typically used third order polynomials and more rational and simplistic equations, of the form dose inversely proportional to pixel value, were both found to provide satisfactory results. Fitting-related uncertainty was clearly identified as a major contributor to the overall film dosimetry uncertainty with up to 40% error on the dose estimate. CONCLUSIONS: The overall uncertainty associated with the use of XR-RV3 films to determine skin dose in the interventional environment can realistically be estimated to be around 20% (k = 1). This uncertainty can be reduced to within 5% if carefully monitoring scanner, film, and fitting-related errors or it can easily increase to over 40% if minimal care is not taken. This work demonstrates the importance of appropriate calibration, reading, fitting, and other film-related and scan-related processes, which will help improve the accuracy of skin dose measurements in interventional procedures.

In vivo EBT radiochromic film dosimetry of electron beam for Total Skin Electron Therapy (TSET).

EBT radiochromic films were used to determine skin-dose maps for patients undergone Total Skin Electron Therapy (TSET). Gafchromic EBT radiochromic film is one of the newest radiation-induced auto-developing photon and electron-beam analysis films available for therapeutic radiation dosimetry in radiotherapy applications. EBT films can be particularly useful in TSET; due to patient morphology, underdosed regions typically occur, and the radiochromic film represents a suitable candidate for monitoring them. In this study, TSET was applied to treat cutaneous T-cell lymphoma. The technique for TSET was implemented by using an electron beam with a nominal energy of 6MeV. The patient was treated in a standing position using dual angled fields in order to obtain the greatest dose uniformity along the patient's longitudinal axis. The electron beam energy was degraded by a PMMA filter. The in vivo dose distribution was determined through the use of EBT films, as well as of thermoluminescent dosimeters for comparison (TLDs). EBT results showed a reasonable agreement with TLDs data.

Use of GAFCHROMIC XR type R films for skin-dose measurements in interventional radiology: Validation of a dosimetric procedure on a sample of patients undergone interventional cardiology.

The Gafchromic XR type R film is a suitable dosimeter to determine the map of the skin dose in patients undergone complex interventional radiological procedures, such as cardiology ones. The need of preventing or locating possible skin injuries due to high doses administered to patients-as recommended by international organizations-wants the introduction in patient dosimetry of a dosimeter easy to handle, with low dependence of the response on energy in the typical radiological range, and extended measurable dose range. XR type R films fulfil all these requirements and moreover may be quickly analyzed by cheap commercial scanners. In order to determine skin-dose values by XR-R, a film calibration curve is required. In this work, validation of the XR-R dosimetry has been performed for the determination of the skin dose: maximum skin-dose values in 14 patients undergone radiofrequency ablation and pacemaker implant procedures have been determined by XR-R calibrated films. A comparison between skin-dose values determined by XR-R films and retrospective ionometric measurements has pointed out some discrepancies in the results, due to difficulties in retrospectively reproducing the real procedure settings, where XR-R film dosimetry is related to the specific patient procedure, even, in very complex interventional settings.

Dosimetric, mechanical, and geometric verification of conformal dynamic arc treatment.

A conformal dynamic arc (CD-arc) technique has been implemented at the S. Giovanni Calibita-Fatebenefratelli Hospital Radiotherapy Center. This technique is performed by rotational beams and a dynamic multileaf collimator (DMLC): during the treatment delivery the gantry rotates and the field shape, formed by the DMLC changes continuously. The aim of this study was to perform dosimetric, mechanical, and geometric verification to ensure that the dose calculated by a commercial treatment planning system and administered to the patient was correct, before and during the clinical use of this technique. Absolute dose values, at the isocenter and at other points placed in dose heterogeneity zone, have been verified with an ionization chamber in a solid homogeneous phantom. In uniform dose regions measured dose values resulted in agreements with the calculated doses within 2%. Isodose distributions have also been determined by radiographic films and compared with those predicted by the planning system. Distance to agreement between calculated and measured isodoses in dose gradient zone was within 2 mm. In conclusion, our results demonstrated the feasibility and the accuracy of the CD-arc technique for achieving highly conformal dose distributions. Up till now 20 patients have been treated with CD-arc therapy.

Radiochromic film dosimetry of a low energy proton beam.

In this work some dosimetric characteristics of MD-55-2 GafChromic films were studied in a low energy proton beam (21.5 MeV) directly in a water phantom. The nonlinearity of the optical density was quantified by a factor P(lin). A correction factor P(en), that accounts for optical density dependence on the energy, was empirically determined. The effects of detector thickness in depth dose measurements and of the film orientation with respect to beam direction were investigated. The results show that the MD-55-2 films provide dose measurements with the films positioned perpendicularly to the proton beam. A dosimetric formalizm is proposed to determine the dose to water at depth d, with films oriented perpendicularly to the beam axis. This formalism uses a calibration factor of the radiochromic film determined directly on the proton beam at a reference depth in water, and the P(lin) factor, that takes into account the nonlinearity of the calibration curve and the P(en) factor that, in turn takes into account the change of proton beam energy in water. The MD-55-2 films with their high spatial resolution and the quasiwater equivalent material are attractive, positioned perpendicularly along the beam axis, for the absolute dose determination of very small beam sizes and modulated proton beams.

Two quality control procedures on radiotherapy beam calibration and treatment planning system implementation.

New challenging dosimetric approaches, such as narrow beams and 3D algorithms, are being used in radiotherapy. In this paper two quality control (QC) procedures are reported. The first one concerns the QC of the dosimetry of small x-ray beams, generally carried out by using silicon detectors. The comparison of dose values obtained by a silicon diode, a diamond detector, and radiochromic films shows that for x-ray beams of high energy, the silicon diode can give an overestimation of the output factors in phantom, up to 4%. This is due to the higher than unit density silicon diode and the surrounding envelope that restore the lateral electron equilibrium. About the 3D algorithms for breast treatment planning, a quality control test has been adopted to verify the accuracy of the computed dosimetry when "loss of scatter" occurs. The results show a sensible agreement (within 1.5%) between computed and experimental data.

[Comparison between measured and computed absorbed dose values in the tangential treatment of the breast]. / Confronto tra i valori misurati e calcolati della dose assorbita nel trattamento tangenziale della mammella.

The calculated absorbed dose values to the reference point of the breast tangential irradiation, obtained by 2-dimensional (2D) Treatment Planning Systems (TPS), were compared with the measured absorbed dose values obtained with a ionization chamber in an anthropomorphic phantom treated with two opposed photon beams of 60Co, 6 MV and 10 MV. This comparison was aimed at assessing the 2D algorithm dose overestimation due to the effect of missing tissue in the irradiated field. The ratio between the computed dose, Dc, and the measured dose, Dm, reached 1.08 for 60Co beams and 1.04 for X-ray beams of linacs. The Clarkson method gives an adequate correction factor for the calculated absorbed dose value in the reference point. The portal films of some patients were studied to estimate the correction factor for the treatment time or monitor units evaluated by TPS using tomographic breast central plane. The values of the correction factors reached 1.10 for 60Co photon beams and 1.04 for X-ray beams. The measurements were carried out with TLDs positioned in the central plane of the breast, covered by a special bolus, of a second anthropomorphic phantom. The dose homogeneity as well as the accuracy of the relative computed dose values in the breast phantom irradiated with 10 MV X-ray beams were assessed.