Calibration of Elekta aSi EPIDs used as transit dosimeter.

The transit in vivo dosimetry performed by the Electronic Portal Imaging Device (EPID), avoids the problem of solid-state detector positioning on the patient. Moreover, the dosimetric characterization of the recent Elekta aSi EPIDs in terms of signal stability and linearity enables these detectors adaptable for the transit in vivo dosimetry with 6, 10 and 15 MV photon beams. However, the implementation of the EPID transit dosimetry requires several measurements. Recently, the present authors have developed an in vivo dosimetry method for the 3D CRT based on correlation functions defined by the ratios between the transit signal, st (w,L), by the EPID and the phantom mid-plane dose, Dm(w,L), at the Source to Axis Distance (SAD) as a function of the phantom thickness, w, and the square field dimensions, L. When the phantom mid-plane was positioned at distance d from the SAD, the ratios st(w,L)/s't(d,w,L), were used to take into account the variation of the scattered photon contributions on the EPID as a function of, d and L. The aim of this paper was the implementation of a procedure that uses generalized correlation functions obtained by nine Elekta Precise linac beams. The procedure can be used by other Elekta Precise linacs equipped with the same aSi EPIDs assuring the stabilities of the beam output factors and the EPID signals. The calibration procedure of the aSi EPID here reported avoids measurements in solid water equivalent phantoms needed to implement the in vivo dosimetry method in the radiotherapy center. A tolerance level ranging between ±5% and ±6% (depending on the type of tumor) was estimated for the comparison between the reconstructed isocenter dose, Diso and the computed dose Diso, TPS by the treatment planning system (TPS)

An EPID Dosimetry Verification During Treatment

Purpose: This work reports the extension of a semiempirical method based on the correlation ratios to convert electronic portal imaging devices transit signals into in vivo doses for the step-and-shoot intensity-modulated radiotherapy Siemens beams. The dose reconstructed at the isocenter point Diso, compared to the planned dose, Diso,TPS, and a γ-analysis between 2-dimensional electronic portal imaging device images obtained day to day, seems to supply a practical method to verify the beam delivery reproducibility. Method: The electronic portal imaging device images were obtained by the superposition of many segment fields, and the algorithm for the Diso reconstruction for intensity-modulated radiotherapy step and shoot was formulated using a set of simulated intensity-modulated radiotherapy beams. Moreover, the in vivo dose-dedicated software was integrated with the record and verify system of the centers. Results: Three radiotherapy centers applied the in vivo dose procedure at 30 clinical intensity-modulated radiotherapy treatments, each one obtained with 5 or 7 beams, and planned for patients undergoing radiotherapy for prostatic tumors. Each treatment beam was checked 5 times, obtaining 900 tests of the ratios R = Diso/Diso,TPS. The average R value was equal to 1.002 ± 0.056 (2 standard deviation), while the mean R value for each patient was well within 5%, once the causes of errors were removed. The γ-analysis of the electronic portal imaging device images, with 3% 3 mm acceptance criteria, showed 90% of the tests with Pγ < 1 ≥ 95% and γmean ≤ 0.5. The off-tolerance tests were found due to incorrect setup or presence of morphological changes. This preliminary experience shows the great utility of obtaining the in vivo dose results in quasi real time and close to the linac, where the radiotherapy staff may immediately spot possible causes of errors. The in vivo dose procedure presented here is one of the objectives of a project, for the development of practical in vivo dose procedures, financially supported by the Istituto Nazionale di Fisica Nucleare

Daily On-Line Set-Up Correction in 3D-Conformal Radiotherapy: Is It Feasible?

Aims and background The aim of this report was to investigate the feasibility in terms of treatment time prolongation of an on-line no-action level correction protocol, based on daily electronic portal image verification. Methods and study design The occupation of a linear accelerator (LINAC) delivering 3-D conformal treatments was monitored for two weeks (from Monday to Friday, 10 working days). An electronic portal image device I-View (Elekta, UK) was used for setup verification. Single-exposure portal images were acquired daily using the initial 8 monitor units delivered for each treatment field. Translational deviations of isocenter position larger than 5 mm or 7 mm, for radical or palliative treatments, respectively, were immediately corrected. In order to estimate the extra workload involved with the on-line protocol, the time required for isocenter check and table correction was specifically monitored. Results Forty-eight patients were treated. In all, 482 fractions had electronic portal images taken. Two hundred and forty-five setup corrections were made (50.8% of all fractions). The occupation of the LINAC lasted 106 h on the whole. Twelve h and 25 min (11.7% of LINAC occupation time) were spent for portal image verification and setup correction. On the average, 4.3 fractions per hour were carried out. Conclusions When used by trained therapists, ideally, portal imaging may be carried out before each fraction, requiring approximately 10% of LINAC occupation time

Dosimetric characterization of a large area pixel-segmented ionization chamber.

A pixel-segmented ionization chamber has been designed and built by Torino University and INFN. The detector features a 24 x 24 cm2 active area divided in 1024 independent cylindrical ionization chambers and can be read out in 500 micros without introducing dead time; the digital charge quantum can be adjusted between 100 fC and 800 fC. The sensitive volume of each single ionization chamber is 0.07 cm3. The purpose of the detector is to ease the two-dimensional (2D) verifications of fields with complex shapes and large gradients. The detector was characterized in a PMMA phantom using 60Co and 6 MV x-ray photon beams. It has shown good signal linearity with respect to dose and dose rate to water. The average sensitivity of a single ionization chamber was 2.1 nC/Gy, constant within 0.5% over one month of daily measurements. Charge collection efficiency was 0.985 at the operating polarization voltage of 400 V and 3.5 Gy/min dose rate. Tissue maximum ratio and output factor have been compared with a Farmer ionization chamber and were found in good agreement. The dose profiles have been compared with the ones obtained with an ionization chamber in water phantom for the field sizes supplied by a 3D-Line dynamic multileaf collimator. These results show that this detector can be used for 2D dosimetry of x-ray photon beams, supplying a good spatial resolution and sensibly reducing the time spent in dosimetric verification of complex radiation fields

Combined use of a transmission detector and an epid-based in vivo dose monitoring system in external beam whole breast irradiation: A study with an anthropomorphic female phantom

We evaluate the combined usage of two systems, the Integral Quality Monitor (IQM) transmission detector and SoftDiso software, for in vivo dose monitoring by simultaneous detection of delivery and patient setup errors in whole breast irradiation. An Alderson RANDO phantom was adapted with silicon breast prostheses to mimic the female anatomy. Plans with simulated delivery errors were created from a reference left breast plan, and patient setup errors were simulated by moving the phantom. Deviations from reference values recorded by both monitoring systems were measured for all plans and phantom positions. A 2D global gamma analysis was performed in SoftDiso for all phantom displacements. Both IQM signals and SoftDiso R-values are sensitive to small MU variations. However, only IQM is sensitive to jaw position variations. Conversely, IQM is unable to detect patient positioning errors, and the R-value has good sensitivity to phantom displacements. A gamma comparison analysis allows one to determine alert thresholds to detect phantom shifts or relatively large rotations. The combined use of the IQM and SoftDiso allows for fast identification of both delivery and setup errors and substantially reduces the impact of error identification and correction on the treatment workflow

Calibration of Elekta aSi EPIDs Used as Transit Dosimeter

The transit in vivo dosimetry performed by the Electronic Portal Imaging Device (EPID), avoids the problem of solid-state detector positioning on the patient. Moreover, the dosimetric characterization of the recent Elekta aSi EPIDs in terms of signal stability and linearity enables these detectors adaptable for the transit in vivo dosimetry with 6, 10 and 15 MV photon beams. However, the implementation of the EPID transit dosimetry requires several measurements. Recently, the present authors have developed an in vivo dosimetry method for the 3D CRT based on correlation functions defined by the ratios between the transit signal, st (w,L), by the EPID and the phantom mid-plane dose, Dm(w,L), at the Source to Axis Distance (SAD) as a function of the phantom thickness, w, and the square field dimensions, L. When the phantom mid-plane was positioned at distance d from the SAD, the ratios st(w,L)/s't(d,w,L), were used to take into account the variation of the scattered photon contributions on the EPID as a function of, d and L. The aim of this paper was the implementation of a procedure that uses generalized correlation functions obtained by nine Elekta Precise linac beams. The procedure can be used by other Elekta Precise linacs equipped with the same aSi EPIDs assuring the stabilities of the beam output factors and the EPID signals. The calibration procedure of the aSi EPID here reported avoids measurements in solid water equivalent phantoms needed to implement the in vivo dosimetry method in the radiotherapy center. A tolerance level ranging between ±5% and ±6% (depending on the type of tumor) was estimated for the comparison between the reconstructed isocenter dose, Diso and the computed dose Diso, TPS by the treatment planning system (TPS)

aSi-EPID transit signal calibration for dynamic beams: a needful step for the IMRT in vivo dosimetry

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Large discrepancies between planned and actually delivered dose in IMRT of head and neck cancer. A case report

The case is reported of a patient with locally recurrent carcinoma of the tongue treated with intensity-modulated radiotherapy (IMRT) (simultaneous integrated boost) plus concurrent chemotherapy, who during the third week of radiotherapy developed grade 3 mucositis. Treatment was interrupted for 10 days until significant resolution of the symptoms. At the time of treatment resumption the patient showed 8% weight loss, and in vivo portal dose verification revealed large discrepancies between the computed and measured doses. A new CT scan showed marked tumor shrinkage and modifications to the critical structures. The comparison between the original plan and the hybrid IMRT showed a minimal dose increase in the new target volumes and a marked dose increase in the organs at risk. This case confirms the need for a robust quality assurance program when using IMRT, the feasibility and efficacy of in vivo dosimetry to detect significant discrepancies between planned and delivered dose, and the need to combine IMRT with 4-dimensional radiotherapy, at least for head and neck cancer

integration between in vivo dosimetry and image guided raditherapy for lung tumors

The article reports a feasibility study about the potentiality of an in vivo dosimetry method for the adaptive radiotherapy of the lung tumors treated by 3D conformal radiotherapy techniques 3D CRTs . At the moment image guided radiotherapy IGRT has been used for this aim, but it requires taking many periodic radiological images during the treatment that increase workload and patient dose. In vivo dosimetry reported here can reduce the above efforts, alerting the medical staff for the commissioning of new radiological images for an eventual adaptive plan. The in vivo dosimetry method applied on 20 patients makes use of the transit signal St on the beam central axis measured by a small ion chamber positioned on an electronic portal imaging device EPID or by the EPID itself. The reconstructed in vivo dosimetry at the isocenter point Diso requires a convolution between the transit signal St and a dose reconstruction factor C that essentially depends on i tissue inhomogeneities along the beam central axis and ii the in-patient isocenter depth. The C factors, one for every gantry angle, are obtained by processing the patient\u2019s computed tomography scan. The method has been recently applied in some Italian centers to check the radiotherapy of pelvis, breast, head, and thorax treatments. In this work the dose reconstruction was carried out in five centers to check the Diso in the lung tumor during the 3D CRT, and the results have been used to detect the interfraction tumor anatomy variations that can require new CT imaging and an adaptive plan. In particular, in three centers a small ion chamber was positioned below the patient and used for the St measurement. In two centers, the St signal was obtained directly by 25 central pixels of an a-Si EPID, equipped with commercial software that enabled its use as a stable detector. A tolerance action level of 6% for every checked beam was assumed. This means that when a difference greater than 6% between the predicted dose by the treatment planning system, Diso,TPS, and the Diso was observed, the clinical action started to detect possible errors. 60% of the patients examined presented morphological changes during the treatment that were checked by the in vivo dosimetry and successively confirmed by the new CT scans. In this work, a patient that showed for all beams Diso values outside the tolerance level, new CT scans were commissioned for an adaptive plan. The lung dose volume histograms DVHs for a Diso,TPS=2 Gy for fraction suggested the adaptive plan to reduce the dose in lung tissue. The results of this research show that the dose guided radiotherapy DGRT by the Diso reconstruction was feasible for daily or periodic investigation on morphological lung tumor changes. In other words, since during 3D CRT treatments the anatomical lung tumor changes occur frequently, the DGRT can be well integrated with the IGRT