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

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)

Nomogram for predicting radiation maculopathy in patients treated with Ruthenium-106 plaque brachytherapy for uveal melanoma

Purpose: To develop a predictive model and nomogram for maculopathy occurrence at 3 years after106Ru/106Rh plaque brachytherapy in uveal melanoma. Material and methods: Clinical records of patients affected by choroidal melanoma and treated with106Ru/106Rh plaque from December 2006 to December 2014 were retrospectively reviewed. Inclusion criteria were: dome-shaped melanoma, distance to the fovea &gt; 1.5 mm, tumor thickness &gt; 2 mm, and follow-up &gt; 4 months. The delivered dose to the tumor apex was 100 Gy. Primary endpoint of this investigation was the occurrence of radiation maculopathy at 3 years. Analyzed factors were as follows: gender, age, diabetes, tumor size (volume, area, largest basal diameter and apical height), type of plaque, distance to the fovea, presence of exudative detachment, drusen, orange pigment, radiation dose to the fovea and sclera. Univariate and multivariate Cox proportional hazards analyses were used to define the impact of baseline patient factors on the occurrence of maculopathy. Kaplan-Meier curves were used to estimate freedom from the occurrence of the maculopathy. The model performance was evaluated through internal validation using area under the ROC curve (AUC), and calibration with Gronnesby and Borgan tests. Results: One hundred ninety-seven patients were considered for the final analysis. Radiation-related maculopathy at 3 years was observed in 41 patients. The proposed nomogram can predict maculopathy at 3 years with an AUC of 0.75. Distance to fovea appeared to be the main prognostic factor of the predictive model (hazard ratio of 0.83 [0.76-0.90], p &lt; 0.01). Diabetes (hazard radio of 2.92 [1.38-6.20], p &lt; 0.01), and tumor volume (hazard radio of 21.6 [1.66-281.14], p = 0.02) were significantly predictive for maculopathy occurrence. The calibration showed no statistical difference between actual and predicted maculopathy (p = 1). Conclusions: Our predictive model, together with its nomogram, could be a useful tool to predict the occurrence of radiation maculopathy at 3 years after the treatment

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)

Online adaptive magnetic resonance guided radiotherapy for pancreatic cancer: state of the art, pearls and pitfalls

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Breast in vivo dosimetry by a portal ionization chamber

This work reports a practical method for the determination of the in vivo breast middle dose value, Dm, on the beam central axis, using a signal St, obtained by a small thimble ion chamber positioned at the center of the electronic portal imaging device, and irradiated by the x-ray beam transmitted through the patient. The use of a stable ion chamber reduces many of the disadvantages associated with the use of diodes as their periodic recalibration and positioning is time consuming. The method makes use of a set of correlation functions obtained by the ratios St Dm, determined by irradiating cylindrical water phantoms with different diameters. The method proposed here is based on the determination of the water-equivalent thickness of the patient, along the beam central axis, by the treatment planning system that makes use of the electron densities obtained by a computed tomography scanner. The method has been applied for the breast in vivo dosimetry of ten patients treated with a manual intensity modulation with four asymmetric beams. In particular, two tangential rectangular fields were first delivered, thereafter a fraction of the dose (typically less than 10%) was delivered with two multi leaf-shaped beams which included only the mammarian tissue. Only the two rectangular fields were tested and for every checked field five measurements were carried out. Applying a continuous quality assurance program based on the tests of patient setup, machine settings and dose planning, the proposed method is able to verify agreements between the computed dose Dm,TPS and the in vivo dose value Dm, within 4%. \ua9 American Association of Physicists in Medicine