The paper reports a feasibility study to carry out the adaptive radiotherapy of the lung tumors, guided by an in-vivo dosimetry method. At the moment the image guided radiotherapy (IGRT) is used for this aim, but it requires many periodic radiological images during the treatment that increase the workload and patient dose. The in-vivo dosimetry method reported here can reduce the above efforts alerting the medical staff for the commissioning of new radiological images for an eventual adaptive plan. The reconstructed in-vivo dosimetry at the isocenter point, Diso, requires a convolution between the transit signal, St, obtained by an Electronic Portal Imaging Device and a dose reconstruction factor, C, obtained by processing the patient’s computed tomography scans and that depend on (i) tissue inhomogeneities along the beam central axis, (ii) the in-patient isocenter depth and (iii) the field dimension. In this work the dose reconstruction was carried out to check the Diso in the lung tumor during the 3D conformal radiotherapy technique (3D CRT), and the results have been used to detect the interfraction tumor anatomy variations that can require new CT image and an adaptive plan. One patient showed, at mid-treatment for all beams, Diso values outside the tolerance level of 6%, and the commissioned new CT scans were used for the elaboration of an hybrid plan. The dose volume histograms for a prescribed dose per fraction Diso,TPS = 2Gy suggested an 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 is feasible for daily or periodic investigation about the morphological lung tumor changes. In other words, since during the 3D CRT treatments the lung tumor anatomical changes occur frequently, the DGRT can be well integrated with the IGRT.

The mostly used in-vivo dosimetry techniques require efforts for their implementation measurements, workload for the detector positioning and for the data analysis. The transit in-vivo dosimetry, performed by the Electronic Portal Imaging Device (EPID), avoids the problem of the detector positioning on the patient. Recently the present authors have developed an in-vivo dosimetry method based on correlation functions F(w,L), defined as the ratio between the transit signal, St (w,L), and the phantom mid-plane dose, Dm(w,L) as a function of the phantom water equivalent thickness, w, and of the field dimensions, L. This paper reports a method to determine the generalized correlation functions F(w,L) for the Varian a-Si EPIDs equipped with a commercial software that allows the reading of 0.01 Calibrated Unit per Monitor Unit when the EPID is positioned at the SAD=100 cm and irradiated by a field size 10x10cm2. These generalized correlation functions are generated for a reference linac with a calibration of 1 cGy/UM at the water depth of the maximum dose, dmax, positioned at the SAD, for a 10x10 cm2 field. This way the use of these functions avoids the measurements for the implementation of the in-vivo dosimetry method with the a-Si EPIDs of the same manufacturer. An example of these functions for the 15 MV photon beams is reported in the present paper. Three Varian a-Si EPIDs have been examined to verify their ability to be used as transit detectors to reconstruct the isocenter dose Diso in patient. The in-vivo dosimetry reconstruction adopted here supplies an accuracy well within 5% (2SD), and when Diso is compared with the Diso,TPS, computed by the TPS, a tolerance level up to ±6% has been adopted for different pathologies as pelvis, thorax and head tumors.

A 2D-array equipped with 729 vented plane parallel ion-charnbers has been calibrated as a portal dose detector for radiotherapy in vivo measurements. The array has been positioned by a radiographic film stand at 120 cm from the source orthogonal to the radiotherapy beam delivered with the gantry angle at 180 degrees. The collision between the 2D-array and the patient's couch have been avoided. In this work, using the measurements of the portal detector, we present a method to reconstruct the dose variations in the patient treated with step and shoot intensity-modulated beams (IMRT) for head-neck tumours. For this treatment morphological changes often occur during the fractionated therapy.In a first step an in-house software supplied the comparison between the measured portal dose and the one computed by a commercial treatment planning system within the field of view of the computed tomography (CT) scanner. For each patient, the percentage R, of chainbers, where the comparison is in agreement within a selected acceptance criteria, was determined 8 times. At the first radiotherapy fraction the gamma-index analysis supplied P-gamma values of about 95%, within acceptance criteria in terms of dose-difference, Delta D, and distance-agreement, Delta d, that was equal to 5% and 4 mm, respectively. These acceptance criteria were taken into account for small errors in the patient's set-up reproducibility and for the accuracy of the portal dose calculated by the treatment planning system (TPS) in particular when the beam was attenuated by inhomogeneous tissues and the shape, of the head-neck body contours were irregular. During the treatment, some patients showed a reduction of the P-gamma below 90% because dueto radiotherapy treatment.there was a change of the patient's morphology.In a second step a method, based on dosimetric measurements that used standard phantoms, supplied the percentage dose variations in a coronal plane of the patient using the percentage dose variations measured by the 2D-array portal detector. The results showed that the dose variations due to the change of the patient's morphology reached 15% and'such discrepancies were displayed on the digitally reconstructed radiography of the patient. The dose discrepancies were confirmed by the hybrid plan obtained by the treatment planning system. The good results here reported show that once it is possible to have the portal dose distributions even for other gantry angles, these tests could be introduced in the clinical protocol to have major support to decide when to repeat the patient's CT scan and to replan the new IMRT dose.calculation. (C) 2008 Elsevier B.V. All rights reserved.

PhoNeS (photo neutron source) is a project aimed at the production and moderation of neutrons by exploiting high energy linear accelerators, currently used in radiotherapy. A feasibility study has been carried out with the scope in mind to use the high energy photon beams from these accelerators for the production of neutrons suitable for boron neutron capture therapy (BNCT). Within these investigations, it was necessary to carry out preliminary measurements of the thermal neutron component of neutron spectra, produced by the photo-conversion of X-ray radiotherapy beams supplied by three LinAcs: 15MV, 18MV and 23MV. To this end, a simple passive thermal neutron detector has been used which consists of a CR-39 track detector facing a new type of boron-loaded radiator. Once calibrated, this passive detector has been used for the measurement of both the thermal neutron component and the cadmium ratio of different neutron spectra. In addition, bubble detectors with a response highly sensitive to thermal neutrons have also been used. Both thermal neutron detectors are simple to use, very compact and totally insensitive to low-ionizing radiation such as electrons and X-rays. The resultant thermal neutron flux was above 106n/cm2s and the cadmium ratio was no greater than 15 for the first attempt of photo-conversion of X-ray radiotherapy beams.

A 2D array (PTW, type 10024), equipped with 729 vented plane parallel ion-chambers, has been calibrated as a detector for the in vivo comparison between measured and predicted portal doses for head–neck tumors. The comparison of absolute portal doses measured to ones predicted by a commercial treatment planning system within the field of view of the CT scanner, can help the delivered dose verification during different treatment fractions, in particular when the patient's present weight loss.This paper reports the preliminary results of the comparison of the portal doses measured by a PTW 2D array during several radiotherapy fractions and the predicted portal doses for seven patients undergoing head–neck tumor radiotherapy. The gamma index analysis supplied an agreement of more than 95% of the dose-point Pγ>95% within acceptance criteria, in terms of dose difference, ΔDmax, and distance-agreement, Δdmax, equal to 5% and 4mm, respectively. After the third week, one patient showed a decrease of Pγ values due to the markedly reduced patient's thickness. Even if the spatial resolution of the 2D array was 1cm, there were two advantages in the use of this 2D array as a portal dose device for IMRT quality control. The first one was the use of a stable and efficient absolute dosimeter for in vivo verification, although its construction and behavior for other gantry angles need to be tested, and the second one was the time efficiency in verifying the correct dose delivery in several fractions of the therapy.This study presents acceptance criteria for the comparison of TPS-predicted portal dose images with in vivo 2D ion-chamber measurements for IMRT. In particular, portal dose measurements offer clues for additional studies as to which indicators can signal the need for replanning during treatment.

In this work a linear array of liquid ion-chambers has been used as a detector for the transmission dose below a Rando phantom. The good dosimetric characteristics of the LA48 linear array, that uses 47 liquid ionization chambers, have suggested its use as a detector to verify computed portal dose, obtained by a treatment planning system (TPS) first for conventional irradiation beams (open beams) and later for step and shoot intensity modulated radiotherapy (IMRT) beams. For this last technique, that often uses many sub beam sequences with a small number of monitor units (MU), the early and recent electronic portal imaging devices (EPID) present deviations at beam start-up that affect the transmitted dose accuracy.Absolute transmitted dose profiles with a spatial resolution of 1 mm have been determined along a direction orthogonal to the beam central axis below an anthropomorphic phantom by the LA48, calibrated in terms of dose to water. Computed portal dose profiles have been obtained by a commercial TPS. For open beams of different sizes, the computed portal dose profiles of 20 anthropomorphic phantom sections have been compared with the experimental dose profiles. The percentage of the 7 index values < 1, P-y<1, is greater than 90%, when DeltaD(max) = 3% and Deltad(max) = 3 mm acceptance criteria were selected. Using the step and shoot IMRT beams, which give sequences with small numbers of MU, the acceptance criteria for the dose computation, DeltaD(max) = 5% and Deltad(max) = 4 mm were selected to obtain a percentage P-y<1 > 90%. These acceptance criteria were obtained by a simulation of step and shoot IMRT for head tumors in anthropomorphic phantom.The simulation procedure was a useful training for physicists and oncologists involved in the step and shoot IMRT Process, to obtain a greater comfort with this technique. indeed the simulation on an initial clinical site supplied useful information about: (i) the correct execution of the beam sequences, (ii) the correct delivery of the doses due to small number of MUS, (iii) the accuracy needed to the beam centering, observing the agreement between computed and measured portal dose profiles also near the inhomogeneities.The results reported here could encourage plans to make 2-D detectors that better simulate a simple phantom for the transmitted dose measurements.

The experimental dosimetry in a water phantom of a 32P linear source, 20 mm in length, used for the brachytherapy of coronary vessels is reported. The source content activity, A, was determined by means of a calibrated well ion-chamber and the value was compared with the contained activity reported in the manufacturer's certification. In this field of brachytherapy dosimetry, radiochromic film supplies a high enough spatial resolution. A highly sensitive radiochromic film, that presents only one active layer, was used in this work for the source dosimetry in a water phantom. The radiochromic film was characterized by electron beams produced by a clinical linac. A Monte Carlo calculation of beta spectra in water at different distances along the source transverse bisector axis allowed to take into account the low dependence of film response from the electron beam energy. The adopted experimental set-up, with the source in its catheter positioned on the film plane inside the water phantom, supplies accurate dosimetric information. The measured dose rate to water per unit of source activity at reference distance, D(r0, theta0)/A, in units of cGy s(-1) GBq(-1), was in agreement with the value reported in the manufacturer's certification within the experimental uncertainty. The radial dose function, g(r), is in good agreement with the literature data. The anisotropy function F(r, theta) is also reported. The analysis of the dose profile obtained at 2 mm from the source longitudinal axis shows that the uniformity is within 10% along 75% of the 20 mm treatment length. The adopted experimental set-up seems to be adequate for the quality control procedure of the dose homogeneity distribution in the water medium.