The purpose of this study was to evaluate setup uncertainties for brain sites with ExacTrac X‐Ray 6D system and to provide optimal margin guidelines. Fifteen patients with brain tumor were included in this study. Two X‐ray images with ExacTrac X‐Ray 6D system were used to verify patient position and tumor target localization before each treatment. The 6D fusion software first generates various sets of DRRs with position variations in both three translational and three rotational directions (six degrees of freedom) for the CT images. Setup variations (translation and rotation) after correction were recorded and corrected before treatment. The 3D deviations are expressed as mean±standard deviation. The random error false(normalΣfalse(σifalse)false), systematic error false(μifalse), and group systematic error false(Mfalse(μifalse)false) for the different X‐ray were calculated using the definitions of van Herk. (1) Mean setup errors were calculated from X‐ray images acquired after all fractions. There is moderate patient‐to‐patient variation in the vertical direction and small variations in systematic errors and magnitudes of random errors are smaller. The global systematic errors were measured to be less than 2.0 mm in each direction. Random component of all patients are smaller ranging from 0.1–0.3 mm small. The safety margin (SM) to the lateral, is 0.5 mm and 2.6 mm for van Herk (1) and Stroom et al., (2) respectively, craniocaudal axis is 1.5 mm and 3.4 mm, respectively, and with respect to the antero–posterior axis, 2.3 mm and 3.9 mm. Daily X‐ray imaging is essential to compare and assess the accuracy of treatment delivery to different anatomical locations.PACS number: 87.55.D

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's 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.

An electronic portal imaging device (EPID) is an effective detector for in vivo transit dosimetry. In fact, it supplies two‐dimensional information, does not require special efforts to be used during patient treatment, and can supply data in real time. In the present paper, a new procedure has been proposed to improve the EPID in vivo dosimetry accuracy by taking into account the patient setup variations. The procedure was applied to the breast tangential irradiation for the reconstruction of the dose at the breast midpoint, normalDnormalm. In particular, the patient setup variations were accounted for by comparing EPID images versus digitally reconstructed radiographies. In this manner, EPID transit signals were obtained corresponding to the geometrical projections of the breast midpoint on the EPID for each therapy session. At the end, the ratios R between normalDnormalm and the doses computed by the treatment planning system (TPS) at breast midpoints, normalDm,TPS, were determined for 800 therapy sessions of 20 patients. Taking into account the method uncertainty, tolerance levels equal to ±5% have been determined for the ratio R.The improvement of in vivo dosimetry results obtained (taking into account patient misalignment) has been pointed out comparing the R values obtained with and without considering patient setup variations. In particular, when patient misalignments were taken into account, the R values were within ± 5% for 93% of the checks; when patient setup variations were not taken into account, the R values were within ± 5% in 72% of the checks. This last result points out that the transit dosimetry method overestimates the dose discrepancies if patient setup variations are not taken into account for dose reconstruction. In this case, larger tolerance levels have to be adopted as a trade‐off between workload and ability to detect errors, with the drawback being that some errors (such as the ones in TPS implementation or in beam calibration) cannot be detected, limiting the in vivo dosimetry efficacy.The paper also reports preliminary results about the possibility of reconstructing a dose profile perpendicular to the beam central axis reaching from the apex to the lung and passing through the middle point of the breast by an algorithm, similar to the one used for dose reconstruction at breast midpoint. In particular, the results have shown an accuracy within ± 3% for the dose profile reconstructed in the breast (excluding the interface regions) and an underestimation of the lung dose.PACS numbers: 87.55.Qr, 87.55.km, 87.53.Bn

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