Integration between in vivo dosimetry and image guided radiotherapy for lung tumors
Authors: Piermattei Angelo , Cilla Savino , Grimaldi Luca , Sabatino Domenico , Fidanzio Andrea , Greco Francesca , Mameli Alessandra , Balducci Mario , Mattiucci Gian Carlo , Frascino Vincenzo , Stimato Gerardina , Gaudino Diego , Ramella Sara , Trodella Lucio , D'Onofrio Guido , Zini Giampaolo , Macchia Gabriella , Digesu’ Cinzia , Morganti Alessio G. , Clemente Stefania , Cozzolino Mariella , Pedicini Piernicola , Fusco Vincenzo 
Published: May 2009

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 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 requires a convolution between the transit signal 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 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 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 measurement. In two centers, the signal was obtained directly by 25 central pixels of an 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, , and the 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 values outside the tolerance level, new CT scans were commissioned for an adaptive plan. Thelung dose volume histograms (DVHs) for a 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 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.