Integration between in vivo dosimetry and image guided radiotherapy for lung tumors

The article reports a feasibility study about the potentiality of an in vivodosimetry method for the adaptive radiotherapy of the lungtumorstreated 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 vivodosimetry 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 vivodosimetry method applied on 20 patients makes use of the transit signal S t 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 vivodosimetry at the isocenter point D iso requires a convolution between the transit signal S t 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 D iso in the lungtumor during the 3D CRT, and the results have been used to detect the interfraction tumor anatomy variations that can require new CTimaging and an adaptive plan. In particular, in three centers a small ion chamber was positioned below the patient and used for the S t measurement. In two centers, the S t 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, D iso , TPS , and the D iso 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 vivodosimetry and successively confirmed by the new CT scans. In this work, a patient that showed for all beams D iso values outside the tolerance level, new CT scans were commissioned for an adaptive plan. Thelung dose volume histograms (DVHs) for a D iso , TPS = 2 Gy for fraction suggested the adaptive plan to reduce the dose in lungtissue. The results of this research show that the dose guided radiotherapy (DGRT) by the D iso reconstruction was feasible for daily or periodic investigation on morphological lungtumor changes. In other words, since during 3D CRT treatments the anatomical lungtumor changes occur frequently, the DGRT can be well integrated with the IGRT.