Margriet Kwint

Intra anatomical changes during radiotherapy 81 4 may have caused a bias in the observation of progression and regression because this could be more pronounced in larger tumors. Besides tumor progression and regression, there was a high incidence of other ITACs during treatment. Møller et. al. [19] found in 23% of 163 lung cancer patients changes in lung density on CBCT due to atelectasis, pleural effusion or infiltrative changes. Tumor volume changes were excluded in this study. This is in accordance to our findings, with an incidence of 28% ITACs due to atelectasis, pleural effusion and infiltrative changes. Due to the high incidence and large variability of ITACs it is therefore important that repeated (CB) CT’s are made during the course of radiotherapy and that RTTs are well trained to evaluate these scans. In current clinical practice, more and more radiotherapy departments are implementing CBCT’s. A clear decision protocol could be helpful in guiding the radiation oncologists and RTTs in evaluating changes visible on CBCT. The decision protocol that was implemented in our institute, contains illustrative examples of each of the four urgency levels (figure 1)[28]. All RTTs are trained in using these urgency levels. There are no practical limitations to implement this protocol in other radiotherapy centers. This study is based on weekly CBCT-imaging. In this study, we were unable to distinguish whether the ITAC occurred on the first day following the weekly CBCT or almost a week later before the next weekly CBCT. From January 2012 on, we have implemented daily CBCT guidance for lung cancer patients in our institute in order to assess ITACs as soon as possible and for accurate patient alignment. This is the first investigation on ITAC during RT on CBCT. This study was done retrospectively with weekly CBCT’s. The results of this study needs to be validated in a prospective study to find out if CBCT’s needs to be part of routine clinical practice in radical irradiated lung cancer patients. The dosimetric impact of ITACs on the dose distribution is subject of further research. In conclusion, ITACs have been observed in 72% of all lung cancer patients during the course of radical radiotherapy. In 12% of the patients, the radiation oncologist was required to respond immediately and in 8% of the patients, a new planning-CT- scan was made to mitigate the risk of tumor under dosage. The clinical relevance of the proposed ITAC classification in the lung radiotherapy needs to be validated in a prospective analysis.