Margriet Kwint

Chapter 4 80 Discussion To the best of our knowledge, this is the first study where ITACs observed on the CBCT have been systematically analyzed and a decision protocol implemented on how to deal with these changes. This study showed that ITACs frequently occurred during radical radiotherapy of lung cancer patients. During radiotherapy, it is important to have knowledge of and act accordingly to these changes. Many studies have described set-up errors, changes in tumor volume or regression during treatment [2-13,17,18]. We observed ITACs in 72% (N=128) of all lung cancer patients during the radical radiotherapy course. Regression during treatment was described by several groups [2-14,18]. Knap et.al. [5] reported that ⅓ of lung cancer patients undergoing (chemo-) radiotherapy achieved significant tumor shrinkage at the end of radiotherapy. This is in accordance with our findings that in 36% of the patients regression was visible on the CBCT during treatment. Siker et.al. [8] studied tumor volume change on MV-CT and found similar results; in 32% of the patients, regression was visible. Bosmans et.al. [2] studied tumor volume changes in 23 patients in the first two weeks of a course of accelerated RT and reported a variation in tumor volume change: a 30% increase and 30% decrease. Decreased volume was observed from week 2 onwards. This is in accordance with our findings, where only in 2% of patients demonstrated regression in week 1. In our experience, informing the patient about tumor regression seen during treatment, helps them to cope with their side- effects during radiotherapy. Bosman et.al . found in 17% (4/23) of the patients an increase in tumor volume, mainly seen in week 1. This corresponds with our findings that in 12% of patients, tumor progression was visible on CBCT, mainly scored in week 1. It is possible that tumor progression may occur between the planning-CT and start of irradiation. The initial volume increase may be due to tumor progression, edema or inflammation. This study did not show a significant relationship in the time interval between planning-CT and start of treatment and tumor volume increase in the whole group. However, in stage III patients there was a significantly higher chance of adapting a treatment plan due to ITACs, if the time interval between the planning- CT and start of the treatment is more than one week. Therefore, it is necessary to keep this time interval as short as possible. Tumor progression correlated highly with the development of atelectasis and tumor regression. This could be explained if atelectasis developed due to progression. Progressive tumors may have a higher proliferation rate and therefore be more radiosensitive, which could explain the correlation with regression[27]. In our study, the CBCT’s were scored visually. This