The impact of tumor regression on the dose distribution of the three dimensional conformal radiotherapy for non-small cell lung cancer

Background and purpose:The dose distribution of tumor may be affected by geometrical changes due to tumor shrinkage during 3-DCRT for non-small cell lung cancer (NSCLC). In this study, we evaluated the effect of tumor regression during treatment on the dose distribution of 3D-CRT for non-small cell lung cancer. Methods:Ten NSCLC patients, who received 3D-CRT in our hospital between Jan 2003 and Dec 2005, were entered into this study. All patients underwent simulation CT scans before radiotherapy and once during radiotherapy, after about 40 Gy. Target volumes and critical organs were delineated on both CT scans. The gross tumor volumes of two CT scans were compared. Treatment plans, plan 1 and plan 2, were generated based on the two simulation CT scans, respectively. To evaluate the impact of tumor regression, the dose distribution of PTV1 and PTV2 were compared when Plan 1 was used, and V20 of lung, mean lung dose (MLD), Dmax of spinal cord, D1cm3 of spinal cord,mean heart dose(MHD),V55 of esophagus、mean esophagus dose (MED)were selected as the end points to compare the dose distribution of normal tissues when plan 1 was used with plan 3(the hybrid plan of plan 1 and plan 2) . Paired-samples t test was used for statistics.Results:The median absolute loss of the GTV was 9.5cm3(1.7-64.4 cm3)after about 40 Gy radiotherapy, and the median relative loss of GTV was 22.35 %(10.05 %-54.81 %).The difference of GTV before treatment and after about 40 Gy radiotherapy have statistical significance(P=0.015). The dose distribution of PTV was not changed after about 40 Gy external beam radiotherapy, but a modified plan that was based on the intratreatment CT image could significantly diminish the treated heart volume.Conclusions:The conformal radiation therapy plan, based on the first simulation CT scan, still had good tumor coverage after about 40 Gy external beam radiotherapy despite the tumor regression. Replanning can significantly diminish the dose to the heart.