Tumor shrinkage evaluation based interfractional cone beam computed tomography for limited disease small-cell lung cancer and relevant impact on dosimetry

Background: the pattern of interfractional tumor changes during limiteddisease small-cell lung cancer (ld-sclc) radiotherapy is not clear, the studywas to evaluate tumor changes based on interfractional cbct images and it’simpact on dosimetry. material and method: we analyzed tumor changes andit’s dosimetry impact for 30 ld-sclc patients who were treated withconcurrent chemoradiotherapy(ccrt). cbct images were acquired for eachpatient every five fractions before each treatment. the grass tumor volume(gtv) and total lung were adapted to create the gtvn and total lungn basedon cbctn. dose was recalculated for every cbct fraction. the impact on targetdose coverage and lung sparing was also evaluated while relevant tissue's ctdensity correction was done on plan ct combined with tumor changesadapted by fraction’s cbct images. results: mean gtv volume of each cbctfraction reduced，and mean gtv volume of 7th cbct fraction shrank nearly10% compared to 1st fraction. the centroid positions of left/right tumorsmoved towards the right/left direction gradually. most left/right tumorborders had a trend of rightward/leftward shrinkage. target dose coverageand lung tissue dose volume increased through fractions. ct image densitycorrection slightly increased the target dose coverage and lung tissue dosevolume. conclusion: tumor shrinkage was seen for ld-sclc patients, it’srelated to the tumor's initial volume and location. it is appropriate that mostld-sclc patients should be intervened at 21st radiotherapy fraction.