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

The pattern of interfractional tumor changes during limited disease small-cell lung cancer (LD-SCLC) radiotherapy is not clear, The study was to evaluate tumor changes based on interfractional CBCT images and it’s impact on dosimetry.

We analyzed tumor changes and it’s dosimetry impact for 30 LD-SCLC patients who were treated with concurrent chemoradiotherapy(cCRT). CBCT images were acquired for each patient every five fractions before each treatment. The grass tumor volume(GTV) and total lung were adapted to create the GTVn and total lungn based on CBCTn. Dose was recalculated for every CBCT fraction. The impact on target dose coverage and lung sparing was also evaluated while relevant tissue's CT density correction was done on plan CT combined with tumor changes adapted by fraction’s CBCT images.

Mean GTV volume of each CBCT fraction reduced，and mean GTV volume of 7th CBCT fraction shrank nearly 10% compared to 1st fraction. The centroid positions of left/right tumors moved towards the right/left direction gradually. Most left/right tumor borders had a trend of rightward/leftward shrinkage. Target dose coverage and lung tissue dose volume increased through fractions. CT image density correction slightly increased the target dose coverage and lung tissue dose volume.

Tumor shrinkage was seen for LD-SCLC patients, it’s related to the tumor's initial volume and location. It is appropriate that most LD-SCLC patients should be intervened at 21st radiotherapy fraction.