SU‐GG‐T‐585: The Impact of Free Breathing versus Average 4D CT Image Data on External Beam Radiotherapy Planning for Liver Tumors

PURPOSE: To investigate the dosimetric impact of using two different 4DCT image data set for the treatment planning on tumor and surrounding normal tissues in liver patients. MATERIALS AND METHODS: Ten patients with primary liver malignancies underwent radiotherapy (4 3D‐conformal and 6 IMRT) treatment. A set of 4DCT images was obtained for target delineation and an average CT (AvCT) was generated by averaging over 10 phases of the complete breathing cycle. Another set of free breathing (FB) CT was acquired for treatment planning. The internal GTV was contoured based on 4DCT, then copied to FB and AvCT image sets. Organs at risk (OAR) DVHs were generated for the liver, left and right kidneys, stomach and duodenum. RESULTS: SI motion of liver ranged from 0.5 to 2.25 cm, with a median of 1.25 cm. The GTV volume ranged from 77 to 966 cc, and the median was 257 cc. When the same beam arrangements and prescription doses were applied to FBCT and AVCT, no statistically significant difference was observed for the minimum and maximum GTV dosage, prescription dose coverage for the GTV/CTV/PTV, or OAR DVHs (V20 for liver and kidneys, V50 for stomach and duodenum) (p<0.001). The maximum difference observed was ∼4% minimum GTV dose and ∼13% GTV prescription dose coverage, which we believe was partly due to change of air distribution in the GI tract between FBCT and AVCT. CONCLUSIONS: There was no statistically significant difference for target coverage and OAR DVHs observed when the plan generated with FBCT was applied to AvCT. Assuming that the patient's breathing pattern is almost identical at simulation as throughout the treatment course, we believe Average CT planning is a more reproducible representation of the patient's anatomy and tumor and therefore will yield a more accurate plan that could potentially improve tumor coverage.