Room: AAPM ePoster Library
Purpose: Motion encompassing SBRT plans use ITV based on tumor motion visualized on simulation 4DCT scans. This approach assumes consistent patient breathing during all fractions. Large tumor motion variations may not be easily detected using 3D-CBCT. We evaluated tumor motion variations and dosimetric consequences using daily 4D-CBCT images.
Methods: Simulation 4DCT and daily 4DCBCT images of twenty lung cancer SBRT patients with abdominal compression were analyzed (total 73). Variation in tumor motion amplitude (distance between centroids of end-of-inhale and exhale GTVs) and ITV (union of all phase GTVs) is defined as the difference between max and min values measured from all 4D images for each patient. Treatment plans (5mm PTV margin) of patients >10mm variations further analyzed dosimetrically using ITV/PTVs from daily 4D-CBCTs.
Results: The mean tumor motion was 9.0 ± 4.1 mm (1.1 to 23.0 mm) in all scans. Thirteen patients had > 25% ITV variations compared to the simulation ITV. Six patients had >5mm and two had >10mm (11.1 and 11.7 mm) tumor motion variations. These two patients (P1 and P2) also had largest ITV variations; 96% and 65% respectively. The min/max ITV volumes for P1 and P2 were 11.0/15.9 cc and 20.2/33.3 cc, respectively. For P1(4 fractions), ITV minDose was 92% for daily 4D-CBCT ITVs (range: 88 - 103%) vs 105% for 4DCT; PTV/ITV coverage was 60/98% for daily 4D-CBCTs (range: 44-85/94-100%) vs 97/100% for 4DCT. For P2 (3fractions); ITV minDose was 78% for daily 4D-CBCT ITVs (range: 51-97%) vs 101% for 4DCT; PTV/ITV coverage was 70/96% for daily 4D-CBCTs (range: 63-78/93-99%) vs 97/100% for 4DCTs.
Conclusion: 4D-CBCT based daily IGRT detected large (>10mm) variations in lung cancer SBRT patient. Further studies are needed to develop adaptive treatment or gating options to mitigate the dosimetric effects caused by these large variations.
Funding Support, Disclosures, and Conflict of Interest: This study was supported by Varian Madical Systems