Evaluation of Motion Effect on Dose Delivery in Tumor Tracking Based Stereotactic Body Radiotherapy of Abdominal Targets

Purpose: To evaluate the effects of respiratory motion on the delivered dose distribution in motion tracking-based stereotactic body radiotherapy of abdominal targets. Methods: Eight patients with tumors in the abdominal region (three with pancreas tumors and five with liver tumors) were retrospectively selected; all of them received 4DCT during simulation. A3D plan was optimized on the endexpiration phase in the Cyber Knife Synchrony Multi Plan treatment planning system, with 40Gy prescribed in 5 fractions. A4D plan was then created using the Multi Plan 4D planning module. The other seven phases of the 4DCT were deformably registered to the end-expiration phase, and dose distribution in each phase was deformably mapped to the end-expiration phase for dose summation. Doses to the target and organs at risk, including duodenum, liver, spinal cord and both kidneys were compared between 3D and 4D plans for each patient. Results: The target coverage in 4D plans which represent the real dose delivered could be compromised by up to 10.3% for some patients. Doses to organs varied depending on the location, with maximum dose differences observed up to 8.3Gy higher or 5.4Gy lower in 4D plans. Doses to the spinal cord and both kidneys were significantly different between 3D and 4D plans. Conclusion: The delivered dose can be significantly different from the 3D planned dose for both target and organs at risk, which is caused by the patient anatomy changes while the radiation beams are chasing the moving target. This study suggests a 4D dose recalculation may be necessary to reliably assess the real doses delivered to patients in the stereotactic treatment of abdominal targets with tumor tracking strategy.