Evaluation of IGRT-Induced Imaging Doses and Secondary Cancer Risk for SBRT Early Lung Cancer Patients In Silico Study.

OBJECTIVES: This study performed dosimetry studies and secondary cancer risk assessments on using electronic portal imaging device (EPID) and cone beam computed tomography (CBCT) as image guided tools for the early lung cancer patients treated with SBRT. METHODS: The imaging doses from MV-EPID and kV-CBCT of the Edge accelerator were retrospectively added to sixty-one SBRT treatment plans of early lung cancer patients. The MV-EPID imaging dose (6MV Photon beam) was calculated in Pinnacle TPS, and the kV-CBCT imaging dose was simulated and calculated by modeling of the kV energy beam in TPS using Pinnacle automatic modeling program. Three types of plans, namely PlanEPID, PlanCBCT and Planorigin, were generated with incorporating doses of EPID, CBCT and no imaging, respectively, for analysis. The effects of imaging doses on dose-volume-histogram (DVH) and plan quality were analyzed, and the excess absolute risk (EAR) of secondary cancer for ipsilateral lung was evaluated. RESULTS: The regions that received less than 50 cGy were significantly impacted by the imaging doses, while the isodose lines greater than 1000 cGy were barely changed. The DVH values of ipsilateral lung increased the most in PlanEPID, followed by PlanCBCT. Compared to Planorigin on the average, the estimated EAR of ipsilateral lung in PlanEPID increased by 3.43%, while the corresponding EAR increase in PlanCBCT was much smaller (about 0.4%). Considering only the contribution of the imaging dose, the EAR values for the ipsilateral lung due to the MV-EPID dose in 5 years,10 years and 15 years were 1.49 cases, 2.09 cases and 2.88 cases per 104PY respectively, and those due to the kV-CBCT dose were about 9 times lower, correspondingly. CONCLUSIONS: The imaging doses produced by MV-EPID and kV-CBCT had little effects on the target dose coverage. The secondary cancer risk caused by MV-EPID dose is more than 8.5 times that of kV-CBCT.

Individualized Fraction Regimen of SBRT Patients With Non-Small Cell Lung Cancer Based on Uncomplicated and Cancer-Free Control Probability.

INTRODUCTION: Stereotactic body radiotherapy (SBRT) currently adopts non-discriminative prescription regimen. This study attempts to investigate an individualized fraction regimen (IFR) method for SBRT patients with non-small cell lung cancer (NSCLC) based on Uncomplicated and Cancer-free Control Probability (UCFCP). METHODS: Twenty patients with NSCLC were retrospectively prescribed with 40 regimens, ranging from 8Gy×5f to 12Gy×5f in step of 0.1 Gy. Taking into consideration of the age and the BMI index of each patient as well, the tumor control probability (TCP), the normal tissue complication probability (NTCP) of the total lung, chest wall and rib, and the secondary cancer probability (SCP) of the total lung were calculated for each plan of the patients. For the 40 regimens, the UCFCP was calculated and the maximum value of UCFCP was the IFR of the specified patient. Besides, IFR of UCP approach which only took account of the TCP and NTCP was also derived and to be compared with the IFR based on the UCFCP method. RESULTS: For all the patients, the UCFCP value showed a bell-shaped trend with the change of prescription dose. Among the 20 patients, the IFRs of 16 patients were different from the original fixed regimen. Of the 16 patients, the IFR of 5 patients exhibited slight changes between UCP and UCFCP methods. CONCLUSION: The method based on the maximum value of UCFCP function may be helpful to provide IFR for specific SBRT patients with NSCLC, differentiating the patient specific characteristics such as anatomical structures and locations.