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Background: Stereotactic body radiotherapy (SBRT) is suggested to pose higher second cancer risk than conventional three-dimensional conformal radiotherapy (3D-CRT) for generating greater scatter/ leakage dose to distant organs. Epidemiological reports indicate the need to include primary beam effect in risk assessment because of its great contribution to total cancer risk.
Objective: To determine the second cancer dosimetric index, organ equivalent dose (OED), for organ in treatment field (planning target volume, PTV), beam border (uninvolved liver), and distant area (stomach and pancreas) in patients with liver cancer treated either by Cyberknife SBRT (Cyber-SBRT) or 3D-CRT.
Methods: Treatment plans for seven patients were optimized and prescription dose of 45 Gy were delivered in 15 Gy x3 fractions for Cyber-SBRT and 1.8 Gy x25 fractions for 3D-CRT. OED for primary beam was calculated from differential dose volume histogram. Image-guided dose and scatter/ leakage dose from each treatment were measured in Rando phantom using thermoluminescence dosimeters.
Results: For primary beam component, OEDs of PTV were comparable for both treatments (p = 0.00003). In organs outside the treatment field, Cyber-SBRT generated much lower OEDs than 3D-CRT (p £ 0.059). OEDs for scatter/ leakage component were smaller for 3D-CRT but their contributions to total OEDs were < 1%. OED from image-guided procedure in Cyberknife SBRT was relatively small. In overall, total OEDs were comparable between Cyber-SBRT and 3D-CRT.
Conclusion: Total OEDs of normal tissues from both treatments were comparable or apparently lower for SBRT than 3D-CRT (p ³ 0.20) while total OED of PTV from Cyber-SBRT is slightly higher than that of 3D-CRT (p < 0.05).
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