(Saturday, 4/7/2018)
Room: Foyer
Purpose: To compare the dosimetric characteristics of larynx SBRT plans generated using LINAC-based non-coplanar VMAT technique and Cyberknife technique.
Methods: 10 patients diagnosed of cTis-T2N0M0 glottic larynx carcinoma were studied. All patients have been planned and treated on the Cyberknife system with a prescribed dose of 42.5Gy/5 fractions to the involved hemilarynx. Plans were delivered using fixed-cone due to small PTVs. For each patient, a four-arc non-coplanar VMAT plan was retrospectively designed in Eclipseâ„¢ to evaluate the feasibility of treatments using conventional LINACs. Dose engine was Monte-Carlo for Cyberknife and AcurosXB for Eclipse to achieve accurate dose calculation of PTVs with air cavity. Dosimetric comparisons were performed between VMAT and Cyberknife plans using metrics including PTV coverage, prescribed isodose lines(normalized to global maximum dose), and maximum doses to various OARs. Statistical significance was assessed using Wilcoxon signed-rank test.
Results: The LINAC-based VMAT technique achieves similar dosimetric endpoints as the Cyberknife planning. The PTV coverages of VMAT and Cyberknife plans are 95.0% and 95.6%, respectively. The corresponding mean prescribed isodose lines are 86.3% and 85.9% (p>0.6). The Cyberknife plans have slightly better conformity index than the VMAT plans (1.19(Cyberknife) vs. 1.29(VMAT), p<0.04). The average maximum doses to carotid arteries (12.3Gy(VMAT) vs. 13.9Gy(Cyberknife), p>0.3) and skin (41.6Gy vs. 41.5Gy, p>0.3) are similar. The VMAT plans better spare the contralateral arytenoid than the Cyberknife plans (16.0Gy(VMAT) vs. 18.1Gy(Cyberknife), p<0.04). The Cyberknife plans incur less dose to the spinal cord (8.1Gy(VMAT) vs. 5.8Gy(Cyberknife), p<0.02), both however are well below the tolerance of 28Gy. VMAT uses less than 1/3 of the total MUs of Cyberknife plans and halves average treatment time (~20min vs. 40min, excluding pretreatment setup). Planning time is much shorter with VMAT.
Conclusion: Larynx SBRT can be conducted on either Cyberknife or conventional LINAC with dosimetrically equivalent target coverage and similar OAR sparing.
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