Poster - 43: Analysis of SBRT and SRS dose verification results using the Octavius 1000SRS detector


  • Cherpak Amanda

    1. Nova Scotia Cancer Centre, Nova Scotia Health Authority, Halifax, NS, Department of Radiation Oncology, Dalhousie University, Halifax, NS, Department of Physics and Atmospheric Sciences, Dalhousie University, Halifax, NS
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The Octavius 1000SRS detector was commissioned in December 2014 and is used routinely for verification of all SRS and SBRT plans. Results of verifications were analyzed to assess trends and limitations of the device and planning methods.


Plans were delivered using a True Beam STx and results were evaluated using gamma analysis (95%, 3%/3mm) and absolute dose difference (5%). Verification results were analyzed based on several plan parameters including tumour volume, degree of modulation and prescribed dose.


During a 12 month period, a total of 124 patient plans were verified using the Octavius detector. Thirteen plans failed the gamma criteria, while 7 plans failed based on the absolute dose difference. When binned according to degree of modulation, a significant correlation was found between MU/cGy and both mean dose difference (r=0.78, p<0.05) and gamma (r=−0.60, p<0.05). When data was binned according to tumour volume, the standard deviation of average gamma dropped from 2.2% – 3.7% for the volumes less than 30 cm3 to below 1% for volumes greater than 30 cm3.


The majority of plans and verification failures involved tumour volumes smaller than 30 cm3. This was expected due to the nature of disease treated with SBRT and SRS techniques and did not increase rate of failure. Correlations found with MU/cGy indicate that as modulation increased, results deteriorated but not beyond the previously set thresholds.