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Evaluation of Factors Affecting Absolute Gamma Passing Rates in ArcCHECK Measurements in Tomotherapy

C.W. Cheung*, C.W. Kong, H.H.F. Choi, K.Y. Cheung, S.K. Yu, Hong Kong Sanatorium & Hospital, Hong Kong


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: In delivery QA (DQA) in Tomotherapy using ArcCHECK (Sun Nuclear Corporation), variabilities of absolute gamma passing rates were observed in different treatment plan conditions. This study aims at identifying and evaluating the factors affecting the absolute gamma passing rates in dose measurements using ArcCHECK in Tomotherapy.

Methods: In total, 35 Tomotherapy helical treatment plans at our hospital were included in this retrospective study. The absolute gamma passing rates with dose difference/distance to agreement criteria of 3%/3mm, 2%/2mm and 1%/1mm and with 10% dose threshold were computed using the SNC Patient software. Four factors which may affect the absolute gamma passing rates in dose measurements were identified: 1. The modulation factor used in plan optimization; 2. The total size of the planning target volumes (PTV); 3. The off-axis distance of the centre of ArcCHECK in the setup for the measurement; 4. The plan complexity index defined as fractional machine units divided by the planned central dose in ArcCHECK and the total PTV size. The correlations between the gamma passing rates and the factors were assessed by Spearman correlation coefficients and the corresponding p-values were calculated.

Results: The results show that the absolute gamma passing rates are positively correlated with the PTV size (3%/3mm:0.26; 2%/2mm:0.39; 1%/1mm:0.52) and negatively correlated with the off-axis distance of the ArcCHECK (3%/3mm:-0.38; 2%/2mm:-0.39; 1%/1mm:-0.29) and the plan complexity index (3%/3mm:-0.42; 2%/2mm:-0.45; 1%/1mm:-0.49). The correlations are statistically significant (p-value < 0.05) except PTV size vs Gamma(3%/3mm) (p-value = 0.067). No significant correlation is observed between the absolute gamma passing rates and the modulation factor.

Conclusion: In this study, factors affecting the absolute gamma passing rates of ArcCHECK DQA measurements in Tomotherapy were identified and their correlations were confirmed statistically. It may help establishment of appropriate set of absolute gamma passing criteria in DQA in Tomotherapy using ArcCHECK.


Tomotherapy, Quality Assurance


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