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Correlation Between 2D-Gamma Index Passing Rate and 3D-DVH Based Patient Specific Quality Assurance

S SHARMA*, S Chander , V SUBRAMANI , P KUMAR , N GOPISHANKAR , S BHASKAR , S PATHY , S THULKAR , R Kumar , M SINGH , A BINJOLA , P KUMAR , N DHAYANETHI , All India Institute of Medical Sciences, New Delhi, Delhi


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Aim is to find correlation between 2D-gamma passing rate and 3D-DVH based pre-treatment patient-specific quality-assurance.

Methods: 21 head-neck and 21 pelvis patients, treated with VMAT were selected for this study. Patients were planned with Elekta VersaHD linear-accelerator using Monaco (5.11) treatment planning system. 2D-planar dose measurements were performed with IBA-I'matriXX evolution detector-array using My-QA-Patients software. For 2D-Gamma index evaluation 3%/3mm criteria was used. 3D-dose measurements were performed using IBA-COMPASS system. For 3D-measurement, Monaco and COMPASS doses were compared in terms of percentage dose differences to PTV and organ at risks. For PTV D95, D2 and D50 (dose received by 95%, 2% and 50% volume), similarly for OARs serial-structure D2 and parallel-structure D50 was noted. Correlation coefficient and its corresponding two tailed p-value (≤0.05, for statistically significant) was calculated for 2D-gamma passing rate and percentage dose differences of 3D-DVH based metrics (Monaco calculated versus COMPASS measured). Strength of correlation will be considered weak or strong based on r value ≤0.59 and ≥0.60 respectively.

Results: 2D-Gamma index passing rate was 98.6±1.8% and 98.5±1.3% for head-neck and pelvis patients respectively. Percentage dose-differences for PTV D95, D2, D50 for head-neck and pelvis were: 4.22±2.09%, 3.25±2.23%, 3.93±1.59 & 0.60±1.96%, 1.53±1.64%, 1.59±1.20% respectively. For Spine and brainstem D2 were -0.84±6.10%, 0.77±2.70%, bladder and rectum D50 were 3.75±3.31%, -2.19±3.60%.Correlation coefficient for head-neck patients, between PTV and 2D-gamma were as: PTV-D95(r=0.416, p=0.061), PTV-D2 (r=0.278, p=0.223) and PTV-D50 (r=0.072, p=0.758). Spine and brainstem D2 (% dose difference) with 2D-gamma showed Correlation coefficient(r=0.102, p=0.677 & r=0.431, p=0.051) respectively.Similarly, for pelvis patients: PTV-D95(r=0.511, p=0.018), PTV-D2 (r=0.387, p=0.083) and PTV-D50 (r=0.327, p=0.148). Bladder and rectum D50 with 2D-gamma showed Correlation coefficient(r=0.039, p=0.866 & r=0.130, p=0.575) respectively.

Conclusion: weak correlation (not statistically significant) was observed between 2D-Gamma index passing rate(3%/3mm) and percentage dose differences of 3D-DVH based pre-treatment quality assurance using COMPASS.


Not Applicable / None Entered.


TH- External beam- photons: Quality Assurance - VMAT

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