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Effect of Gantry and Collimator Positional Errors On Volumetric Modulated Arc Therapy(VMAT)

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R Holla

R Holla*, V Anand Ravikumar, B Pillai, Amrita Institute of Medical Sciences, Cochin, Ernakulam,India

SU-E-T-706 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Gantry and Collimator positional errors occur during VMAT delivery. The purpose of this work is to understand the impact of such errors on VMAT dose distribution for selected head and neck cases.

Methods:Five head and neck single arc (360 degree) VMAT treatment plans were created. Two types of systematic errors were simulated for an error magnitude of 1 degree on gantry and Collimator rotations respectively. For gantry, the systematic errors were,
1)Gantry angle is shifted in the same direction of VMAT arc by one degree
2)Gantry angle is shifted in the opposite direction of VMAT arc by one degree.

In case of collimator, the systematic errors of 1 degree in the collimator angle in both positive and negative directions were simulated.The gantry and collimator angles were shifted for all the control points of VMAT arc. The dose was calculated and DVH was computed.

Results:Conformity index (CI), D2, D98 and Dmean for PTV did not show significant variations for gantry and collimator errors in both positive and negative directions for all the 5 patients. For a positive gantry error, the average decrease in Dmean to the left parotid was found to be 80cGy and the same found to be increased for right parotid by 60cGy.
However for both the directions of collimator error, the average increase in D0.01 for spinal cord was 100cGy. For a positive collimator error, the average increase in Dmean for left parotid was 90cGy and the same was decreased by 40cGy for right parotid. For a negative collimator error, the right parotid dose was increased and left parotid dose was decreased.

Conclusion:Effect of one degree collimator error was found to be more significant compared to one degree Gantry error since collimator error increased the maximum dose to spinal cord and the mean dose to the parotids.

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