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Evaluation of Velocity Dependent Positional Error of Dynamic Multi Leaf Collimator During VMAT Delivery Using a Well Defined Mathematical Function


b sarkar

A Manikandan1, B Sarkar2*, M Nandy3, N Sujatha4, S Mallik5, J Goswami6, (1) Inlaks and Budhrani Hospitals, ,,(2) Advance Medicare and Research Institute Hospitals, Kolkata, West bengal, (3) Saha Institute of Nuclear Physics, ,,(4) Government Medical College, Guntur, ,AP, (5) West bank Hospital, Howrah, West bengal, (6) West bank Hospital, ,West bengal

SU-E-T-51 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To investigate the correlation between Multileaf Collimator (MLC) positional errors against its speed and dose rate during volumetric modulated arc therapy (VMAT) delivery by an Elekta Beam Modulator LINAC, using Am-Si EPID while MLC's were moving in a mathematically well defined velocity.
Methods: VMAT is delivered by the control points (CP). Each CP, is characterized by a definite aperture ?=f(?, dXi /dt) , a dose rate defined by monitor unit M=f(?, dXi /dt ) (?- gantry angular speed and Xi position of ith MLC). The MLC motion can be written as an explicit function either of time only or of gantry angle and time Xi =f (t) or f (?, t). The MLCs were moved with linear, parabolic and elliptical velocity defined as a function of time. Beam delivery was interrupted at certain points and portal images ware acquired and MLC positional error (MLCPE) was calculated.
Results: The velocity dependent MLCPE were evaluated for all combinations of discrete dose rates 35, 70, 140, 280, 560 MU/min and MLC speeds 0.5, 1, 1.5, 2 and 2.5 cm/sec. Maximum MLCPE is 1.59 mm for 2.5 cm/sec-70Mu/min. Mean MLCPE was 0.25 mm (SD 0.02). There was significant relation of MLC positional error with the velocity in 7th (p=0.017) ,12th (p=0.11),15th (p=0.26), 22nd(p=0.045), 24th (p=0.011),26th (p=0.043), 27th (p=0.007), 28th (p=0.042), 29th (p=0.01), 31st (p= 0.017), 33rd (p=0.01),34th (p=0.008), 35th (p=0.030), 36th (p=0.035), 37th (p=0.042), 39th (p=0.046), 40th (p=0.017)MLC's . where p is probability coefficient. There was no association of dose rate with MLCPE. On sub group analysis we found MLCE was more correlated with velocity when dose rate was 35 MU/m with R2 value of 0.8 compared with other dose rates (70MU/min, 140MU/min, 280MU/min, 560MU/min).
Conclusions: MLCPE never exceeded clinically significant value of 2mm, showing a strong velocity dependency but no significant dose rate dependency.

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