Maximum leaf speed max?
Sean Frigo
For Varian TrueBeam machines, Philips recommends a lower value than the Varian specification for the Maximum MLC Leaf Speed parameter, 2.25 cm/s versus 2.50 cm/s. The rationale is that this will help ensure VMAT plan deliverability. We feel that by doing so, one is limiting the optimizer solution space, i.e. fewer segments are available.
Has anyone used a value of 2.50 cm/s with success?
How about other conservative recommendations?
Maximum Jaw Speed: 2.25 vs. 2.50 cm/s
Thanks in advance,
Sean Frigo
John Shakeshaft
Hi Sean,
I am not familiar with the particular example of the TrueBeam, so the comments below are a general point.
In coming to a decision as to how to proceed, you need to consider carefully the information that is sent from Pinnacle to the linac, and how the linac then interprets or implements the request. In the DICOM RTPLAN (exported from any TPS) the information that is sent is the start-of-segment and end-of-segment positions of all components and the number of MU to deliver during the segment. The speed that Pinnacle predicts that the linac will use for a motion of a component (eg leaf speed/gantry speed) is *NOT* sent within the DICOM RTPLAN, and so may not be used in reality. On receiving the RTPLAN, the linac control software decides on to deliver the requested segment. There are obviously a number of solutions to the problem given gantry speed, leaf speed, dose rate and possibly jaw speed (if these move) can all be varied. In the past (it’s a while since I have done any detailed work on this), one solution that was adopted by some linacs was to deliver at the maximum gantry speed possible given dose-rate constraints. (Therefore the gantry was only slowed if it was not possible (because max dose rate was insufficient) to deliver the required MU within the segment at the maximum gantry speed.) In this case setting the *ratio* of max gantry speed and max leaf speed incorrectly in Pinnacle could lead to plans that were not deliverable (or at least generated a warning) on the linac due to leaf speed violations.
I suspect because of the many possible ways (with variable gantry speed/leaf speed/dose rate) of delivering X MU between point A and point B, you are not effectively limiting your solution space by using the Pinnacle recommended parameters (unless you have binned dose rate, which you should not).
Regards
John
Dr John Shakeshaft
Director - Therapeutic Physics | Radiation Oncology
Princess Alexandra Hospital | Metro South Health
199 Ipswich Rd Woolloongabba QLD 4102 Australia
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