[p3rtp] Pinnacle\Elekta Users - Picket Fence
Mohr, Dr. Peter
Let me comment on the picket fence / rounded leaf problem:
1) The usual AutoCal MLC calibration is a DOSIMETRIC calibration, i.e. the picket fence test should not show 1mm stripes (neither overdose nor underdose).
2) The pinnacle leaf position is defined as GEOMETRIC position.
Obviously, these definitions are different! The rounded leaf offsets in pinnacle are used to adjust the geometric definition to the dosimetric calibration. I have plotted (approximately to scale!) the geometry at different leaf positions: 200 mm (l200.pdf), 125 mm (l125.pdf), ... -125 mm (overtravel, lm125.pdf). There you can easily see what the offset in pinnacle has to do:
A) At positions close to 0: the leaf tip is too thin to provide a 50% isodose. You have to move the leaf in (0.5 - 1mm) to find the calculated 50% isodose at position 0.
B) At increasing leaf positions from a few mm up to about 125 mm: the leaf cuts more and more from the edge of the beam: you have to move the leaf out (approximately increasing with the square of the position)
C) At further increasing leaf positions beyond 125 mm: the rounded leaf offsets further increase, but because of the finite leaf height the slope of the rounded leaf does not further increase quadratically.
This leads to my suggestions for the "Leaf Offset Calibration" in Pinnacle (see attached "rlo_pinn.pdf", red line). The blue line was recommended by our Philips consultant in 2005; in 2007 this suggestion was slightly modified (green line). The new curve (red line) is a fit to the red data points; these points have been calculated from the shift of the 50% isodose for an empty leaf offset table (offsets = 0 at all positions). The results have been presented at the German Pinnacle Users Meeting in 2009.
Please contact me if you are interested in the numerical values of the red curve. From my point of view, this curve should work for all Elekta LINACs with the Elekta standard 80-leaf MLC (as used in our LINAC which was manufactured in 2005).
Please note (i) that the offsets are large (several mm) for large leaf positions, and (ii) the significant change between the different curves at large positions.
Peter
Dr. Peter Mohr
Strahlentherapie
Diakonie-Klinikum Schwäbisch Hall gGmbH
Tel. 0791 753 4845
Fax 0791 753 4911
Peter...@dasdiak.de
www.dasdiak.de
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Eoin Toomey
Many thanks for your post. This is exactly what I've got my MSc
student working on at the minute. The idea was to put everything back
to zero, and build up an offset curve based on experimental data. I
would be very interested in the numerical values for your red curve.
It would be great if a few centres could compare independent results
with the intention of creating a new, more appropriate default table
in Pinnacle.
Thanks again for your help.
Best wishes,
Eoin
KT
Titus
.......................>>
Sent from a Cloud Device
Hi Peter,
Best wishes,
Eoin
Peter
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Sean White
Thanks for your excellent post.
I would be interested in your offset curve data also. Eoin makes a good suggestion regarding multi-centre collaboration on this project. We are now looking at performing a major calibration on our synergy so that we get no undershoot or overshoot (zero).
Best Regards
Sean White
Senior Medical Physicist Ph: +612 47341401
Nepean Cancer Care Centre Fax: +612 47343570
PO BOX 63 whi...@wahs.nsw.gov.au
Penrith NSW 2751
>>> eoint...@gmail.com 21/02/2011 7:44 pm >>>
Hi Peter,
Best wishes,
Eoin
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Ileana
Hello Peter,
I know I am arriving to the discussion with more than a year of delay, but I
just relatively recently joined the group of pinnacle users. We are trying to
commission Pinnacle for IMRT and the offset table is one of the subjects where
we believe is place for improvement from the previous more general model. I
would be interested in the numerical values of the red curve. We also have the
Elekta machine with the standard 80-leaf MLC.
Thanks in advance for your time
Ileana
Ileana Silvestre
NPL
Douglas Drake
Wouldn't it be great if Philips and Elekta would actually work together and release a standard table for each type of Elekta MLC? Elekta knows the relationship betweeen geometric and dosimetric positions for their MLCs; it's basically hard-coded into their control systems from what I understand. This uncertainty is so many years old. I envy Varian users on this.
=====
Douglas G. Drake, M.S., DABR
Staff Physicist
Dept. of Radiation Oncology
Beaumont Health System
3601 West 13 Mile Rd.
Royal Oak, MI 48073
Phone: 248-551-7035 Fax: 248-551-3784
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