Pinnacle & ScandiDos Delta 4 (systematic dose error)

[Janssen, Rogier]

L.S.

At our institute we recently bought a ScandiDos Delta 4 phantom for
IMRT/VMAT QA purposes. Unfortunately, right after the calibration
process (and of course a recalibration) we were disappointed to find
considerable deviations in just simple 6MV and 15MV 10x10 box-plans:
Both Pinnacle and our manual MU calculations underestimate the dose at
the isocenter by respectively 2.2% and 1.6%. I understand that Pinnacle
has some known issues in calculating dose in non-water materials like in
our case PMMA (mass density: 1.19 / electron density 1.147). It looks
like we have to handle this problem by either tweaking the calibration
factor or using a different CT2density table e.g. using the electron
density as mass density seems to work fine. I was wondering if there are
other Pinnacle/Delta4 users that experienced the same problem and of
course how they worked around it?

Thank you,
Rogier

R.W.J. Janssen, MSc Medical Physicist
Isala klinieken Zwolle, Department of Radiotherapy
H: Dr. van Heesweg 2; *: P.O. Box 10400, 8000 GK Zwolle, the Netherlands

': +31 (0) 38 4244633; 7: +31 (0) 38 4247604; *: r.w.j....@isala.nl

[Feygelman, Vladimir]

Delta4 works with Pinnacle exceptionally well.

There are a few points however.

The absolute calibration procedure suggested by ScandiDos is flawed. It is theoretically difficult to do an absolute calibration in PMMA with an ion chamber calibrated to dose to water. Instead, I recommend calculating in Pinnacle the dose at the reference point (diode) using the same SDD and scaling the depth by ELECTRON density. Use this number to in the Delta4 reference dose box during absolute calibration.

I also recommend using bulk electron density of PMMA for the phantom representation.

Use the Delta4 daily correction calculation facility. It works very well. You can do as simple as two opposed beams or as elaborate (for VMAT in particular) as a full arc.

Model the couch explicitly.

When all of this is done, with the proper commissioned Pinnacle system one should get gamma(2/2) in excess of 95% for VMAT, and gamma 3/3 consistently close to 100%

_____________________________________________
From: pinnacl...@googlegroups.com [mailto:pinnacl...@googlegroups.com] On Behalf Of Janssen, Rogier
Sent: Wednesday, April 07, 2010 8:02 AM
To: pinnacl...@googlegroups.com
Subject: Pinnacle & ScandiDos Delta 4 (systematic dose error)

L.S.

At our institute we recently bought a ScandiDos Delta 4 phantom for IMRT/VMAT QA purposes. Unfortunately, right after the calibration process (and of course a recalibration) we were disappointed to find considerable deviations in just simple 6MV and 15MV 10x10 box-plans: Both Pinnacle and our manual MU calculations underestimate the dose at the isocenter by respectively 2.2% and 1.6%. I understand that Pinnacle has some known issues in calculating dose in non-water materials like in our case PMMA (mass density: 1.19 / electron density 1.147). It looks like we have to handle this problem by either tweaking the calibration factor or using a different CT2density table e.g. using the electron density as mass density seems to work fine. I was wondering if there are other Pinnacle/Delta4 users that experienced the same problem and of course how they worked around it?

Thank you,

Rogier

R.W.J. Janssen, MSc Medical Physicist
Isala klinieken Zwolle, Department of Radiotherapy

H: Dr. van Heesweg 2; +: P.O. Box 10400, 8000 GK Zwolle, the Netherlands
': +31 (0) 38 4244633; 7: +31 (0) 38 4247604; 8: r.w.j....@isala.nl

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[Yevgeny Fridkin]

Does anyone know about open position for physicist in South Florida?

[Alex Nguyen, UMC.]

Hi Yevgeny,

 

How do you exactly use the daily correction factor?  If I read you correctly, that means running a simple 4-box, or even a AP open field and then read the "Factor derived from measurement" and use it as the daily correction factor for the real measurements?

 

Thanks,

Alex

[Feygelman, Vladimir]

You are correct. We use two 10x10 POP.

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