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Linac Output Reference Point

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Q1 w
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What is the geometry which defines the output reference point calibrated to be 1 cGy/MU at your facility?

  • Answered: 127
  • Skipped: 4
Created with Highcharts 10.3.30%10%20%30%40%50%60%70%80%90%100%10…
SSD at DmaxSSD at D5SSD at D10SAD at DmaxSAD at D5SAD at D10
Answer Choices –
Responses –
–
SSD at Dmax
60.63%
77
–
SSD at D5
0.00%
0
–
SSD at D10
1.57%
2
–
SAD at Dmax
29.13%
37
–
SAD at D5
4.72%
6
–
SAD at D10
3.94%
5
TOTAL127
Comments(4)
Q2 w

If at Dmax, why?

  • Answered: 67
  • Skipped: 64
Institutional history. Varian Spec
7/29/2014 10:14 PM
Historical. Place where chamber depth accuracy makes the least difference.
7/17/2014 06:56 PM
Tradition
7/11/2014 11:28 AM
Historical standard
7/10/2014 12:49 AM
Convention
7/10/2014 12:32 AM
makes sense to me
7/9/2014 06:02 PM
Tradition and manufacrurer's recommendation
7/9/2014 05:39 PM
Consistent with how we were trained and how we calibrated according to TG21.
7/9/2014 05:31 PM
electrons are done SSD and about half the clinical treatments with photons are delivered SSD
7/9/2014 03:48 PM
Tradition.
7/9/2014 12:20 PM
Q3 w

If not at Dmax, why not?

  • Answered: 15
  • Skipped: 116
Dmax has no clinical relevance
7/10/2014 07:55 PM
Dmax depends on energy. Therefore fixed depth easier to measure.
7/8/2014 01:50 PM
TG51
7/8/2014 01:46 PM
historically: for 3D Plans the MUs per Gy are quite close to 100 setup easy to check: Lasers at ref.point, opt.distance meter at surface 90
7/8/2014 01:17 PM
The reference depth in water 5cm (4-10MV) were recommended by ICRU (1973) Measurement of absorbed dose in a phantom irradiated by a single beam of x or gamma rays. Report 23. Weekly output calibration in solid water phantom is also performed at 5cm depth.
7/8/2014 01:00 PM
If you think about it, the greatest uncertainty in output is around dmax, due to electron contamination. If you normalize to dmax, that uncertainty is transferred to all points. If you normalize at 10 cm, then only the dmax region is subject to that uncertainty, rest of dose calculation is more accurate.
7/8/2014 11:12 AM
It's closer to clinical setup.
7/8/2014 09:35 AM
Because trs-398 reasons aré good enough.
7/8/2014 09:13 AM
Just seems a more straightforward way to calibrate and check output. Less chance of an incorrect factor being applied when correcting back to Dmax. (Also a little bit of "that's how it's always been done here").
7/8/2014 08:00 AM
electron scatter contribution in dmax 5 cm SSD 95 corresponds to a clinical situation
7/8/2014 06:56 AM
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