physics treatment margins · bujold et al, image-guided radiotherapy: has it influenced patient...
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Physics – Treatment MarginsLaurence Court
University of Texas MD Anderson Cancer Center
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Disclosure
• Employer: UT MD Anderson Cancer Center
• Grants from: NCI, CPRIT, Varian, Elekta, Mobius
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Learning Objectives
• Understand how systematic and random uncertainties contribute to overall treatment uncertainties and how these relate to delivered dose
• Understand how to calculate PTV margins
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Figures from Khan
A reminder…..
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Random and systematic uncertainties
Systematic uncertainty
Random uncertainty
low
low
low
high low
high high
high
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Systematic vs. random uncertainties
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Bujold et al, Image-Guided Radiotherapy: Has It Influenced Patient Outcomes?, Seminars in Radiation Oncology, 22, 50, 2012
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Systematic vs. random uncertainties
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Bujold et al, Image-Guided Radiotherapy: Has It Influenced Patient Outcomes?, Seminars in Radiation Oncology, 22, 50, 2012
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Residual setup uncertainties
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Bujold et al, Image-Guided Radiotherapy: Has It Influenced Patient Outcomes?, Seminars in Radiation Oncology, 22, 50, 2012
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Effect of uncertainties on delivered dose
9McCarter et al, PMB 45: 923-931, 2001
• High isodoses move in• Low isodoses move out
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There are lots of margin formulae
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It’s not this bad!
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Calculating uncertainties
Van Herk, Sem. Rad. Onc. 14(1), 52-64, 2004
For the scenario where we want minimum dose to CTV of 95% for 90% of patients, then:
• For lung, dose gradient is more shallow, so smaller margins needed for random errors• If dose prescription is for lower percent (e.g.80%), then smaller margins
McCarter et al, PMB 45: 923-931, 2001
M=2.5+0.7
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Calculating uncertainties
Van Herk, Sem. Rad. Onc. 14(1), 52-64, 2004
M=2.5+0.7
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Estimating margins -a simple spreadsheet
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5= 0.7=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
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Examples of delineation uncertainties
White et al, Clin. Onc. 21, 32-38 2009 (CBCT images)
Persson et al, Brit. J. Radiol. 85, e654-660, 2012
s.d. = 1.8, 2.1, 3.6mm in RL, AP, SI directions
s.d. = 1.5, 2.6mm in transverse and SI directions
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Estimating margins for prostate (no IGRT)
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5= 0.7=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
2.5mm3mm 3mm
2mm1mm
1mm
4.0mm 3.7mm10.0mm 2.9mm
13mm
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Estimating margins for prostate (perfect IGRT)
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5= 0.7=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
2.5mm3mm 3mm
2mm1mm
1mm
2.5mm 1mm6.2mm 0.7mm
7mm
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Now improve the delineation
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5= 0.7=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
1.5mm3mm 3mm
2mm1mm
1mm
1.5mm 1mm3.8mm 0.7mm
4.5mm
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Calculating uncertainties
Van Herk et al, IJROBP 47(4) 1121-1135, 2000
For the scenario where we want minimum dose to CTV of 95% for 90% of patients, then:
But shouldn’t we be aiming for something better than 90% of patients with modern immobilization, localization etc?
M=2.5+0.7
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But this is only for 90% of patients…..What about increasing to 95%?
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5= 0.7=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
1.5mm3mm 3mm
2mm1mm
1mm
1.5mm 1mm4.2mm 0.7mm
4.9mm
2.8
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But what about deformations?
• SD of inter-fraction variation after marker-based rigid translation
Van der Wielen,, IJROBP 72(5), 1604-1611, 2009
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So?
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5= 0.7=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
1.5mm2mm 2mm
2mm1mm
1mm
2.5mm 2.2mm6.2mm 1.6mm
7.8mm
(accounting for some deformation, but still assuming super-high quality delineation)
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Estimating margins – H&N
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5= 0.7=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
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Estimating margins – H&N
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
2mm
Lets start with a small, perfectly immobilized and localized target
0mm0mm0mm
0mm0mm0mm
2mm 0mm5mm 2.8= 5.6mm
5-6mm, even with a dream-like IGRT process
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Van Kranen et al, IJROBP 73(5), 1566-1573, 2009
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Estimating margins – H&N
• Total error………………………………………………………………..
• Error margin…………………………………………………………….
• Total margin……………………………………………………………..
2.5=
• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….
Systematic errors
Random errors
2mm
If we include subject deformations….
1.5mm0mm0mm
1.5mm0mm0mm
1.5mm 1.5mm3.8mm
5mm with deformations (and perfect delineation)0.7= 1.1mm
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27
Court and Tishler, IJROBP 69(2) 607-613
PTV(3mm pullback)
PTV(5mm pullback)
What about planning tricks?
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Conclusions/Summary• Random uncertainties tend to average out
• Systematic uncertainties (for individual patients) dominate margins
• With IGRT, margins can be dominated by delineation uncertainties
• Uncertainties should be understood – generally most benefit is achieved if focus on systematic uncertainties