physics treatment margins · bujold et al, image-guided radiotherapy: has it influenced patient...

Physics – Treatment MarginsLaurence Court

University of Texas MD Anderson Cancer Center

Disclosure

• Employer: UT MD Anderson Cancer Center

• Grants from: NCI, CPRIT, Varian, Elekta, Mobius

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

Figures from Khan

A reminder…..

Random and systematic uncertainties

Systematic uncertainty

Random uncertainty

low

low

low

high low

high high

high

Systematic vs. random uncertainties

6

Bujold et al, Image-Guided Radiotherapy: Has It Influenced Patient Outcomes?, Seminars in Radiation Oncology, 22, 50, 2012

Systematic vs. random uncertainties

7


Residual setup uncertainties

8


Effect of uncertainties on delivered dose

9McCarter et al, PMB 45: 923-931, 2001

• High isodoses move in• Low isodoses move out

There are lots of margin formulae

It’s not this bad!

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


Van Herk, Sem. Rad. Onc. 14(1), 52-64, 2004

M=2.5+0.7

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

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

Estimating margins for prostate (no IGRT)

• Total error………………………………………………………………..

• Error margin…………………………………………………………….

• Total margin……………………………………………………………..

2.5= 0.7=


Systematic errors

Random errors

2.5mm3mm 3mm

2mm1mm

1mm

4.0mm 3.7mm10.0mm 2.9mm

13mm

Estimating margins for prostate (perfect IGRT)

• Total error………………………………………………………………..

• Error margin…………………………………………………………….

• Total margin……………………………………………………………..

2.5= 0.7=


Systematic errors

Random errors

2.5mm3mm 3mm

2mm1mm

1mm

2.5mm 1mm6.2mm 0.7mm

7mm

Now improve the delineation

• Total error………………………………………………………………..

• Error margin…………………………………………………………….

• Total margin……………………………………………………………..

2.5= 0.7=


Systematic errors

Random errors

1.5mm3mm 3mm

2mm1mm

1mm


4.5mm


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

But this is only for 90% of patients…..What about increasing to 95%?

• Total error………………………………………………………………..

• Error margin…………………………………………………………….

• Total margin……………………………………………………………..

2.5= 0.7=


Systematic errors

Random errors

1.5mm3mm 3mm

2mm1mm

1mm


4.9mm

2.8

But what about deformations?

• SD of inter-fraction variation after marker-based rigid translation

Van der Wielen,, IJROBP 72(5), 1604-1611, 2009

So?

• Total error………………………………………………………………..

• Error margin…………………………………………………………….

• Total margin……………………………………………………………..

2.5= 0.7=


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)

Estimating margins – H&N

• Total error………………………………………………………………..

• Error margin…………………………………………………………….

• Total margin……………………………………………………………..

2.5= 0.7=


Systematic errors

Random errors


• Total error………………………………………………………………..

• Error margin…………………………………………………………….

• Total margin……………………………………………………………..

2.5=


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

Van Kranen et al, IJROBP 73(5), 1566-1573, 2009


• Total error………………………………………………………………..

• Error margin…………………………………………………………….

• Total margin……………………………………………………………..

2.5=


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

27

Court and Tishler, IJROBP 69(2) 607-613

PTV(3mm pullback)

PTV(5mm pullback)

What about planning tricks?

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

physics treatment margins · bujold et al, image-guided radiotherapy: has it influenced patient...

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