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

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

Residual setup uncertainties

8

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

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

Calculating uncertainties

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=

• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….

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=

• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….

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=

• Delineation ………….. ………….• Organ motion ………….. ………….• Setup error ………….. ………….• Intrafraction motion ………….. ………….

Systematic errors

Random errors

1.5mm3mm 3mm

2mm1mm

1mm

1.5mm 1mm3.8mm 0.7mm

4.5mm

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

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

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=

• 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)

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

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

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

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

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