Physics Treatment Margins. Laurence Court University of Texas MD Anderson Cancer Center

Transcription

1 Physics Treatment Margins Laurence Court University of Texas MD Anderson Cancer Center

2 Disclosure Employer: UT MD Anderson Cancer Center Grants from: NCI, CPRIT, Varian, Elekta, Mobius

3 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

4 A reminder.. Figures from Khan

5 Random and systematic uncertainties Systematic uncertainty low low high high Random uncertainty low high low high

6 Systematic vs. random uncertainties Bujold et al, Image-Guided Radiotherapy: Has It Influenced Patient Outcomes?, Seminars in Radiation Oncology, 22, 50,

7 Systematic vs. random uncertainties Bujold et al, Image-Guided Radiotherapy: Has It Influenced Patient Outcomes?, Seminars in Radiation Oncology, 22, 50,

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

9 Effect of uncertainties on delivered dose High isodoses move in Low isodoses move out McCarter et al, PMB 45: ,

10 There are lots of margin formulae

11 It s not this bad!

12 Calculating uncertainties For the scenario where we want minimum dose to CTV of 95% for 90% of patients, then: M= For lung, dose gradient is more shallow, so smaller margins needed for random If dose prescription is for lower percent (e.g.80%), then smaller margins Van Herk, Sem. Rad. Onc. 14(1), 52-64, 2004 McCarter et al, PMB 45: , 2001

13 Calculating uncertainties M= Van Herk, Sem. Rad. Onc. 14(1), 52-64, 2004

14 Estimating margins - a simple spreadsheet Total error = Systematic 0.7 = Random Delineation... Organ motion... Setup error... Intrafraction motion... Error margin. Total margin..

15 Examples of delineation uncertainties s.d. = 1.8, 2.1, 3.6mm in RL, AP, SI directions White et al, Clin. Onc. 21, (CBCT images) s.d. = 1.5, 2.6mm in transverse and SI directions Persson et al, Brit. J. Radiol. 85, e , 2012

16 Estimating margins for prostate (no IGRT) Delineation... Organ motion.. 3mm. 3mm Setup error.. 1mm. 2mm Intrafraction motion... 1mm Total error = Systematic 2.5mm 0.7 = Random Error margin. 13mm 4.0mm 10.0mm 3.7mm 2.9mm Total margin..

17 Estimating margins for prostate (perfect IGRT) Delineation... Organ motion.. 3mm. 3mm Setup error.. 1mm. 2mm Intrafraction motion... 1mm Total error = Systematic 2.5mm 0.7 = Random Error margin. 7mm 2.5mm 6.2mm 1mm 0.7mm Total margin..

18 Now improve the delineation Delineation... Organ motion.. 3mm. 3mm Setup error.. 1mm. 2mm Intrafraction motion... 1mm Total error = Systematic 1.5mm 0.7 = Random Error margin. 4.5mm 1.5mm 3.8mm 1mm 0.7mm Total margin..

19 Calculating uncertainties For the scenario where we want minimum dose to CTV of 95% for 90% of patients, then: M= But shouldn t we be aiming for something better than 90% of patients with modern immobilization, localization etc? Van Herk et al, IJROBP 47(4) , 2000

20 But this is only for 90% of patients.. What about increasing to 95%? Delineation... Organ motion.. 3mm. 3mm Setup error.. 1mm. 2mm Intrafraction motion... 1mm Total error = Systematic 1.5mm 1.5mm 4.2mm 0.7 = Random 1mm 0.7mm Error margin. 4.9mm Total margin..

21 But what about deformations? SD of inter-fraction variation after marker-based rigid translation Van der Wielen,, IJROBP 72(5), , 2009

22 So? (accounting for some deformation, but still assuming super-high quality delineation) Systematic 1.5mm Random Delineation... Organ motion.. 2mm. 2mm Setup error.. 1mm. 2mm Intrafraction motion... 1mm Total error = 2.5mm 6.2mm 0.7 = 2.2mm 1.6mm Error margin. 7.8mm Total margin..

23 Estimating margins H&N Total error = Systematic 0.7 = Random Delineation... Organ motion... Setup error... Intrafraction motion... Error margin. Total margin..

24 Estimating margins H&N Lets start with a small, perfectly immobilized and localized target Systematic 2mm Random Delineation... Organ motion.. 0mm. 0mm Setup error.. 0mm. 0mm Intrafraction motion.. 0mm. 0mm 2mm 0mm 2.5 = 5mm 2.8 = 5.6mm 5-6mm, even with a dream-like IGRT process Total error.. Error margin. Total margin..

25 Van Kranen et al, IJROBP 73(5), , 2009

26 Estimating margins H&N If we include subject deformations. Systematic 2mm Random Delineation... Organ motion.. 1.5mm. 1.5mm Setup error.. 0mm. 0mm Intrafraction motion.. 0mm. 0mm 1.5mm 1.5mm 2.5 = 3.8mm 0.7 = 1.1mm 5mm with deformations (and perfect delineation) Total error.. Error margin. Total margin..

27 What about planning tricks? PTV(3mm pullback) PTV(5mm pullback) Court and Tishler, IJROBP 69(2)

28 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