1 Centre for Medical Radiation Physics, University of Wollongong, Australia.2 Liverpool & Macarthur Cancer Therapy Centres & Ingham Institute, Liverpool, Australia. 3 SWSCS, University of New South Wales, Australia.4 Institute of Medical Physics, University of Sydney, Australia.
Defining and Assessing a Delineation Margin for Modern Radiation Therapy
L.R.Bell1,2, E.M.Pogson1,2, P.Metcalfe1,2, L.Holloway1,2,3,4
~50% patients have an indication for RT 1
Conform ionising radiation to cancer
Radiation Therapy
2
Motion Set up Delineation errors
Random uncertainties BLUR the dose distribution
Systematic uncertainties SHIFT the dose distribution
Uncertainties
New technologies and imaging reduces impact of motion and set up errors◦ High precision
Set up and motion margins can be/are being/have been be reduced
Uncertainty Margins
Delineation ErrorsSet up ErrorsMotion Errors
Delineation uncertainty is a current limiting factor in radiotherapy accuracy- weakest link!
We need precision AND accuracy for most effective radiotherapy
Delineation Uncertainty
Based on first systematic term of margin recipes◦ Average SD in contour delineation multiplied by a weighting
factor ANISOTROPIC to account for spatially varying
uncertainty
Delineation Uncertainty Margin
21 CT whole breast datasets 3
CTVs delineated by 8 observers
Small, medium and large volume categories for both left and right breast patients
Dataset
Small 0-700 cm3
Medium 701-1400 cm3
Large 1401-2100 cm3
Defined in cylindrical and spherical coordinates for anisotropic approach; and cartesian to compare to current practice
Origin defined average COM of CTVs
Margin=2*SDav ◦ ‘Leave one out approach’
Margin Definition
1. Generated consensus contour (STAPLE) for each patient
2. Applied margin to STAPLE, smallest CTV and largest CTV
3. Assessed encompassment of CTVs (% Overlap)
4. Assessed extra and missed tissue
Margin Assessment
Overlap: >90%
Non-critical MT: <10% of CTV union Critical MT: 0% of CTV union Non-critical EIT: No limit Critical EIT: <1/3 union of CTVs
Margin Assessment
<90% overlap in ◦ 35/504 (6.9%) cases for cylindrically defined margin◦ 17/504 (3.4%) cases for spherically defined margin◦ 72/504 (14.3%) cases for the clinically defined margin
Results
Small STAPLE
Large
Critical ET within tolerance
Non-Critical ET small (<11%)
Negligible critical MT
Non-critical MT exceeds tolerance in 52/54 (96.3%) cases
Extra and Missed TissueSmall STAPLE
Large
Anisotropic margin has greater encompassment in all cases
Anisotropic margin includes less EIT for large target volumes
Anisotropic margin misses more non-critical tissue– potentially due to coordinate system failures
Anisotropic margin is necessary to account for the weak link in RT that is delineation uncertainty
Conclusions
References
1. Barton, M.B., S. Jacob, J. Shafiq, K. Wong, S.R. Thompson, T.P. Hanna and G.P. Delaney, Estimating the demand for radiotherapy from the evidence: A review of changes from 2003 to 2012. Radiother. Onc., 2014.
2. Njeh, C.F., Tumor delineation: The weakest link in the search for accuracy in radiotherapy. Journal of Medical Physics / Association of Medical Physicists of India, 2008. 33(4): p. 136-140.
3. Jameson, M.G., et al., Correlation of contouring variation with modeled outcome for conformal non-small cell lung cancer radiotherapy. Radiotherapy and Oncology, 2014. 112(3): p. 332-336.
Hemi-spherical shape of breast CTVs means cylindrical/spherical coordinates are not always suitable
SD limited to ½ max hausdorff distance
Coordinate System Failures
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