tibor major, csaba polgár - brachyacademy€¦ · 3d treatment planning: tricks and tips . tibor...
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3D treatment planning: tricks and tips
Tibor Major, Csaba Polgár National Institute of Oncology, Budapest
Euro-Asian Breast Brachytherapy School, October 9-10. 2014, Erlangen
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Aims of interstitial implants
good dose coverage of the target volume
uniform dose distribution inside the target volume
steep dose fall-off outside the target volume
Both the dose homogeneity and conformality can be manipulated by using individual source dwell times in the different dwell positions
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0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0 20 40 60 80 100 120 140 160 180 200 220
DNR
Relative reference dose (%)
single planedouble plane - triangulardouble plane - square
Catheter separation: 15 mm
DNR as a function of prescription isodose
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DNR in the function of reference isodose at different number of catheters
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DNR in the function of reference isodose at different active lengths
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0,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
Step size / catheter separation
DN
Rm
in
single plane
double plane - triangular
double plane - square
DNRmin as a function of ratio of step size and cath. separation
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COIN in the function of active length (single-plane implant, LPTV=5 cm, d=1 cm, GOS, CI=0.95)
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Optimal active lengths (La) for the dosimetry systems
Paris Dosimetry System La = 1.3-1.6 x LPTV Geometrically Optimized System La = LPTV Conformal Dosimetry System La = LPTV – (0.5-1.0) cm Stepping Source Dosimetry System La = LPTV – (0.5-1.0) cm
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Effect of dose prescription on target coverage and
dose homogeneity in interstitial brachytherapy
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Reference isodose = 85% of BD
Basal dose points with dose of BDx
5BD...BDBDBD 521 +++
=
Dose prescription in the Paris dosimetry system
catheter
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CI = 0.94 DNR = 0.33
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CI = 0.90 DNR = 0.26
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CI = 0.84 DNR = 0.22
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CI = 0.77 DNR = 0.19
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CI = 0.68 DNR = 0.18
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CI = 0.57 DNR = 0.19
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CI = 0.45 DNR = 0.20
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Effect of dose optimization on target coverage, dose
homogeneity and conformality in interstitial brachytherapy
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CI = 0.68 DNR = 0.18
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CI = 0.75 DNR = 0.19
Graphical optimization in one plane
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CI = 0.90 DNR = 0.29
Graphical optimization in multiple planes
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CI = 0.90 DNR = 0.29 COIN = 0.75
Graphical optimization in multiple planes
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CI = 0.92 DNR = 0.39 COIN = 0.83
Dose point optimization on points on target
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CI = 0.92 DNR = 0.39 COIN = 0.83
Dose point optimization on points on target
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Highly conformal, but inhomogeneous dose distribution
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1. The uniformity of dose distribution in interstitial implants can be improved by increasing the active lengths in the catheters using more catheters/needles applying optimal ratio of step size/catheter separation (0.75) selecting appropriate isodose for prescription
2. The active lengths in the catheters have to be adapted to the type of
optimization. Keep the dwell positions inside the PTV when dose point
optimization is used.
3. The conformal dosimetry system can provide highly conformal dose
distributions but the dose homogeneity may deteriorate.
4. In image-based brachytherapy use DVH-based dose prescription and apply
volumetric parameters for plan evaluation.
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Our current planning protocol
1. Keeping active source positions inside the PTV
2. Geometrical optimization followed by graphical optimization
3. Definition of basal plane and basal dose points
4. Normalization (100 %) on mean central dose (MCD)
5. Dose prescription on individual isodose line to get CI ≥ 0.9 and DNR ≤ 0.35
6. Keeping the maximal skin dose below 70 % of prescribed dose