The Role of Monte Carlo and other Advanced Dose Calculation Algorithms for Breast APBI

Transcription

1 The Role of Monte Carlo and other Advanced Dose Calculation Algorithms for Breast APBI Luc Beaulieu, Ph.D. Associate Prof, Département de physique, Université Laval Researcher, Centre de recherche en cancérologie de l Université Laval Medical physicist and Head of medical physics research, CHUQ CE Therapy SAMS Session, Tuesday July 20

2 Disclosure CHUQ is a Nucletron center of excellence I am part of a standing research agreement with Nucletron

3 Learning Objectives Understanding the current limitation related to breast brachytherapy dosimetry. Identify situations where Monte Carlo or other advanced dose calculation algorithms could be useful. Identify the underlying dosimetric impact of the choice of treatment isotope or source energy (ebx) in breast brachytherapy.

4 Topics What s wrong with TG43? Scatter conditions Heterogeneities Effect of source energy Alternatives and Challenges

5 What s wrong with TG43?

6 What s wrong with TG43? Source position within a 15 (40) cm radius water sphere

7 The Reality of Breast Brachytherapy! Interstitial Contura Mammo SAVI

8 Limitation of TG43 in relation to clinical/physical parameters Rivard, Venselaar and Beaulieu, Med Phys 36 (2009)

9 Limitation of TG43 for Breast Brachytherapy Energy Range Effect 192 Ir Scatter condition Shielding (applicator related) 103 Pd/eBx Absorbed dose (μ en /ρ) Attenuation (μ/ρ) Shielding (applicator, source)

10 Tissue Heterogeneity

11 Tissue Heterogeneity From NIST website

12 Tissue Heterogeneity: Adipose vs Water G Landry et al., submitted to Med Phys

13 Tissue Heterogeneities not a problem at 192 Ir energy

14 103 Pd Breast Brachytherapy Tissue effects can be large 14 H Afsharpour et al., PMB (in press)

15 Spectral measurements AXXENT electronic source Ag K CdTe K-edge W L Y K [Liu et al. Spectroscopic characterization of a novel electronic brachytherapy system. PMB 53, 61, 2008]

16 Tissue Heterogeneities Energy dependant Less than 1% for 192 Ir over useful distances 10-40% effect for 103 Pd Similar for ebx Difference increase with Decreasing energy. Increasing distance from ref point.

17 Scatter Condition

18 Scatter Condition Interstitial CONTURA Dashed line = TG43 ; Full line = MC

19 Scatter Condition

20 Scatter Condition Finite dimension of the breast for 192 Ir No large effect on the highest isodoses Reduces skin doses by about 5% Pentalis et al., IJROBP 2005 For lower energy such as 169Yb Skin and lung doses overestimated by 15-30% Lymperopoulou et al., Med Phys 2006 For even lower energy Not the most important effect anymore

21 Shielding: Contrast Agent

22 Interstitial Contrast and Air Contura Mammo SAVI

23 Article Agents de contrast Med Phys 31

24 Med Phys 37 (2010)

25 0-4% 0-6% 0-9%

26 Contrast (15%) with an ebx source? 192 Ir ebx

27 Contrast with an ebx source (15%) Effect of contrast agent Contrast + tissues The ballon for Xoft source does not use contrast

28 Alternatives

29 If not TG43, what? Model-Based Dose Calculation Algorithms Stochastic methods: Monte Carlo Deterministic approaches: GBBS (Acuros) Primary/Scatter separation: CCC Analytical Methods Ray tracing with scaling Hybrid approaches

30 Alternatives to TG43 Rivard, Beaulieu and Mourtada, Vision 20/20, Med Phys 2010

31 and Challenges TPS Dose Calc Dose parameters, DVHs, isodoses

32 Going Beyond TG43 Must maintain inter-institution consistency Must high QC/QA standards Must be able to Define for each voxel: material and density Define the dose scoring medium Might have to address retrospectively and prospectively dose-toxicity and dose-outcome relationships

33 In the meantime TG186 is working toward providing guidelines Look for it at a conference near you in 2011 For the time being, we recommend that All clinical plans be generated with TG43 If using a MBDCA for dose recalculation Be sure to read Rivard et al., Vision 20/20 (2010) MBDCA should fall back to TG43 in well controlled conditions (15 or 40 cm radius water geometry) Transport should be carried out in the medium (CT #) if no artifacts Full documentation of tissue (Z eff ) assignments

34 TG-186 Luc Beaulieu, CHUQ (Chair) Äsa Carlsson-Tedgren, Linköping University Jean-François Carrier, CHUM Stave Davis, UW Maddison Firas Mourtada, MD Anderson Mark Rivard, Tuft University Rowan Thomson, Carleton University Frank Verhaegen, Maastro Todd Waering, Transpire inc (consultant) Jeff Williamson, VCU

35 Quiz Time

36 For 192Ir breast brachytherapy, what is the most important factor influencing the dosimetry relative to TG43? 33% 1. Tissue heterogeneities 33% 2. None 33% 3. Scatter condition 10

37 For 192Ir breast brachytherapy, what is the most important factor influencing the dosimetry relative to TG43? 1. Tissue heterogeneities 2. None 3. Scatter condition

38 In your clinical practice, what role do you see for advanced dose calculation algorithms? 33% 1. I do not want to use these yet. 33% 33% 2. For dose recalculation after the fact. 3. In parallel to TG43, allowing plan modification. 10

39 What do you think will be the biggest impact of moving away from TG43? 33% 33% 33% 1. Affect dose prescription levels and dose-outcome studies. 2. Will provide better tx (in term of outcome). 3. High risk of losing center-to-center uniformity that we have with TG43. 10

40 Discussion The limitation of TG43 are modulated by Calculation geometry Applicator composition Source energy Energy Range Effect 192 Ir Scatter condition Shielding (applicator related) 103 Pd/eBx Absorbed dose (μ en /ρ) Attenuation (μ/ρ) Shielding (applicator, source)

41 Conclusions TG43 has numerous useful qualities but is limited MBDCA (MC, Acuros, ) promises more accurate dose calculations Higher chance of GIGO AAPM recommendations are in the work