1. Dose Overestimation in Balloon Catheter Brachytherapy for Breast Cancer
Ye, Sung-Joon, Ph.D.
Ove, Roger, M.D., Ph.D.; Shen, Sui, Ph.D.
Russo, Suzanne, M.D.; Brezovich, Ivan A., Ph.D.
Radiation Oncology, Univ of Alabama
School of Medicine, Birmingham, AL USA

2. MammoSite™ Brachytherapy
A part of breast conservation therapy for early-stage patients using a balloon catheter and an 192Ir-HDR source
Balloon placed into lumpectomy cavity at time of surgery
Balloon inflated with sterile saline with iodine-containing radiographic contrast medium
Prescription dose at 1.0 cm from balloon surface, in a plane transverse to balloon axis at its center
10 fractions of 3.4 Gy per fraction, b.i.d.

3. Treatment Plans in Clinical Use
Assume that a source is located in a large sphere of water (30 cm-diam) cm
Edmundson GK, et al., Int J Radiat Oncol Biol Phys 2002;52:1132–39

4. Motives of This Study
Potential under-dosage to the breast tumors is a major concern because of
Proximity to both the lung tissue and the breast skin  less lateral and back scatter
Iodine-containing contrast medium in the balloon  preferentially absorbing low-energy photons (attenuation) of 192Ir

5. Monte Carlo (MC) Simulations
MCNP5 and Photon Cross-Section Library, MCLIP04 (from EPDL97)
0.3 g/cm3 (lung)
Water (tissue)
Water (breast)
Contrast medium (balloon)
Air
192Ir source
4.0 cm
4.5 cm
-x (anterior)
+x (posterior)
+z (lateral)

6. Absolute Monte Carlo Dosimetry
Air kerma per primary photon from MC, k(d), in a voxel at a transverse distance, d (cm)
Air kerma strength per unit activity, cGy cm2 h-1 Bq-1
Activity for MC dose calculations
MC dose rate from raw MC voxel dose at space
In-Air
Sk = TPS air kerma strength traceable to NIST
In breast/lung phantom
To be published in Int J Radiat Oncol Biol Phys

7. TPS Dosimetry
TPS (Plato™, Nucletron Corp.) based on AAPM TG-43 formalism
along transverse plane
along longitudinal axis (+z)
r = distance from source, G = geometry factor for the line source, g = radial dose function,  = dose rate constant at reference point = 1.115, F = anisotropy function from Williamson and Li, Med. Phys. 22, (1995)

8. %differences of MC doses in breast/lung phantom from TPS doses at various contrast concentrations
Balloon
Lung
Tissue
Anterior
Posterior

9. %differences of MC doses in breast/lung phantom from TPS doses along four different directions
Prescription distance from the balloon surface
Balloon

10. Skin Doses v.s. Skin-to-Balloon Separation (Distance)
Because
in TPS, scatter dose component increases with distances from the source
but, in reality no scatter medium exists near skin
Skin-to-balloon distance
MC dose rate
TPS dose rate
%diff
5 mm
1.63 cGy/s
1.75 cGy/s
-7.1%
7 mm
1.40 cGy/s
1.52 cGy/s
-7.8%
10 mm
1.14 cGy/s
1.25 cGy/s
-8.4%
12 mm
1.00 cGy/s
1.10 cGy/s
-9.7%
15 mm
0.84 cGy/s
0.93 cGy/s
-9.9%

11. Summary
Compared to MC, conventional TPS overestimates doses to prescription line and skin by 10% or even more, depending on concentration of contrast medium and tissue point
Omission of attenuation by contrast medium contributes up to 5% to dose error
Limited scatter accounts for the remaining dose error of up to 6%

12. Conclusive Remarks
In general, conventional TPS overestimate superficial doses and skin doses, an issue that is of concern for breast brachytherapy and brachytherapy for other tumor sites
In clinical practice, TPS overestimation of the skin dose indicates
target between balloon and skin may be inadequately treated
cosmetic problems and erythema seen on trials occurred at a lower dose than previously thought
However, 5% increase of dwell time reduces ~10% overestimation to < 5% over all directions

13. Thank You for Your Attention!