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

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.

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

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

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)

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

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, 809-19 (1995)

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

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

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%

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%

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

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