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Dose Overestimation in Balloon Catheter Brachytherapy for Breast Cancer

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, Uni v of Alabama School of Medicine, Birmingham, AL USA.

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Dose Overestimation in Balloon Catheter Brachytherapy for Breast Cancer

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  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, ALUSA

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

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

  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, 809-19 (1995)

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

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

  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

  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!

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