Dose Overestimation in Dose Overestimation in Balloon Catheter Brachytherapy Balloon Catheter Brachytherapy

for Breast Cancerfor Breast Cancer

Ye, Sung-Joon, Ph.D. Ye, Sung-Joon, Ph.D. Ove, Roger, M.D., Ph.D.; Shen, Sui, Ph.D. Ove, Roger, M.D., Ph.D.; Shen, Sui, Ph.D.

Russo, Suzanne, M.D.; Brezovich, Ivan A., Ph.D.Russo, Suzanne, M.D.; Brezovich, Ivan A., Ph.D.

Radiation Oncology, UniRadiation Oncology, Univv of Alabama of Alabama School of Medicine, Birmingham, ALSchool of Medicine, Birmingham, AL USAUSA

MammoSite™ BrachytherapyMammoSite™ Brachytherapy

A part of breast conservation therapy for early-A part of breast conservation therapy for early-stage patients using a balloon catheter and an stage patients using a balloon catheter and an 192192Ir-HDR sourceIr-HDR source

Balloon placed into lumpectomy cavity at time of Balloon placed into lumpectomy cavity at time of surgerysurgery

Balloon inflated with sterile saline with iodine-Balloon inflated with sterile saline with iodine-containing radiographic contrast medium containing radiographic contrast medium

Prescription dose at 1.0 cm from balloon surface, Prescription dose at 1.0 cm from balloon surface, in a plane transverse to balloon axis at its centerin a plane transverse to balloon axis at its center

10 fractions of 3.4 Gy per fraction, b.i.d. 10 fractions of 3.4 Gy per fraction, b.i.d.

Treatment Plans in Clinical UseTreatment Plans in Clinical UseAssume that a source is located in a large Assume that a source is located in a large sphere of water (30 cm-diam)sphere of water (30 cm-diam)

cm

Edmundson GK, et al., Int J Radiat Oncol Biol Phys 2002;52:1132–39

Motives of This StudyMotives of This Study

Potential under-dosage to the breast Potential under-dosage to the breast tumors is a major concern because of tumors is a major concern because of

– Proximity to both the lung tissue and the Proximity to both the lung tissue and the breast skin breast skin less lateral and back scatter less lateral and back scatter

– Iodine-containing contrast medium in the Iodine-containing contrast medium in the balloon balloon preferentially absorbing low-energy preferentially absorbing low-energy photons (attenuation) of photons (attenuation) of 192192Ir Ir

Monte Carlo (MC) SimulationsMonte Carlo (MC) Simulations

MCNP5 and Photon Cross-Section MCNP5 and Photon Cross-Section Library, MCLIP04 (from EPDL97)Library, MCLIP04 (from EPDL97)

0.3 g/cm3 (lung)

Water (tissue)

Water (breast)

Contrast medium (balloon)

Air

192Ir source

4.0 cm

4.0 cm

4.5 cm

-x (anterior)

+x (posterior)

+z (lateral)

Absolute Monte Carlo DosimetryAbsolute Monte Carlo Dosimetry

Air kerma per primary photon from MC, Air kerma per primary photon from MC, kk((dd), in a voxel at ), in a voxel at a transverse distance, a transverse distance, dd (cm) (cm)

Air kerma strength per unit activity, cGy cmAir kerma strength per unit activity, cGy cm2 2 hh-1-1 Bq Bq-1-1

Activity for MC dose calculations Activity for MC dose calculations

MC dose rate from raw MC voxel dose at space MC dose rate from raw MC voxel dose at space

23MC )(10507.8 ]/[ ddkASk

MCMC ]//[ ASSA kk

Sk = TPS air kerma strength traceable to NIST

r

In-Air

In breast/lung phantom

)(2.363 )( MCMCMC rDArD raw

To be published in Int J Radiat Oncol Biol Phys

TPS DosimetryTPS Dosimetry

TPS (Plato™, Nucletron Corp.) based on TPS (Plato™, Nucletron Corp.) based on AAPM TG-43 formalismAAPM TG-43 formalism

),()90,cm 1(

)90,( )(

0TPS rg

rG

rGSrD k

along transverse plane

),0,()()90,cm 1(

)0,( )(

0TPS

rFrgrG

rGSrD k

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 %differences of MC doses in breast/lung phantom

from TPS doses at various contrast concentrationsfrom TPS doses at various contrast concentrations

-10%

-9%

-8%

-7%

-6%

-5%

-4%

-3%

-2%

-1%

0%

1%

2%

-4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Transverse distance (x), cm

%d

iffe

ren

ce f

rom

TP

S

Water

5%

12.5%

25%

Balloon LungTissue

Anterior Posterior

Tissue

%differences of MC doses in breast/lung phantom %differences of MC doses in breast/lung phantom from TPS doses along four different directionsfrom TPS doses along four different directions

-10%

-9%

-8%

-7%

-6%

-5%

-4%

-3%

-2%

-1%

0%

2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4

Distance from the source, cm

%d

iffe

ren

ce f

rom

TP

S

Anterior (-x)Posterior (+x)Sagittal (±y)Lateral (+z)

Prescription distance from the balloon surfaceBalloon

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%

Skin Doses v.s. Skin-to-Balloon Skin Doses v.s. Skin-to-Balloon Separation (Distance)Separation (Distance)

BecauseBecause– in TPS, scatter dose component increases with in TPS, scatter dose component increases with

distances from the source distances from the source – but, in reality no scatter medium exists near skinbut, in reality no scatter medium exists near skin

SummarySummary

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

Conclusive RemarksConclusive Remarks

In general, conventional TPS overestimate In general, conventional TPS overestimate superficial doses and skin doses, an issue that superficial doses and skin doses, an issue that is of concern for breast brachytherapy and is of concern for breast brachytherapy and brachytherapy for other tumor sites brachytherapy for other tumor sites In clinical practice, TPS overestimation of the In clinical practice, TPS overestimation of the skin dose indicatesskin dose indicates– target between balloon and skin may be inadequately target between balloon and skin may be inadequately

treatedtreated– cosmetic problems and erythema seen on trials cosmetic problems and erythema seen on trials

occurred at a lower dose than previously thoughtoccurred at a lower dose than previously thought

However, 5% increase of dwell time reduces However, 5% increase of dwell time reduces ~10% overestimation to < 5% over all directions~10% overestimation to < 5% over all directions

Thank You for Your Attention!Thank You for Your Attention!