Organ S values and effective doses for family members ... · In the Society of Nuclear Medicine and Molecular Imaging (SNMMI) procedure guideline 1 for radioiodine therapy, com- mon

Organ S values and effective doses for family members exposed to adultpatients following I-131 treatment: A Monte Carlo simulation study

Eun Young Hana)

Department of Radiation Oncology, University of Arkansas Medical Sciences, Little Rock, Arkansas 72205

Choonsik LeeDivision of Cancer Epidemiology and Genetics, National Cancer Institute, National Institute of Health,Bethesda, Maryland 20852

Lynn Mcguire and Tracy L. Y. BrownDepartment of Radiology, Division of Nuclear Medicine, University of Arkansas Medical Sciences,Little Rock, Arkansas 72205

Wesley E. BolchJ. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida,Gainesville, Florida 32611

(Received 4 October 2012; revised 5 June 2013; accepted for publication 6 June 2013; published 3July 2013)

Purpose: To calculate organ S values (mGy/Bq-s) and effective doses per time-integrated activity(mSv/Bq-s) for pediatric and adult family members exposed to an adult male or female patient treatedwith I-131 using a series of hybrid computational phantoms coupled with a Monte Carlo radiationtransport technique.Methods: A series of pediatric and adult hybrid computational phantoms were employed in the study.Three different exposure scenarios were considered: (1) standing face-to-face exposures between anadult patient and pediatric or adult family phantoms at five different separation distances; (2) an adultfemale patient holding her newborn child, and (3) a 1-yr-old child standing on the lap of an adultfemale patient. For the adult patient model, two different thyroid-related diseases were considered:hyperthyroidism and differentiated thyroid cancer (DTC) with corresponding internal distributionsof 131I. A general purpose Monte Carlo code, MCNPX v2.7, was used to perform the Monte Carloradiation transport.Results: The S values show a strong dependency on age and organ location within the family phan-toms at short distances. The S values and effective dose per time-integrated activity from the adultfemale patient phantom are relatively high at shorter distances and to younger family phantoms. Ata distance of 1 m, effective doses per time-integrated activity are lower than those values based onthe NRC (Nuclear Regulatory Commission) by a factor of 2 for both adult male and female patientphantoms. The S values to target organs from the hyperthyroid-patient source distribution stronglydepend on the height of the exposed family phantom, so that their values rapidly decrease with de-creasing height of the family phantom. Active marrow of the 10-yr-old phantom shows the highest Svalues among family phantoms for the DTC-patient source distribution. In the exposure scenario ofmother and baby, S values and effective doses per time-integrated activity to the newborn and 1-yr-oldphantoms for a hyperthyroid-patient source are higher than values for a DTC-patient source.Conclusions: The authors performed realistic assessments of 131I organ S values and effective doseper time-integrated activity from adult patients treated for hyperthyroidism and DTC to family mem-bers. In addition, the authors’ studies consider Monte Carlo simulated “mother and baby/child” ex-posure scenarios for the first time. Based on these results, the authors reconfirm the strong conser-vatism underlying the point source method recommended by the US NRC. The authors recommendthat various factors such as the type of the patient’s disease, the age of family members, and thedistance/posture between the patient and family members must be carefully considered to providerealistic dose estimates for patient-to-family exposures. © 2013 American Association of Physicistsin Medicine. [http://dx.doi.org/10.1118/1.4812425]

Key words: I-131, S value, effective dose, thyroid, hybrid phantoms, Monte Carlo transport

1. INTRODUCTION

In the Society of Nuclear Medicine and Molecular Imaging(SNMMI) procedure guideline1 for radioiodine therapy, com-mon methods to prescribe the amount of I-131 activity have

been introduced. For the patients treated for hyperthyroidism,one may use the size of thyroid gland and the results of a 24-hI-131 uptake test. Based on these SNMMI guidelines, an ac-tivity in the range of 2775–5550 MBq (75–150 mCi) is gener-ally administered for the ablation of the thyroid bed remnants

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following thyroidectomy for thyroid cancer patients. For dif-ferentiated metastatic thyroid cancer (DTC) patients, thereare two common approaches to determine the administered I-131 activity. The first approach is an empirical fixed-activityparadigm where 7400 MBq (200 mCi) is given to all can-cer patients. The second approach is a dosimetric scheme thatestimates the absorbed dose to bone marrow based on serialblood and whole-body counting to ensure that the dose doesnot exceed 2 Gy.2, 3 Using the second approach, the prescribedactivity could be more than 7400 MBq (200 mCi).

After I-131 is administered to a patient, licensees can re-lease the patient if they can confirm that the effective dose(the regulation in the US specifically refer to the TEDE, andit is defined as the total effective dose equivalent) to any otherindividual exposed to the patient is not likely to exceed 5 mSvbased on the following equation recommended in 10 CFR Part35.75:4

D = 34.6 �QoTpOF

r2, (1)

where D is the total dose from exposure to gamma radiation,34.6 is the conversion factor of 24 h/day multiplied by thetotal integration of decay (1.44), � (mGy-m2/Bq-s) is the ex-posure rate constant for a point source, Qo (MBq) is the ad-ministered activity, Tp (days) is the physical half-life of theradionuclide, r is the distance from the point source to thepoint of interest in meters, and OF is the occupancy factor,which represents the fraction of total time that an individualspends at a distance of r (i.e., 1 m) from the patient.

The intrinsic conservatism underlying Eq. (1) has been un-derscored in several previous studies. Siegel et al.5 found thatthe point source method is not suitable up to a certain distancebetween radioactive patients and exposed family members.Sparks et al.2 reported that the point source method overes-timates dose equivalent per time-integrated activity by morethan a factor of 2. Han et al.3 calculated the maximum re-leasable activity under the regulatory limit and effective doseper time-integrated activity using a series of the revised OakRidge National Laboratory (ORNL) stylized phantoms6 as afunction of distance, for two different exposure scenarios andthree different types of distributions of I-131. The authorsreported that the overestimation of the point source/targetmethod as compared to anthropomorphic phantom-based cal-culations is more than twofold. The overestimation may becaused by the assumption that the human body involved in theexposure scenario was simulated as a point in space, whichignores photon attenuation and scattering in both patient andexposed family members. The built-in conservatism in theNuclear Regulatory Commission (NRC) formation, thus, mayresult in an unnecessary increase of the length of patient’s stayin the hospital. Although these previous studies2, 3 used morerealistic approaches-–modeling patient and family memberusing stylized computational phantoms—these phantoms arestill limited in their ability to accurately represent humananatomy, as the organs in stylized phantoms are described bysimple mathematical surface equations such as planes, cylin-ders, cones, ellipsoids, and spheres.

There have been some efforts to overcome the anatomicallimitations of the stylized phantoms in the simulations of thepatient-to-family member through the use of voxel phantomsbased on patient CT images. de Carvalho et al.7 reported or-gan and effective doses for a caregiver or bystander exposedto a radioactive patient using two standing adult female voxelphantoms at three different separation distances and orienta-tions. More recently, they reported the dose to the exposed in-dividuals from three therapeutic regions represented by point,line, and volume source models, and they emphasized theconservatism inherent in the point source model.8

In the present study, we simulated the conventional face-to-face exposure scenario as well as more realistic scenario,including a baby/child exposed to its radioactive mother whilethe mother is closely holding the baby. Zanzonico et al. re-ported the measured dose rate based algorithm to determinethe time of release and the duration of postrelease precautionssuch as a patient not holding a child after radioiodine therapy.They used a distance of 0.3 m for the child held by the patientand 1 mSv as the maximum permissible effective dose to pe-diatrics and pregnant women and their work was also includedin NCRP Report No. 155.9, 10

To perform the realistic exposure simulations, we em-ployed a series of hybrid phantoms developed in collaborationbetween the University of Florida (UF) and the NationalCancer Institute (NCI), which can be easily morphed intorealistic postures. Organ S values and effective doses pertime-integrated activity to pediatric and adult family membersexposed to adult male or female patients were calculated bycoupling the hybrid phantom series with a Monte Carlo ra-diation transport technique. The results were compared withthe US NRC regulatory limit and the values we previouslyreported using stylized phantoms.

2. METHODS

2.A. UF/NCI hybrid phantom series

In this study, we employed the UF/NCI hybrid phantomseries including the newborn, 1-, 5-, and 10-yr-old male phan-toms, and 15-yr-old and adult male and female phantoms.The original Non-Uniform Rational B-Spline (NURBS) andPolygon Mesh (PM) format of the phantoms was convertedinto voxel format using a procedure called voxelization forMonte Carlo radiation transport. The adult male and femalephantoms representing patients treated with I-131 were vox-elized at the resolution of 3 mm3, whereas the pediatric andadult phantoms representing family members exposed to thepatients were voxelized at the resolution of 2 mm3, withan exception of the newborn and 1-yr-old male phantomswhich were voxelized at the resolution of 1 mm3 to bet-ter describe the details of their organs and skeletal structure.Only male phantoms were used for pediatric family phantomsfrom newborn to 10-yr-old as their anatomy is identical tothat of female phantoms with the exception of the gender-specific organs. The heights of the phantoms are 76 cm(1-yr-old), 109 cm (5-yr-old), 138 cm (10-yr-old), 161 cm

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FIG. 1. The three-dimensional lateral views of the adult male patient phantom facing the 1-, 5-, 10-, and 15-yr-old male family phantoms at the distances of 10,50, 100, and 200 cm. The distance of 75 cm is not included for clear presentation.

(15-yr-old female), 167 cm (15-yr-old male), 163 cm (adultfemale), and 176 cm (adult male).

2.B. Patient-to-family exposure scenario

One conventional and two more realistic exposure scenar-ios were considered in this study. First, conventional exposurescenarios were simulated where the released adult male or fe-male patient is facing family members (1-, 5-, and 10-yr-oldmale, 15-year-old male and female, and adult male and fe-male) at five different separation distances (10, 50, 75, 100,and 200 cm). Figure 1 shows the lateral views of the adultmale patient phantom facing the 1-, 5-, 10-, and 15-yr-oldmale family phantoms at different distances.

Second, two additional scenarios were simulated including(a) a radioactive mother cuddling a newborn baby (distancefrom the mother’s neck to the baby’s abdomen is 18 cm) and(b) 1-yr-old child standing on the mother’s lap (distance fromabdomen to abdomen is 13 cm) as shown in Fig. 2. The orig-inal NURBS/PM files of the newborn and 1-yr-old phantomswere merged with that of the adult female phantom within the3D NURBS/PM modeling software, RhinocerosTM (McNeel,Seattle, WA). The arms, legs, and head of the adult femalephantom including skeleton were reoriented to represent thedifferent postures shown in Fig. 2. The mother and baby phan-toms in NURBS/PM format were voxelized together to be im-plemented in Monte Carlo transport simulation. The previousorgan and tissue in the female phantom were adjusted to avoidconflict with the baby and child phantoms.

2.C. Modeling of the source distribution

Two I-131 source distributions were considered to rep-resent the two main types of treatments: hyperthyroidismand metastatic differentiated thyroid cancer (DTC). First, the

FIG. 2. The three-dimensional views of the adult female patient phantomsholding (a) the newborn phantom in its chest and (b) the 1-yr-old phantom onits lap.

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source region for hyperthyroid-patient was the thyroid gland,given that the thyroid gland may demonstrate an uptake ofthe order of 80% or more of available circulating radioiodinein hyperthyroid-patients.11, 12 A thyroid source distribution isalso applicable to the approximation of radioiodine uptake inthe postoperative thyroid gland remnant in patients undergo-ing I-131 remnant ablation after thyroidectomy or in patientswith residual iodine-avid metastatic lymph node involvementwithin the neck.

Second, patients with DTC who have undergone total ornear-total thyroidectomy typically have a neck I-131 uptakeof less than 5% of the administered activity, with metastasesmost frequently appearing in the lungs (49% of metastases),skeleton (25%), or both (15%).16 Thus, in order to be con-servative, the source regions for simulating DTC includedall sites of significant iodine localization such as the lungs,thyroid bed (simulating the gland remnants or neck lymphnodes), salivary glands, stomach (contents and wall), smallintestine (contents and wall), colon (contents and wall), liver,kidneys, and urinary bladder (contents and wall).11 We ex-cluded bone metastases in this study because they may oc-cur randomly throughout the whole body.13 The I-131 photonspectrum reported by International Commission on Radiolog-ical Protection (ICRP) Publication No. 107 was employed inthe Monte Carlo simulation.14

In both source definitions, we first extracted the locationof voxels tagged with the index of the source organ (or or-gans for DTC source) from the patient phantoms. Then werandomly sampled a voxel location from the series of ex-tracted voxel locations of the source organs. The source voxelwas uniformly sampled within a single organ (e.g., the thy-roid source). However, as for the DTC source, the samplingwas technically weighted by the size of the source organs sothat the bigger the organ was the more sampling was con-ducted. This approach, thus, implicitly assumes a uniformactivity concentration of I-131 in all source tissues for theDTC patient scenarios. The approach can, thus, be alteredgiven clinically relevant biokinetic models of I-131 for cancerpatients.

2.D. Monte Carlo radiation transport

We used a general purpose Monte Carlo transport code,MCNPX v2.7,15 for the simulations of the exposure sce-narios. The UF/NCI hybrid phantoms in voxel format wereimplemented in the MCNPX code. Reference elementalcompositions and mass densities were obtained from theInternational Commission on Radiation Units and Measure-ments (ICRU) Report No. 46 (Ref. 16) and ICRP PublicationNo. 89,17 and were implemented in the MCNPX materialcards.

In the face-to-face exposure scenarios, particle tracks andenergy information at a virtual planar surface positioned infront of the patient phantom were first recorded in the file(called as a “phase-space file”). The file was then subse-quently sampled to continue the transport of emitted photonsto the various family phantoms located at different distancesfrom the patient phantom. For the two mother-child exposure

scenarios, direct photon transport from mother to child wassimulated. Kerma approximation was used when calculatingorgan average doses considering the energy of 364 keV forthe primary photons of I-131. In order to ensure the applica-bility of the approximation, we evaluated differences of organS values within various target phantoms calculated using thekerma approximation as well as using full secondary electrontransport. The differences were less than 2% with the excep-tion of small organs in which the differences were still lessthan 5%.

To calculate dose to the active marrow and bone endos-teum (shallow marrow), recently published fluence-to-doseresponse functions18 developed at the University of Floridawere adopted. Energy-dependent photon fluence to the spon-giosa volume in a total of 34 bone sites was scored in 25 en-ergy bins ranging from 0.01 to 10 MeV. Active marrow andendosteum doses in each bone site were then calculated asthe energy-summed product of the photon fluences and thefluence-to-dose response functions for active marrow and en-dosteum, respectively. Skeletal averaged active marrow andendosteum doses were calculated as target mass-weighted av-erage of the bone-specific doses.

A total of 500 million photon histories were used to de-crease the relative errors within the major organs of the fam-ily phantoms to less than 2%, with the exception of smallorgans such as the prostate, testes, adrenals, and gall blad-der, which showed relative errors up to 3% for the adult and15-yr-old family phantoms. Organ doses in the pediatric fam-ily phantoms showed a relative error of less than 3% (10-yr-old) and 5% (5-yr-old). Since the height of the 1-yr-old phan-tom (76 cm) is much shorter than the height of both sourceregions, the relative errors of major organ doses were upto 10%.

2.E. Calculation of S values and effective dose pertime-integrated activity

The MIRD dosimetry schema19 shown in Eq. (2) was mod-ified to calculate the absorbed dose and effective dose to tar-get phantoms by replacing the target organ rT with an organin family phantoms, rFM,T, and replacing the source organ rS

within patient phantoms, rPT, as shown in Eq. (3):3

D(rT ) = As · S(rT ← rS), (2)

E(rFM ) = APT •∑

T

wT S(rFM,T ← rPT ), (3)

where APT is a total time-integrated activity in the source re-gion of patient phantoms and S(rFM, T ← rPT) is the mean ab-sorbed (or equivalent) dose to a target region within familyphantoms per time-integrated activity (mGy/Bq-s). The effec-tive dose for each family phantom was calculated as a summa-tion of the organ equivalent doses in family phantoms multi-plied by tissue weighting factors reported in ICRP PublicationNo. 103.20 Equation (3) may be expanded to show the details

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of the S value calculation:∑

T

wT S(rFM,T ← rPT )

=∑

T

wT

∑

i

EiYiϕ(rFM,T ← rPT )imT

, (4)

where Ei is the emitted photon energy from I-131, Yi is theradiation yield of the ith photon, mT is the target organ mass(kg), and ϕ(rFM, T ← rPT)i is the absorbed fraction defined asthe fraction of radiation energy emitted from a source regionin the patient phantom that is deposited in a given organ inthe exposed family phantom. It is assumed that the sum of the

wT-weighted organ doses for the 1-, 5-, and 10-yr-old malephantoms is a reasonable estimation of the gender-averagedeffective dose which is recommended by ICRP PublicationNo. 103.

3. RESULTS AND DISCUSSIONS

Organ-specific S values (mGy/Bq-s) and effective doseper unit time-integrated activity (mSv/Bq-s) were calculatedfor pediatric and adult family phantoms (1-, 5-, and 10-yr-old male phantoms and 15-yr-old and adult male and femalephantoms) facing the adult male and female patients. The

FIG. 3. S values (mGy/Bq-s) to the (a) lungs and (b) active marrow of family phantoms from the adult male hyperthyroid-patient phantom as a function ofdistance.

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values were also calculated for more realistic exposurescenarios of the adult female phantom closely holding thenewborn or 1-yr-old male phantoms. Two different I-131 dis-tributions, characteristic of hyperthyroidism and DTC, weresimulated within both the adult male and female patient phan-toms. Illustrative dose comparisons were presented for lungsand active marrow for hyperthyroid-patient source and colonand active marrow for DTC-patient source because of theirrelatively high tissue weighting factor, 0.12, in ICRP Publica-tion No. 103.

3.A. Hyperthyroid-patient source

Figures 3(a) and 3(b) shows the age-dependent S val-ues (mGy/Bq-s) to lungs and active marrow, respectively,in the family phantoms when exposed to the adult malehyperthyroid-patient phantom as a function of distance.S values decrease rapidly with increasing interphantom dis-tances and converge to a single value at the distance of 2 m.The S values show a strong dependency on the age and organsof the family phantoms at shorter distance beyond which the

FIG. 4. S values (mGy/Bq-s) to the (a) lungs and (b) active marrow of family phantoms from the adult female hyperthyroid-patient phantom as a function ofdistance.

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FIG. 5. Effective dose per time-integrated activity (mSv/Bq-s) to family phantoms from the adult male and female hyperthyroid-patient phantoms as a functionof distance.

dependency decreases following the inverse square law. The1- and 5-yr-old phantoms are relatively less exposed to theradiation from the adult patient phantom in the face-to-faceexposure scenario due to their shorter heights as compared tothe older and larger family phantoms.

Figures 4(a) and 4(b) show the S values to lungs and activemarrow, respectively, in the family phantoms when exposedto the adult female hyperthyroid-patient. A similar trend toFig. 3 is observed with the exception that the S values tothe lungs from the adult female [Fig. 4(a)] are higher by 1.6fold than the values from the adult male at distances less than50 cm. S values to the active marrow from the adult female[Fig. 4(b)] are higher by a factor of 1.4. This is attributedto the fact that the location of thyroid source is lower in theadult female phantom than the adult male so that the distancefrom the adult female source (thyroid) to target phantomsis shorter than that from the adult male phantom to familyphantoms.

Effective doses per time-integrated activity (mSv/MBq-s)for the 1-, 5-, 10-, 15-yr-old and adult phantoms are shown inFig. 5 as a function of distance when the I-131 distribution ofthe hyperthyroidism adult male and female patient phantomsis simulated. The solid and dotted lines represent the valuesfor the adult male and female patient phantoms, respectively.At the distance of 10 cm, the effective doses to the 15-yr-oldand adult phantoms are highest, showing that the height of thefamily phantoms greatly impacts the effective dose in the caseof the hyperthyroid-patient source. The values from the adultfemale hyperthyroid-patient are up to a factor of 1.5 higher(5- and 10-yr-old) than those from the adult male patient.

The value recommended by NRC (based on the pointsource/target method at 1 m distance) is displayed as a hor-izontal solid line for comparison in Fig. 5. At the distanceof 1 m, effective doses from both adult male and female are

lower than the NRC value by factors of 2.0, 1.6, 1.5, and 1.6for the exposed 5-, 10-, 15-yr-old and adult phantoms, respec-tively. If an activity of 50 mCi is administered to a hyperthy-roidism adult male patient, the effective dose to a 10-yr-oldchild spending most of the time at the distance of 1 m with theadult male patient will be 4.7 mSv, whereas the value based onthe NRC point source/target method [Eq. (1)] will be 7.7 mSv,which means that the patient cannot be released right after ra-dioiodine treatment. The effective dose at the distance of 2 mis very close to the value from the point source/target methodif 2 m is used in the calculation instead of 1 m regardlessof the ages of the family phantoms and the type of the treat-ments, which suggests that the point source/target method isstill valid at 2 m.

We reported a similar trend of the age-dependent effectivedose values in our previous study which was based on styl-ized phantoms for both source and target phantoms.3 Effectivedoses per time-integrated activity to the 5-, 10-, 15-yr-old, andadult hybrid phantoms were higher than the values from thestylized phantoms at the corresponding age by factors of 2.3,2.7, 1.9, and 1.6, respectively, at the separation distance of10 cm when the I-131 is localized in the thyroid of the adultmale patient phantom. The differences are attributed to themore realistic body contour of the hybrid phantoms as com-pared to those of the stylized phantoms in which the anteriorbody contour is flat from the top to the bottom of the torso.

3.B. DTC patient source

Figures 6 and 7 show the S values (mGy/Bq-s) to the(a) colon and (b) active marrow of the pediatric and adultfamily phantoms exposed to the adult male and female pa-tient phantoms, respectively, when the I-131 distribution forthe DTC patient is simulated. The overall behavior of the S

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FIG. 6. S values (mGy/Bq-s) to (a) colon and (b) active marrow of family phantoms from the adult male DTC patient phantom as a function of distance.

values is similar to the values for the hyperthyroid-patients(Figs. 3 and 4) where the values to the 1-yr-old phantom againare relatively small as compared to those to other older familyphantoms. The active marrow of the 10- and 5-yr-old phan-toms shows the first and second highest S values in Fig. 7(b).The S values from the adult female DTC patient phantom(Fig. 7) are up to a factor of 1.6 higher than the values fromthe adult male phantom (Fig. 6).

Figure 8 shows effective doses per time-integrated ac-tivity (mSv/MBq-s) to the 1-, 5-, 10-, and 15-yr-old andadult phantoms from the adult male and female DTC pa-

tient phantoms as a function of distance. The 10- and 15-yr-old phantoms show the highest values because the loca-tions of major organs in those phantoms contributing to ef-fective dose are at the similar level of the source regions inthe DTC patient phantoms. At the distance of 10 cm, thevalues for the adult female DTC patient phantom are higherthan those from the adult male by a factor of up to 1.5 (1-yr-old). The values are lower than the NRC-recommended valueby factors of 2.3, 2.0, 2.0, and 2.2 for the exposed 5-, 10-,15-yr-old, and adult phantoms, respectively, at the distanceof 1 m.

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FIG. 7. S values (mGy/Bq-s) to (a) colon and (b) active marrow of family phantoms from the adult female DTC patient phantom as a function of distance.

Effective doses that we previously reported using the styl-ized phantoms are higher than the values in the current studyby a factor of up to 1.8 at the distance of 10 cm. It is, how-ever, difficult to perform a fair comparison between the tworesults because the source definitions are completely differentbetween the two calculations. The source region in the styl-ized phantom was designed across the whole lower abdomi-nal volume with the dimension of 35 (height) × 33 (width)× 20 (length) cm3 following the definition used by Sparkset al.2 However, the source region within the hybrid adultmale phantom is distributed throughout the abdomen with

the dimension of 75.5 (height) × 28.5 (width) × 19.4(length) cm3 as previously described in Sec. 2.C.

3.C. Realistic exposure scenario of mother and baby

Figure 9 shows (a) S values (mGy/Bq-s) of four major or-gans and tissues (colon, lungs, stomach, and active marrow)on a logarithmic scale and (b) effective dose per unit time-integrated activity (mSv/Bq-s) in the newborn and 1-yr-oldphantoms that are realistically positioned with the adult fe-male patient phantom as shown in Fig. 2.

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FIG. 8. Effective dose per time-integrated activity (mSv/Bq-s) to family phantoms from the adult male and female DTC patient phantoms as a function ofdistance.

FIG. 9. (a) S values (mGy/Bq-s) to colon, lungs, stomach, and active marrowand (b) effective dose per time-integrated activity (mSv/Bq-s) to the newbornand 1-yr-old phantoms from the adult female hyperthyroidism (HT) and DTCpatient phantoms.

S values of the organs in the newborn phantom from thehyperthyroid-patient phantom are higher than the values fromthe DTC patient phantom by factors of 1.1, 1.4, 1.4, and 1.2for colon, lungs, stomach, and active marrow, respectively. Ef-fective dose per unit time-integrated activity to the newbornphantom from the hyperthyroid-patient is 1.3 higher than thevalue from the DTC patient. This is attributed to the fact thatmost of the major organs in the newborn phantom are directlyirradiated by the hyperthyroid-patient source rather than theDTC-patient source which is distributed across the abdomi-nal region, as shown in Fig. 2(a).

S values and effective dose per time-integrated activity tothe 1-yr-old baby phantom from the hyperthyroid-patient arehigher than those from the DTC patient by factors of 1.2, 2.6,1.9, 2.1, and 1.9 for colon, lungs, stomach, active marrow, andeffective dose, respectively, for the same reasons as discussedabove.

The trend in S values and effective dose per time-integratedactivity calculated using the realistic exposure scenarios ofmother and babies shown in Fig. 2 are not observed in thesimplified exposure geometry shown in Fig. 1.

4. CONCLUSIONS

We evaluated organ S values and effective dose per time-integrated activity to pediatric and adult family members ex-posed to adult male or female patients treated with I-131 forhyperthyroidism and DTC using Monte Carlo radiation trans-port methodology. In addition to the conventional face-to-facegeometry that has been studied by previous authors, morerealistic exposure scenarios involving babies exposed to a

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radioactive mother patient were simulated for the first timeusing position morphed hybrid phantoms.

Overall, S values show a strong dependency on the giventarget organ and the age of target family phantoms relative tothe source region as defined in the adult male and female pa-tient phantoms. S values from the adult female patient phan-tom are higher by a factor of 1.6 than the values from the adultmale at shorter distance and in younger family phantoms. Ef-fective doses per time-integrated activity based on anatomi-cally realistic hybrid phantoms are lower by twofold than thevalues of NRC recommendations. The realistic exposure sce-nario of a radioactive mother holding a baby revealed that theeffective dose per unit time-integrated activity to the newbornand 1-yr-old phantoms from the hyperthyroid-patient phan-tom is higher than the value from the DTC patient whichwas not be observed in the simplified face-to-face exposuregeometries.

In this study, we reconfirmed the significant conservatismunderlying the point source/target method recommended bythe NRC. The study also revealed that the exposures of 10-and 15-yr-old children were higher than those in younger chil-dren because major organs in those phantoms are distributedat the similar level with the hyperthyroidism and DTC-patientsources. Therefore, it might be helpful to provide licenseeswith a conversion factor of dose rate measurement at 1 m fromthe patient to age-specific effective doses to family membersin the realistic exposure scenarios. Also, the current study willbe expanded to the effective dose-based guideline for a sitephysicist to determine the time of patient release and the du-ration for radiation precaution to family members.

Since the results also showed that the radiation exposure tothe family member strongly depends on the I-131 distributionwithin the patient, current efforts in our research groups willexplore similar studies using patient-specific voxel phantomsand more realistic activity distributions based on SPECT-CTimages. In the meantime, the results of this study may be usedto better inform physicians and hospital staff on recommen-dations for patient release following radioiodine therapies.

a)Author to whom correspondence should be addressed. Electronic mail:[email protected]; Telephone: (501) 526-5929; Fax: (501) 686-7285.

1D. A. Meier, D. R. Brill, D. V. Becker, E. M. Clarke, E. B. Silberstein,H. D. Royal, and H. R. Balon, “Procedure guideline for therapy of thyroiddisease with 131Iodine,” J. Nucl. Med. 43, 856–861 (2002).

2R. B. Sparks, J. A. Siegel, and R. L. Wahl, “The need for better methods todetermine release criteria for patients administered radioactive material,”Health Phys. 75, 385–388 (1998).

3E. Y. Han, C. Lee, and W. E. Bolch, “TEDE per cumulated activity forfamily members exposed to adult patients treated with 131I,” Radiat. Prot.Dosim. 153, 448–456 (2012).

4USNRC, “NRC: 10 CFR 35.75 Release of individuals containing unsealedbyproduct material or implants containing byproduct material,” USNRCReport No. 10CFR35.75, 2007.

5J. A. Siegel, C. S. Marcus, and R. B. Sparks, “Calculating the absorbed dosefrom radioactive patients: The line-source versus point-source model,” J.Nucl. Med. 43, 1241–1244 (2002).

6E. Y. Han, W. E. Bolch, and K. F. Eckerman, “Revisions to the ORNL se-ries of adult and pediatric computational phantoms for use with the MIRDschema,” Health Phys. 90, 337–356 (2006).

7A. B. de Carvalho, Jr., J. Hunt, A. X. Silva, and F. Garcia, “Use of a voxelphantom as a source and a second voxel phantom as a target to calculate ef-fective doses in individuals exposed to patients treated with 131I,” J. Nucl.Med. Technol. 37, 53–56 (2009).

8A. B. de Carvalho, Jr., M. G. Stabin, J. A. Siegel, and J. Hunt,“Comparison of point, line and volume dose calculations for expo-sure to nuclear medicine therapy patients,” Health Phys. 100, 185–190(2011).

9P. B. Zanzonico, J. A. Siegel, and J. St. Germain, “A generalized algorithmfor determining the time of release and the duration of post-release radia-tion precautions following radionuclide therapy,” Health Phys. 78, 648–659(2000).

10NCRP, “Management of Radionuclide Therapy Patients,” NCRP ReportNo. 155 (National Council on Radiation Protection and Measurements,2006).

11R. W. Leggett and K. F. Eckerman, “Dosimetric Significance of the ICRP’sUpdated Guidance and Models, 1989-2003, and Implications for U.S. Fed-eral Guidance,” Oak Ridge National Laboratory Report No. ORNL/TM-2003/207, 2003.

12S. Lamart, A. Bouville, S. L. Simon, K. F. Eckerman, D. Melo, and C. Lee,“Comparison of internal dosimetry factors for three classes of adult com-putational phantoms with emphasis on I-131 in the thyroid,” Phys. Med.Biol. 56, 7317–7335 (2011).

13M. M. Muresan, P. Olivier, J. Leclere, F. Sirveaux, L. Brunaud, M. Klein,R. Zarnegar, and G. Weryha, “Bone metastases from differentiated thyroidcarcinoma,” Endocr. Relat. Cancer 15, 37–49 (2008).

14ICRP, “Nuclear decay data for dosimetric calculations,” ICRP PublicationNo. 107, 2009.

15D. B. Pelowitz, “MCNPX_TM User’s Manual V 2.7.0,” 2002.16International Commission on Radiological Units and Measurements, ICRU

Report No. 46, 1992.17ICRP, “Basic anatomical and physiological data for use in radiological pro-

tection: Reference values,” ICRP Publication No. 89 (Oxford/PergamonPress, 2003).

18P. B. Johnson, A. A. Bahadori, K. F. Eckerman, C. Lee, and W. E. Bolch,“Response functions for computing absorbed dose to skeletal tissuesfrom photon irradiation—An update,” Phys. Med. Biol. 56, 2347–2365(2011).

19W. E. Bolch, K. F. Eckerman, G. Sgouros, and S. R. Thomas,“MIRD Pamphlet No. 21: A generalized schema for radiopharmaceuticaldosimetry–Standardization of nomenclature,” J. Nucl. Med. 50, 477–484(2009).

20ICRP, “Recommendations of the ICRP: Annals of the ICRP Volume 37/2-4,” ICRP Publication No. 103 (International Commission on RadiologicalProtection, 2007).

Medical Physics, Vol. 40, No. 8, August 2013