Close contact restriction periods for patients who received iodine-131 therapy for diﬀerentiated thyroid cancer Jake C. Forster12 Daniel Badger1 and Kevin J. Hickson13

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Close contact restriction periods for patients who received

iodine-131 therapy for diﬀerentiated thyroid cancer

Jake C. Forster1,2, Daniel Badger1, and Kevin J. Hickson1,3

1Medical Physics & Radiation Safety, South Australia Medical Imaging,

Adelaide SA 5000, Australia

2Department of Physics, School of Physical Sciences, University of Adelaide,

Adelaide SA 5005, Australia

3Allied Health & Human Performance, University of South Australia,

Adelaide SA 5001, Australia

26 October 2022

Abstract

Objective. Patients treated with radionuclide therapy may require restrictions on

certain activities for a period of time following treatment to optimise protection of the

public and ensure the legal dose limit is not exceeded. Software may be used to calculate

necessary restriction periods for an individual based on longitudinal dose rate measure-

ments from the time of radiopharmaceutical administration. A spreadsheet program has

been used for this purpose in Australian hospitals for the last two decades. However,

this spreadsheet has a limitation in that it uses an approximation in the calculation of

dose from a contact pattern, which aﬀects the calculated restriction period. A computer

program called Dorn was developed that provides the same functionality as the spread-

sheet but without this approximation. Proﬀered radiation safety advice from Dorn and

the spreadsheet were compared. Approach. Advice from the spreadsheet and Dorn were

compared for 55 patients who underwent iodine-131 therapy for diﬀerentiated thyroid

cancer. Main results. The restriction periods for caring for infants, close contact with

children and sleeping with a partner were typically about 13 hours longer in Dorn than

in the spreadsheet, but in some cases were over a week shorter or a month longer. Signif-

icance. If the Dorn program is used clinically in place of the spreadsheet, some patients

will enjoy shorter restriction periods and the therapy provider can be more conﬁdent in

their compliance with regulatory requirements and best practice. Dorn is freely available

from https://doi.org/jg5f.

1 Introduction

Patients who undergo a diagnostic or therapeutic nuclear medicine (NM) procedure may

expose persons who make contact with them to ionising radiation, for example, through work,

travel, social, or domestic activities. Radiation protection of exposed members of the public

should be optimised [1]. Additionally, in all Australian states and territories, the provider of a

NM procedure is legally required to ensure that no member of the public receives an eﬀective

dose greater than the prescribed limit as a result of the procedure. The eﬀective dose limit

for a member of the public is 1 mSv in a year for planned exposure situations, in line with

the recommendations of the International Commission on Radiological Protection [2]. For

therapeutic NM procedures in particular, this may necessitate the patient withdraw from

certain activities for a period of time following administration of the radiopharmaceutical.

arXiv:2210.14065v1 [physics.med-ph] 25 Oct 2022

The Australian Radiation Protection and Nuclear Safety Agency has published recom-

mendations for the discharge of NM therapy patients (RPS 4) [3]. They emphasise the need

for patient-speciﬁc radiation safety instructions and expertise from a medical physicist: “In-

dividualised instructions relevant to the patient’s medical and social circumstances should be

provided to each patient by the licensed medical specialist responsible for the treatment, in

consultation with an experienced medical physicist. The instructions . . . should be designed

to suit the patient’s own particular travel and domestic arrangements.”

In situations where patient-speciﬁc dose estimates are not available, RPS 4 recommends

that, in an eﬀort to meet the 1 mSv public limit, the external ambient dose equivalent

rate at 1 m from the patient should not exceed 25 µSv h−1at the time of discharge from

hospital. RPS 4 also recommends maximum activities of 111In, 131I, 32P, 188Rh, 153Sm, 89Sr,

and 90Y which may be administered in unsealed forms to outpatients, which can be taken

as recommended maximum activities retained by the patient at the time of discharge from

inpatient therapy. NM therapy providers may wrongly assume that these discharge criteria

ensure compliance with the legal dose limit. Even if these discharge recommendations are

followed, the patient may still require restrictions after being discharged to ensure that no

person’s dose limit is exceeded.

J. Cormack and J. Shearer developed software using Microsoft Excel (97-2003) and Visual

Basic for Applications called Radionuclide Therapy Close Contact Dose Program (RNTCCDP

or “the spreadsheet” for short), which can be used to estimate radiation exposures and provide

instructions to patients following NM procedures [personal communication]. The spreadsheet

has been used clinically in NM departments across Australia for the last two decades. Brieﬂy,

the user enters into the spreadsheet measurements of dose rate at a ﬁxed distance from the

patient (either 1, 2, or 3 m) at multiple time points, starting from the time of administration.

An exponential or biexponential curve is ﬁtted to the measurements to obtain the whole body

clearance function in terms of dose rate. The instantaneous activity retained by the patient

is estimated by multiplying the instantaneous dose rate by the ratio of the administered

activity to the initial dose rate (called the baseline method). Thus, in the spreadsheet,

a recommended discharge time is calculated based on when the retained activity reaches

the maximum administered activity for outpatient therapy as recommended by RPS 4 (e.g.

600 MBq for 131I). If RPS 4 does not contain such a value for the radionuclide being used,

the recommended discharge time can be calculated based on when the dose rate at 1 m

reaches 25 µSv h−1. The clearance function in terms of dose rate can be used to calculate

the cumulated dose to a person who shares a particular contact pattern with the patient,

resumed after a given period of restriction following administration. The required restriction

period is then determined such that the dose to the contacted person is less than the dose

constraint.

RNTCCDP is limited by using an approximation in the calculation of dose from a contact

pattern, which aﬀects the calculation of required restriction period. A computer program

called Dorn (Delay or not) was developed that provides the same functionality as the spread-

sheet, including a graphical user interface, but implements the full, rigorous calculation of

dose from a contact pattern and hence obtains the required restriction periods with greater

accuracy than in the spreadsheet. To investigate diﬀerences in proﬀered radiation safety ad-

vice between the RNTCCDP and Dorn programs, Dorn was applied retrospectively to data

recorded during treatments of patients who underwent 131I therapy for diﬀerentiated thyroid

cancer [4], who were managed at the time based on advice from the spreadsheet.

2 Methods

2.1 Program overview

The ﬂowchart in Fig. 1 illustrates the main actions of the Dorn and RNTCCDP programs.

The RNTCCDP spreadsheet was obtained via File Transfer Protocol. Version 14.0 of the

spreadsheet was compared against in this work.

Input

Clearance

curve ﬁt

Administered

activity

Patient dose rate

measurements

RPS 4

discharge

criteria

Contact

patterns

Recommended

discharge time

Restriction

periods

Patient

information

sheet

Figure 1: Overview of the Dorn and RNTCCDP programs, either of which can be used to

estimate radiation exposures and provide instructions to patients following NM procedures.

Dorn was written in Python 3. The code was developed on top of well-established Python

libraries such as NumPy [5], SciPy [6], pandas [7], and Matplotlib [8]. The graphical user

interface was created using Python’s Tkinter library. Data entered by a user is written to

and read from XML ﬁles. A patient information sheet and treatment summary report are

generated as Microsoft Word ﬁles. For the restrictions part of the program, a separate,

standalone Python package called glowgreen was written and integrated into Dorn. The

glowgreen package stores the contact patterns and calculates doses to contacted persons and

restriction periods.

Dorn and glowgreen are open-source software distributed under the MIT License [9, 10].

This work uses version 1.9.6 of Dorn and version 0.0.1 of glowgreen.

2.2 Dose rate curve ﬁt

In both the RNTCCDP spreadsheet and Dorn program, an exponential or biexponential curve

is ﬁtted to the longitudinal measurements of dose rate at a ﬁxed distance rfrom the patient,

between 1 and 3 m. A least-squares ﬁt is performed to an exponential of the form

D(t) = ˙

D(0)e−tln 2/T ,(1)

which has 2 ﬁt parameters, or to a biexponential

D(t) = ˙

D(0)f1e−tln 2/T1+ (1 −f1)e−tln 2/T2,(2)

with 4 ﬁt parameters. In the spreadsheet, the Generalised Reduced Gradient nonlinear

method was used subject to constraints

D(0) ≥0,(3)

0≤f1≤1,(4)

and

0.36 s ≤T, T1, T2≤Tp,(5)

where Tpis the physical half-life of the radionuclide used (8 d for 131I).

Dorn uses the curve ﬁt method from the optimize module of the SciPy library [6], with

the Trust Region Reﬂective algorithm. The initial dose rate is constrained to

0<˙

D(0) < A0Γ1 m

r1.5

,(6)

Figure 2: Example of a contact pattern that repeats with a period of 24 h. There is contact

every day from noon to 3 PM at a distance of 1 m and from 6:30 to 7:30 PM at 30 cm. The

heights of the bars are drawn such that the bar area would be proportional to the dose from

the contact if the dose rate was constant.

where A0is the administered activity and Γ is the radionuclide’s speciﬁc gamma or bremsstrahlung

dose rate constant at 1 m. The value used for 131I is 7.647 ×10−2µSv h−1MBq−1[11]. The

upper bound corresponds to an unattenuated point source, except distance correction from

1 m to ris performed using an “inverse 1.5 power” relationship rather than the inverse square

law because it gives a larger upper bound and is more appropriate for an extended patient

source [3]. The constraint

0< f1<1 (7)

is applied, and the clearance component half-lives are constrained to

2.5 s < T < Tp(8)

and

12.5 s < T1, T2< Tp.(9)

Values of Tpare taken from the Evaluated Nuclear Structure Data File Database main-

tained by the National Nuclear Data Center [12]. For example, Tp= 8.0252 d for 131I. The

lower bounds assist the algorithm to ﬁnd optimal parameters. Initial guesses of ˙

D(0) =

(1/2)A0Γ(1 m/r)1.5,f1= 0.5 and T=T1=T2=Tp/3 are provided.

2.3 Contact patterns

A contact pattern consisting of mcontacts/elements is denoted by {θj, cj, dj}for j= 1, . . . , m,

where θjis the time at which the jth element starts, cjis the duration of that element, and

djis the distance at which it takes place. For a contact pattern that repeats with period p,θj

is deﬁned w.r.t. a reference time, Tr. For diurnal patterns, midnight is used for the reference

time by convention. An example of a repeating contact pattern is provided in Fig. 2 for

demonstration. For a contact pattern that does not repeat, called a once-oﬀ contact pattern,

θjis instead deﬁned w.r.t. when the contact begins (i.e. θ1= 0).

Cormack and Shearer created 10 unique, repeating, diurnal patterns of close contact for

various groups of exposed persons [13], which they implemented in the RNTCCDP spread-

sheet. The same contact patterns were implemented in the Dorn program via version 0.0.1

of the glowgreen package. They are provided in Supplementary Material for reference.

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Closecontactrestrictionperiodsforpatientswhoreceivediodine-131therapyfordierentiatedthyroidcancerJakeC.Forster1,2,DanielBadger1,andKevinJ.Hickson1,31MedicalPhysics&RadiationSafety,SouthAustraliaMedicalImaging,AdelaideSA5000,Australia2DepartmentofPhysics,SchoolofPhysicalSciences,UniversityofAdelaide...

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Close contact restriction periods for patients who received iodine-131 therapy for diﬀerentiated thyroid cancer Jake C. Forster12 Daniel Badger1 and Kevin J. Hickson13

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