Physical design

There is no reason why a radioiodine facility cannot be located in a general ward area, however the location and design both need consideration. The facility should be away from public areas to allow control of access (including from any balconies), and because the patients usually require low levels of nursing care, does not have to be close to the nurses’

station. Use of external walls can reduce radiation shielding requirements, especially if the facility is above ground level. If at ground level, public access to external areas should also be controlled.

Design and finish

Ideally, the radioiodine facility should have not only a patient bedroom (or more than one), but also a dedicated shower/toilet for each room and a waste storage/dose preparation area.

The location and orientation of the patient’s bed should allow for easy observation by nursing staff with optimum safety particularly from a distance.

The finish of the rooms must allow for easy decontamination in case of accidental spillage of contaminated fluids, etc. This means that:

• All floors must have a smooth, waterproof, continuous finish (such as vinyl), with coving to the walls;

• The bathroom floor must also be of a non-slip type;

• Walls must be finished with a washable material such as gloss paint;

• Furniture must be similarly waterproof and washable, i.e. no cloth surfaces;

• The door to the bedroom should have a glass viewing window, and be at least 2 metres from the bed;

• Waste storage bins must have removable plastic liners so the contents do not have to be handled.

As the patient is to be kept in semi-isolation for the period of their stay in hospital, they should have all the necessary facilities such as telephone and television available to them in their room.

If capsules are used for the dose delivery, special ventilation of radioiodine rooms is not required. However, if open liquids are administered, this must be done in a well-ventilated area.

Radiation shielding

As the activity level can be significant, and the photon energy of ¹³¹I is reasonably high, shielding of a radioiodine facility is important. This is particularly so if the facility adjoins other patient, staff or public areas. Shielding design is not difficult, and will take into account the following issues:

• Source term;

• The maximum expected activity;

• The maximum expected number of patients per year in the room being considered;

• The expected effective half-life;

• The expected length of hospital stay;

• The expected radiation dose rate from the patient;

• The distances to, and occupancies of, adjacent accessible areas;

• The target design dose in accessible areas (including any constraints required by the local regulatory authority);

• Attenuation of possible shielding materials for ¹³¹I photons.

The following uses an example to illustrate the principles of shielding design.

Source term

Current practice is for a typical administered activity of around 6 GBq, with a range of 2 GBq to 7.5 GBq. For the example, take 6 GBq.

Assume the effective half-life is 1 day (24 hours), and a hospital stay of 3 days (72 hours).

The integrated activity is then 9 GBq-days, which means for calculation purposes that there is a continuous activity in the patient of 3 GBq during their stay.

Let us also assume that the facility is expected to take an average of one patient per week.

Dose rate from patient

There is a range of dose rate data published for ¹³¹I, from 51 to 76 microSv.GBq^-1 hr^-1 at 1 metre. Note that the dose rate from a patient will usually be less due to absorption in their body, and has been measured at around 45 microSv.GBq^-1 hr^-1 at 1 metre, so this value should be used.

For a point source, normally the inverse square law is used to correct the dose rate for distance, but in the case of a source distributed in the body, this does not apply for short distances (up to 3 metres). A good approximation is that the dose rate falls according to (distance)^-1.5.

Distances to calculation points, and occupancies

These can be measured from plans, and should be taken from where the boundary of where the patient is expected to spend their time in the bedroom, to 0.5 metre on the far side of each wall or barrier.

The occupancy is an estimation of the fraction of time an area is expected to be occupied.

These are usually quite conservative, and include values of 1 for offices, 0.25 for corridors and 0.06 for toilets.

Target or design dose

This is the dose in mSv per week, which the shielding will be expected to limit radiation dose.

For areas occupied by radiation workers this is the occupational dose limit specified in the BSS [12.4] of 20 mSv per year averaged over five consecutive years, and for the public, 1 mSv in a year. An additional dose constraint should also be considered [12.4]. For example, a constraint of 0.2 is often used for medical radiation workers.

Attenuation data for shielding materials

There is a large amount of data on the shielding properties of standard materials such as lead and steel. The shielding designer must however, be aware that attenuation for other materials such as bricks or cement blocks can vary greatly. It is assumed lead is to be used as the shielding material. The half value thickness for ¹³¹I photons in lead is 3 mm [12.1].

Sample calculation

Let us assume that the room next to the patient’s bedroom is an office (occupancy = 1), and that the distance from the patient is 2.5 metres.

The distance-corrected dose over a 3 day stay per patient (see source term above) is therefore given by:

(3 GBq × 3 days × 24 hours × 50 patients × 45 microSv.GBq^-1 hr^-1) / 2.5^1.5 microSv/year

= 122 950 microSv/year

= 123 mSv/year

If the target dose is 1 mSv per year, then the required attenuation is 1/123= 0.008. This equates to 7 half value thicknesses, or approximately 27 mm lead.

This is only an example — each facility should be designed according to the criteria and assumptions required by the local regulatory authority.

Shielding construction

Normally, shielding need only extend to 2100 mm above the floor level. There should however be no gaps or holes in the shielding, and doors may need to be shielded.

It is highly recommended that installation of shielding be supervised, and be tested before the facility is used.