Phantom Measurements

The dose calculations were based on the assumption that 10 megabecquerels (MBq) of tracer is retained at the injection depot at the moment of surgery. The effects of two variables were estimated: the depth of the depot, which was placed in tissue equivalent material (i.e., water), and the distance from the depot to the dose-rate monitor.

A sphere with a diameter of 2 cm was filled with 10 MBq of 99mTc pertech-netate and submersed in water at a depth of 1, 2, and 4 cm as a model for peritu-moral injections in breast cancer. A 1-mL syringe filled with 10 MBq of 99mTc pertechnetate in 0.4 mL of saline served as a model for intracutaneous injections such as used in sentinel lymphadenectomy in melanoma. Triple-dose-rate measurements were performed at distances of 10, 15, 25, 50, and 100 cm with a Berthold LB123 monitor (Berthold GmbH, Bad Wildbad, Germany).

We found that the distance from the source to the dose-rate monitor is the single most important determining factor (Fig. 1). From 50 cm to 10 cm distance, the average dose rate increased by a factor of 16.7 for the spherical phantom and 16.8 for the melanoma phantom.

1

pi

1

1

til T^

10 cm 15 cm 25 cm 50 cm 100 cm Distance to phantom

10 cm 15 cm 25 cm 50 cm 100 cm Distance to phantom phantom depth: DO cm SI cm ■ 2 cm 14 cm

Figure 1 Dose rates (in ^Sv/h/10 MBq) at different distances from a breast and melanoma phantom.

Table 2 Dose Rates (in ^Sv/h/10 MBq) as Function of Distance from the Source in Breast Cancer (n = 10, average ± SD) and melanoma patients (n = 10, average ± SD) and phantoms

10 cm

15 cm

25 cm

50 cm

100 cm

Breast phantom

6.9

3.2

1.5

0.4

0.2

Melanoma phantom

8.4

5.0

1.9

0.7

0.3

Breast patient

11.7a

5.8a

1.9 ± 0.4

0.7 ± 0.3

0.25 ± 0.07

(range)

(1.4-2.8)

(0.4-1.3)

(0.16-0.39)

Melanoma patient

9.8 ± 2.4

4.7 ± 1.1

2.0 ± 0.8

0.6 ± 0.3

0.28 ± 0.12

(range)

(7.9-12.5)

(3.7-6.0)

(1.0-3.5)

(0.2-0.9)

(0.16-0.50)

a Calculated values, based on phantom measurements.

a Calculated values, based on phantom measurements.

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