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British Journal of Radiology (2003) 76, 818-823
© 2003 British Institute of Radiology
doi: 10.1259/bjr/57254925
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Full Paper

Patient effective dose from sentinel lymph node lymphoscintigraphy in breast cancer: a study using a female humanoid phantom and thermoluminescent dosemeters

M Law, PhD 1 K C Cheng, MBBS, FHKCR 2 P M Wu, PhD 1 W Y Ho, MBBS, MSc, FHKCR 2 and L W C Chow, MBBS, MS, FRCS 3

1 Departments of Clinical Oncology and 3 Surgery, 2 Nuclear Medicine Unit, Queen Mary Hospital, 102 Pokfulam Road Hong Kong


   Abstract
Top
 Abstract
Introduction
 Method and materials
 Results
 Discussion
 Conclusion
 References
 
The aim of this study was to measure the dose delivered to patients undergoing sentinel lymph node lymphoscintigraphy by taking into account both the transmission scan dose using a 57Co flood source and the 99Tcm internal emission dose. An adult female humanoid phantom and a set of thermoluminescent dosemeters were used in the measurements. The choice of measurement organs in the humanoid was guided by the recommendations described in the International Commission on Radiological Protection report number 60. A 57Co flood source was used in external transmission to irradiate the humanoid at posterior, left lateral, left posterior oblique, right lateral and right posterior oblique positions. Four 99Tcm deposits as internal emission sources were used to simulate patient peritumoural injection. The individual effective doses for external transmission and internal emission, normalized to the cumulated activity and expressed in µSv(MBq·h)-1 were then calculated. The effective dose for a transmission scan was on average 0.061 µSv(MBq·h)-1 for each 57Co flood source position and for internal emission 0.312 µSv(MBq·h)-1 and 0.291 µSv(MBq·h)-1 for left and right breast injection, respectively. Using these results, the effective dose from both transmission and emission sources can be calculated according to the nuclear medicine scanning protocol and surgical procedure of the individual institution. For our protocols, the patient receives a maximum effective dose of 52 µSv for the 1 day protocol (18 MBq injection) and 204 µSv for the 2 day protocol (74 MBq injection) if only the sentinel lymph node is excised. If other tissues containing radioactivity are removed, the patient effective dose will be reduced by about 50% and 6%, respectively, for the 1 day protocol and 2 day protocol. Although the doses are low compared with other radiological examinations, the results are informative for patients concerned about radiation exposure for this new imaging technique.


   Introduction
Top
 Abstract
 Introduction
Method and materials
 Results
 Discussion
 Conclusion
 References
 
The technique of sentinel lymph node (SLN) biopsy is gaining wide acceptance in advancing the clinical management of breast cancer [1–4]. The technique requires the injection of 99Tcm labelled colloid into tissue lying adjacent to the primary tumour. As pointed out by Waddington et al [1], the radiation dose to the patient should be quantitatively determined in order to reassure the patient.

More recently, there has been an increasing trend to use a 57Co flood source, as a transmission scan source, to facilitate the topological localization of the body contour [4–6]. The combined transmission and emission scan images can help the physician identify the SLN location. Therefore the patient receives a radiation dose from both the internal emission dose due to the injected 99Tcm and from the external transmission dose due to the 57Co flood source, the latter of which may be significant if the flood source is a new one of nominal radioactivity 370 MBq.

The patient effective dose (ED) in SLN lymphoscintigraphy has been estimated for a 99Tcm internal emission source based on the Medical International Radiation Dose (MIRD) scheme [7, 8]. By assuming that the radioactive tracer is distributed homogeneously throughout the entire breast tissue in the MIRD calculation, the ED has been estimated at 320 µSv for a 15 MBq administration of tracer [1]. However, this estimation represents a worst case for dosimetric purposes. From SLN imaging observations [1], greater than 95% of the administered radioactivity remains as a localized source within the injection site with negligible biological clearance of radioactivity. The mean tracer uptake in the SLN is about 1% of the injected radioactivity. Hence the MIRD scheme is not a true calculation for the SLN technique. Therefore it is necessary to measure the ED due to internal emission appropriate to the clinical situation. Moreover, the patient ED due to external transmission in SLN lymphoscintigraphy has not been considered as part of the patient dosimetric estimation.

In the present study, direct measurements were made for patient ED for both transmission and emission sources, using a reference female humanoid phantom and a set of thermoluminescent dosemeters (TLDs). The choice of organs within the phantom for TLD placement followed the recommendations of International Commission on Radiological Protection report number 60 (ICRP 60) [9]. The results obtained from the study can be generally applied to evaluate the ED received by patients undergoing SLN lymphoscintigraphy.


   Method and materials
Top
 Abstract
 Introduction
 Method and materials
Results
 Discussion
 Conclusion
 References
 
Phantom
An Alderson-Rando standard adult female phantom (The Phantom Laboratory, New York, USA), constructed with a natural human skeleton and tissue equivalent cast materials, was used in the study. The phantom is divided into 35 contiguous slices, each of which is 25 mm thick and contains a matrix of holes for TLD placement within the phantom.

Thermoluminescent dosemeters
The dose delivered to the phantom was measured with lithium fluoride (LiF) TLDs (type TLD-100H, The Harshaw Chemical Company, Solon, OH) having the form of small square wafers approximately 3.2 mm long and 0.6 mm thick. According to the manufacturer's specifications, the TLDs are characterized by small size, a linear dose response between 1 µGy and 10 Gy and negligible information loss at room temperature. After annealing, the TLDs can be used again to perform a new measurement.

Prior to each exposure, the TLDs were annealed at 240°C for 10 min using a PTW oven (PTW Company, Freiberg, Germany). The temperature profile of the TLD reader (Harshaw, model QS5500) in this study was as follows: pre-heat from room temperature to 135°C for 10 s, read cycle 135°C to 240°C with heating rate 15°C s-1 and at 240°C for 16.6 s according to the manufacturer's instructions. Readout, annealing and placement of TLDs were performed in the absence of radioactive sources.

Calibration of TLDs
The batch of TLDs used in the study was calibrated against a 137Cs source, traceable to a secondary standard, with known air kerma rate at the time of calibration. The batch of TLDs was divided into subsets, which were irradiated by the 137Cs source for different durations. After correction for ambient noise, the signal collected by the glow curve of each subset of TLDs was measured. The relationship between the absorbed doses and the collected signals from the TLDs was found to be linear and a calibration factor was derived from the slope of the linear relationship.

TLD-100H have the same radiation response for 137Cs, 99Tcm and 57Co photons. In order to confirm the energy independence of the TLD response to these isotopes, subsets of the TLDs were irradiated by a 99Tcm point source, the radioactivity of which had been accurately measured with a dose calibrator. The calibration factor for 99Tcm, and hence implied for 57Co as well, was found to be in good agreement with that using 137Cs source.

Measurement organs
The choice of measurement organs was guided by the recommendations of ICRP 60 to evaluate the ED. For the determination of anatomical positions and depth of individual organs, an atlas of sectional anatomy was consulted [10]. At least two TLDs per organ were used, providing the possibility of verifying and comparing the measured values and accounting for possible damage to TLDs. These organ positions were consistently used for TLD placement throughout the study. A total of 54 TLDs were distributed in various organs of interest in the Rando phantom (Table 1Go). The remainder organ, as described in ICRP 60, included the brain, eye, myocardium, small intestine and kidney.


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  		Table 1. Distribution of thermoluminescent dosemeters (TLDs) in various organs

 
To measure the radiation dose for the positions at the humanoid surface, namely the eye and skin, the TLDs were placed inside small black plastic bags. They were then attached to the eye surface and to the skin surface covering the same transversal slice of the humanoid's breast at the bilateral scapular lines, bilateral mid-axillary lines and mid-sternal line.

Irradiation geometry of the transmission source
A rectangular 57Co flood source of active dimensions of 440 mm x 580 mm and containing 77 MBq of radioactivity at the time of measurement was used. The flood source was mounted onto one of the detectors of a large field of view dual headed gamma camera in the same position as used for patient imaging. The humanoid phantom, already loaded with TLDs but without any 99Tcm internal sources, was positioned in the same position as for patient imaging. The field of view irradiated by the flood source covered from the neck to the abdomen of the humanoid. The same measurement procedures were repeated for the 57Co flood source placed at posterior, left lateral, left posterior oblique, right lateral and right posterior oblique positions of the humanoid phantom in order to cover all transmission scan positions used in patient imaging. Each measurement position took 15 h of irradiation to ensure adequate signals were received by the TLDs.

Irradiation geometry of the emission source
Four plugs, each containing about 60 MBq of 99Tcm radioactivity in a volume of 0.2 ml, were embedded into one of the breasts of the humanoid phantom in four deposits to simulate the peritumoural injection. The same measurement setup (without the use of 57Co flood source) was repeated for the other breast of the phantom, after the TLDs had been read and then annealed. Each emission measurement took 15 h of irradiation. The amount of 99Tcm radioactivity and the prolonged irradiation time were used to increase the signals received by the TLDs in the measurements.

Protocols for lymphoscintigraphy at our institution
Two SLN lymphoscintigraphy protocols, namely the 1 day and 2 day protocol, are used in our institution. Patients have peritumoural injection of 2 ml unfiltered 99Tcm sulphur colloid (Hepatocis, CIS bio-international, MA) in 4 deposits. In the 1 day protocol, 18 MBq 99Tcm sulphur colloid was injected at 4 h before the surgery schedule. 74 MBq of 99Tcm sulphur colloid was injected at 24 h before the surgical schedule for the 2 day protocol [11]. The injection sites were gently massaged for 1 min to aid the clearance of colloid particles. The massaging did not cause any radiation contamination to the patient as the injection sites were covered with Elastoplast plastic strips.

Under both protocols, dynamic images are acquired in anterior projection for 20 min without the use of the 57Co flood source. Static images, each of which takes 5 min, are simultaneously acquired at 0.5 h and 2 h post injection in anterior, anterior oblique and lateral projections with the 57Co flood source placed at the respective conjugate view [11].

Based on these imaging protocols, the patient effective dose will be quantitatively determined for irradiation by the transmission and emission sources.

Calculation of effective dose
The values of ED were calculated according to ICRP 60 recommendations using the following expression: Go


where wR is the radiation weighting factor (being unity for photons), DT,R is the mean absorbed dose to an organ for radiation type R and wT is the tissue weighting factor for organ or tissue T as listed in ICRP 60. The values of signal obtained from TLD readout, after correction for ambient noise, were converted into absorbed doses by applying the calibration factor. In both transmission and emission scans, all organ absorbed doses, and hence the ED given by Equation 1Go, were normalized by the cumulated activity given by [12]: Go


where A0 is the initial radioactivity expressed in MBq and t is the elapsed time used in the phantom measurement. {lambda} is the decay constant given by


where T1/2 is the physical half life of the source (T1/2=6.03 h for 99Tcm and T1/2=271 days for 57Co).

The inherent variation of the TLD-100H in dose response was measured to be ±6%. In addition, the accuracy of measurements was affected by other factors, namely the derived calibration factor error (±3%) between 137Cs and 99Tcm and the directionality error associated with the edge and surface of TLDs in relation to the direction of the radiation field (±2%) [13]. Owing to these factors, the final results of ED for this study will have an error of ±11%.


   Results
Top
 Abstract
 Introduction
 Method and materials
 Results
Discussion
 Conclusion
 References
 
Transmission source
Table 2Go tabulates the values of normalized absorbed doses measured with the 57Co flood source located at different phantom positions. The value of normalized ED was calculated for each 57Co flood source position relative to the phantom.


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  		Table 2. Normalized absorbed doses (DT,R) received by various organs of interest for a transmission scan using a 57Co flood source, and the calculated normalized effective dose (ED)

 
Normalized absorbed doses by contralateral symmetry organs, namely lung, ovary, breast and thyroid, were found to be in good agreement when the flood source was placed at the left posterior oblique and right posterior oblique position relative to the phantom, similarly at the left lateral and right lateral position. When the flood source was placed at the posterior position of the phantom, the average normalized absorbed dose to the breast was 0.023 µGy(MBq·h)-1 due to the attenuation of the transmission photons by the humanoid body. However, when the source was placed at the lateral position, the average normalized absorbed dose received by the breast was increased by 4 times. It should be noted that the reported normalized absorbed dose (Table 2Go) was the mean value averaged over the two breasts. In all positions of the flood source, although the normalized dose received by the skin was the greatest among the other organs of interest, its contribution to the normalized ED is not significant as the skin weighting factor is 0.01.

For left breast injection, values of the normalized ED corresponding to the flood source position at right posterior oblique, right lateral and posterior are used. The total of these normalized ED equals 0.177 µSv(MBq·h)-1. For right breast injection, values of the normalized ED corresponding to flood source position at left posterior oblique, left lateral and posterior are used. The total of these normalized ED equals 0.182 µSv(MBq·h)-1.

Emission source
Table 3Go tabulates the normalized absorbed doses for various organs of interest for each breast injection. The normalized ED was calculated as 0.312 µSv(MBq·h)-1 and 0.291 µSv(MBq·h)-1 for left and right breast injection, respectively. Due to the symmetry of the location of the 99Tcm sources in each breast, the normalized ED was observed to be approximately the same. When the 99Tcm sources were located in the left breast, radiation dose would contribute more to the myocardium and thus slightly increase the normalized ED.


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  		Table 3. Normalized absorbed doses (DT,R) received by various organs of interest by internal emission from each breast injection and the calculated normalized effective dose (ED)

 

   Discussion
Top
 Abstract
 Introduction
 Method and materials
 Results
 Discussion
Conclusion
 References
 
The ED delivered to patients undergoing SLN lymphoscintigraphy should be evaluated. We have addressed this issue under current imaging conditions using a reference adult female humanoid phantom and TLDs suitable for dose monitoring in nuclear medicine. ICRP 60 recommendations were followed to calculate independently the normalized ED for transmission and emission scans.

The patient ED was estimated from the measured ED for different clinical protocols. Under both protocols, the patient undergoes emission lymphoscintigraphy at 0.5 h post injection. For the left breast injection, the transmission scan is taken simultaneously with each emission scan for 5 min with the 57Co flood source placed at the patient position of posterior, right lateral and right posterior oblique. For the right breast injection, the transmission scan is taken simultaneously with each emission scan for 5 min in the posterior, left lateral and left posterior oblique positions. The same simultaneous transmission and emission static images are repeated at 2 h post injection. Therefore a total of 10 min of transmission irradiation is delivered to the patient at each flood source position. The 57Co flood source is chosen so that the source strength is just strong enough to give a good body contour image, but not strong enough to expose the patient unnecessarily. Our experience shows that source radioactivity in the range 50 MBq to 150 MBq is sufficient to define a good body contour. If a 100 MBq 57Co flood source is used and the total irradiation time is 10 min, the cumulated activity is 16.67 MBq·h, obtained from Equation 2Go. Multiplying this cumulated activity by the normalized ED (Table 2Go) corresponding to each breast injection, the ED delivered to the patient due to the transmission scan will be 2.95 µSv and 3.03 µSv, respectively, for left and right breast injection. These effective doses are equivalent to half a day natural background radiation dose in our institution area (6 µSv per day).

The ED due to internal emission not only depends on the cumulated activity but also on the schedule and procedure for surgery. If only the SLN is excised, the irradiation time t tends to infinity and Equation 2Go reduces to Go


Therefore for the 1 day protocol, in which the initial injected activity A0 is 18 MBq, the cumulated activity from Equation 4Go is 157 MBq·h. Multiplying this cumulated activity by the normalized ED (Table 3Go) gives an ED due to internal emission of 49 µSv and 46 µSv, respectively, for left and right breast injection, after rounding off the ED to an integer value. Similarly for the 2 day protocol, in which the initial activity A0 is 74 MBq, the ED due to internal emission will be 201 µSv and 187 µSv for left and right breast injection, respectively.

If the tissues containing radioactivity are going to be removed, the surgical procedure is normally performed at 6 h post injection for the 1 day and at 24 h post injection for the 2 day protocol. Together with the initial injected radioactivity of 99Tcm, these surgical times have to be input into Equation 2Go in order to calculate the cumulated activity and then the ED for both protocols. This gives EDs of 24 µSv and 23 µSv for left and right breast injection, respectively, for the 1 day protocol, and 188 µSv and 175 µSv for left and right breast injection, respectively, for the 2 day protocol. Table 4Go shows the total ED, after taking into account both transmission and emission irradiation, for both protocols under different surgical procedures.


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  		Table 4. Effective dose (ED) for both transmission and emission imaging under different protocols. Values in parentheses are the ED if the tissues containing radioactivity are excised at the surgery

 
It is interesting to note that patients undergoing the 2 day protocol will receive about the same total ED, regardless of whether the tissues containing radioactivity are excised at the surgery or not. The total ED for patients undergoing the 1 day protocol will be reduced by about 50% if the excision includes the radioactivity-retaining tissues.

The patient ED due to the emission source, as calculated using the MIRD scheme, has been reported as 320 µSv for a 15 MBq administration of tracer if only the SLN is excised [1]. This is about 7 times higher than our calculated ED for the 1 day protocol with the same surgical procedure. However, the assumption of uniform tracer distribution within the entire breast in the MIRD scheme is not appropriate to the SLN technique. Therefore the ED obtained by the MIRD scheme has been regarded as a worst case for dosimetric purposes when results from measurements are not available [1]. As presented in this study, the use of a standard female humanoid phantom and a set of high sensitivity TLDs allowed precise measurements of the radiation doses received by all organs of interest for an accurate evaluation of ED. Due to the phantom's standard dimensions and tissue equivalent cast materials, the effects of photon attenuation and scattering within the phantom body have been taken into account in the dose measurements of transmission and emission. Thus the accuracy of ED from SLN lymphoscintigraphy has been greatly improved with the results obtained by this study when compared with the ED calculated by the MIRD scheme. Using the concept of cumulated activity as described in the current study, the ED can be calculated for patients undergoing different lymphoscintigraphy protocols and surgical procedures.


   Conclusion
Top
 Abstract
 Introduction
 Method and materials
 Results
 Discussion
 Conclusion
References
 
Despite the relatively small amount of radioactivity administered to patients in SLN nuclear medicine imaging, the patient effective dose, in terms of ICRP 60 recommendations, is a useful assessment of risk to compare with other radiological examinations for the purpose of providing information to the patient. The ED due to the external transmission 57Co flood source is half the daily natural background radiation dose in our institution area if a flood source of 100 MBq is used. Nevertheless, the effective dose due to the external transmission scan is proportional to the strength of the source, and avoiding the use of a new source is recommended to reduce the patient dose.

The ED due to the internal emission is about 4 times higher for patients undergoing the 2 day protocol than the 1 day protocol because of the higher radioactivity of 99Tcm colloid administered to patients.

It has been suggested that the risk associated with a nuclear medicine procedure should be explained to patients by comparison with a more familiar radiological examination [14]. The results obtained from the current study indicate that the patient total ED for the 1 day protocol is slightly more than that from a plain chest X-ray exposure (40 µSv) [14] if only the SLN is going to be excised, and even less if the radiation-containing tissue is going to be removed. For the 2 day protocol, the patient total ED is equivalent to 5 plain chest X-ray exposures, regardless of whether radiation-containing tissues are to be excised or not. The quantitative values of effective dose determined by the current study further confirm that SLN lymphoscintigraphy is a radiologically safe imaging procedure even with the use of a 57Co flood source of moderate radioactivity as the transmission source.

Received for publication February 24, 2003. Revision received June 4, 2003. Accepted for publication July 14, 2003.


   References
Top
 Abstract
 Introduction
 Method and materials
 Results
 Discussion
 Conclusion
 References
 

  1. Waddington WA, Keshtgar MRS, Taylor I, Lakhani SR, Short MD, Ell PJ. Radiation safety of the sentinel lymph node technique in breast cancer. Eur J Nucl Med 2000;27:377–91.[CrossRef][Medline]
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  3. Krausz Y, Ikeda DM, Jadvar H, Langleben D, Birdwell R, Strauss HW. Non-visualization of sentinel lymph node in patients with breast cancer. Nucl Med Comm 2001;22:25–32.[CrossRef][Medline]
  4. Mariani G, Moresco L, Viale G, Villa G, Bagnasco M, Canavese G, et al. Radioguided sentinel lymph node biopsy in breast cancer surgery. J Nucl Med 2001;42:1198–215.[Abstract/Free Full Text]
  5. Birdwell RL, Smith KL, Betts BJ, Ikeda DM, Strauss HW, Jeffrey SS. Breast cancer: variables affecting sentinel lymph node visualization at preoperative lymphoscintigraphy. Radiology 2001;220:47–53.[Abstract/Free Full Text]
  6. Mateos JJ, Vidal-Sicart S, Zanon G, Pahisa J. Fuster D, Martin F, et al. Sentinel lymph node biopsy in breast cancer patients: subdermal versus peritumoural radiocolloid injection. Nucl Med Comm 2001;22:17–24.[Medline]
  7. Loevinger R, Budinger TF, Watson EE. MIRD primer for absorbed dose calculations. New York: Society of Nuclear Medicine, 1991.
  8. Stabin MG. MIRDOSE: personal computer software for internal dose assessment for nuclear medicine. J Nucl Med 1996;37:538–46.[Free Full Text]
  9. International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection, Publication 60. Oxford: Pergamon Press, 1991.
  10. McGrath P, Mill P. Atlas of sectional anatomy: head, neck and trunk. New York: Kargen, 1984.
  11. Yeung HWD, Cody HS, Turlakow A, Riedel ER, Fey J, Gonen M, et al. Lymphoscintigraphy and sentinel node localization in breast cancer patients: A comparison between 1-day and 2-day protocols. J Nucl Med 2001;42:420–3.[Abstract/Free Full Text]
  12. Zanzonico PB, Brill AB, Becker DV. Radiation dosimetry. In: Wagner HN, Szabo Z, Buchanan JW, editors. Principles of nuclear medicine (2nd edn). Philadelphia: Saunders, 1995:106–34.
  13. Wagner GS, Batey SE, Mosleh-Shirazi MA. Directionality of extruded lithium fluoride thermoluminescent dosemeters in a cobalt-60 beam. Br J Radiol 2000;73:1007–9.[Abstract]
  14. Mountford PJ. Risk assessment of nuclear medicine patient. Br J Radiol 1997;70:671–83.[Abstract]



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