EuropeanJournalof

Nuclear Medicine

Original article

Evaluation of a technique for the intraoperative detection of a radiolabelled monoclonal antibody against colorectal cancer* Wendy A. W a d d i n g t o n 1, B r i a n R. D a v i d s o n 2, A n d r e w T o d d - P o k r o p e k 1, Paul B. B o u l e s 2, M i c h a e l D. S h o r t 1 Departments of 1 Medical Physics and 2 Surgery, University College Hospital and University College and Middlesex School of Medicine, London WC1E 6AJ, UK Received 18 May 1991 and in revised form 8 July 1991

Abstract. Occult tumour deposits may be localised at operation with a radiation detecting probe following the administration of a radiolabelled monoclonal antibody (MoAb) recognising a tumour-associated antigen. We have recently evaluated the clinical usefulness of this technique in detecting primary colorectal tumours targetted with an indium-] ] 1 MoAb. In the present study the physical characteristics of the two detector systems used were investigated; a sodium iodide [NaI(T1)] scintillation detector and a cadmium telluride (CdTe) semiconductor probe. Limitations of the technique in use have been examined by testing the statistical significance of tumour detection using an abdominal phantom based on the currently available clinical biodistribution data for tumour uptake of radiolabelled MoAbs. The effect of tumour volume, antibody uptake, collimation and counting conditions was examined. Results indicate that tumours of 10 ml volume may be detected with the NaI(TI) probe at the lowest levels of radiolabelled antibody uptake currently reported in the literature but that at higher published levels, lesions as small as 1 ml may be identified with both detector systems. Detector sensitivity and limited antibody specificity restrict the usefulness of the technique, although moderate improvements in tumour uptake may allow the detection of tumour deposits not clinically apparent. The statistical significance criterion used for this study could be an accurate and reliable indicator for tumour detection in vivo.

Introduction

Radiolabelled monoclonal antibodies (MoAbs) have been used to image both primary and recurrent colorectal cancers, but the detection of small and deeply sited tumour deposits has been limited by both physical factors and the relatively low specificity of current antibodies. Deposits less than 2 cm in diameter are rarely detectable (Colcher et al. 1987), representing a significant limitation, since the early and accurate detection of metastatic or recurrent growths is critical to prognosis. Smaller tumours may be localised if the scatter and attenuation due to overlying tissue can be minimised by reducing source to detector distance, and this may be achieved by detecting regions of increased antibody uptake directly at the time of operation, using a suitable handheld sterile radiation probe (Fig. 1). The technique has the potential to identify smaller deposits not normally palpable at operation, and during resection, repeated measurements may be made to ensure complete removal of tumours. Intraoperative detection of radiolabelled monoclonal antibodies was first reported by Aitken et al.

Key words: Radioimmunolocalisation - Monoclonal antibodies - Colorectal cancer - Intraoperative detection - Semiconductor detectors

Eur J Nucl Med (1991) 18:964-972

Offprint requests to: W. Waddington

* This work was presented at the 2nd Annual Meeting of the European Association for Nuclear Medicine, Strasbourg, France, August 28th-September 2nd 1989

Fig. 1. Clinical use of a radiation-detecting probe during the resection of a primary colorectal cancer

© Springer-Verlag 1991

965 in 1984, a n d s u b s e q u e n t studies r e p o r t the l o c a l i s a t i o n o f d e p o s i t s n o t d e t e c t a b l e b y b a r i u m studies, c o m p u t e d t o m o g r a p h y (CT) o r l a p a r o t o m y ( S i c k l e - S a n t a n e l l o et al. 1987; M a r t i n et al. 1988; N i e r o d a et al. 1989; S a r d i et al. 1989) a n d the c o n s e q u e n t m o d i f i c a t i o n o f p l a n n e d surgical p r o c e d u r e s ( N i e r o d a et al. 1989; C o h e n et al. 1991). A clinical s t u d y ( D a v i d s o n et al. 1991) e v a l u a t e d the usefulness o f this t e c h n i q u e w i t h a n indium-1 l l - l a b e l l e d MoAb ICR2, raised against epithelial membrane antigen ( E M A ) , in p a t i e n t s w i t h p r i m a r y c o l o r e c t a l cancer. A t u m o u r to n o r m a l c o l o n c o u n t r a t i o o f g r e a t e r t h a n 1.5:1 was o b t a i n e d in 8/14 p a t i e n t s w i t h cancer, w i t h u p t a k e r a t i o s o f 0.96:1 a n d 1.06:1 r e c o r d e d for two p a t i e n t s w i t h b e n i g n t u m o u r s . L y m p h n o d e to n o r m a l c o l o n r a t i o s were h i g h e r for all n o d e s f o u n d m e t a s t a t i c relative to t h o s e n o t c o n t a i n i n g t u m o u r . This clinical s t u d y suggests t h a t the i n t r a o p e r a t i v e d e t e c t i o n o f r a d i o labelled M o A b s m a y a l l o w o c c u l t t u m o u r d e p o s i t s to be localised, b u t a m o r e d e t a i l e d e v a l u a t i o n o f the technique is n o t p o s s i b l e f r o m these d a t a alone. A s t u d y was t h e r e f o r e c a r r i e d o u t to p r o v i d e a full q u a n t i t a t i v e a s s e s s m e n t o f the sensitivity o f this technique. T h e p h y s i c a l c h a r a c t e r i s t i c s o f the two types o f d e t e c t o r s y s t e m u s e d clinically for the d e t e c t i o n o f a n l l l I n - l a b e l l e d M o A b , n a m e l y a s o d i u m i o d i d e [NaI(T1)] scintillation d e t e c t o r a n d a c a d m i u m telluride (CdTe) s e m i c o n d u c t o r p r o b e , were e x a m i n e d a n d c o m p a r e d . To investigate the f a c t o r s affecting the p e r f o r m a n c e o f each p r o b e system in clinical use, a r e p r e s e n t a t i v e p h a n t o m m o d e l was d e v i s e d f r o m h u m a n b i o d i s t r i b u t i o n d a t a for l l a i n - l a b e l l e d a n t i b o d i e s to d e t e r m i n e the limits o f tum o u r d e t e c t i o n u n d e r a r a n g e o f p h y s i c a l a n d clinical conditions.

Materials and m e t h o d s

Fig. 2. Sodium iodide [NaI(T1)] detector system, comprising probe with pinhole collimator, NIM EHT and amplifier and a multichannel analyser (Tracor 7200). The probe incorporates extra lead side shielding to reduce penetration of radiation in clinical use

Fig. 3. Cadmium telluride (CdTe) detector system, comprising CdTe probe with two detachable collimators, battery powered counter-timer and visual ratemeter display (RMD Inc.)

Physical characteristics o f detectors NaI(Tl) scintillation detector. The NaI(T1) detector system (Fig. 2) incorporates a NaI(T1) probe with a 26 mm diameter and 20 mm thickness crystal fitted with a conical-bore pinhole collimator, 20 mm depth and 3 mm exit aperture to give good spatial resolution and sensitivity. Energy resolution of the system was examined using a point source of 111in in air, held 10 mm from the detector in a scatter-free geometry. A multi-channel analyser recorded the full energy spectrum, with the energy resolution calculated as the full width halfmaximum (FWHM) of the lower, 17] keV photopeak. All further measurements were recorded using an energy window of 120300 keV to include the two gamma photopeaks (]71 and 247 keV) of 111in whilst excluding scattered radiation. Spatial resolution, calculated as the FWHM of the line spread function, was investigated at depth in tissue-equivalent scattering medium using a line source of 1~*In. Perspex was used, as its linear attenuation coefficient is very similar to that of tissue at ~ I n gamma energies. The probe was held in contact with, and perpendicular to, a block of scattering plates of 100 mm depth, with a

1 mm bore capillary tube line source placed between two plates at the depth studied. Detector response was recorded for a number of points as the plates were displaced laterally, interrogating the line source across its cross-section and determining the FWHM resolution from the line spread function plotted for each depth. A full isoresponse characteristic for 111In in tissue-equivalent medium was also determined by normalising the cross-sectional response at each depth with respect to the central peak response.

CdTe semiconductor detector. The cadmium telluride detector (Fig. 3) is designed to be used intraoperatively, allowing sterilisation of the probe by a low temperature ethylene oxide process. The detector comprises three 2 mm thick CdTe wafers, each 6 mm in diameter, within a tungsten alloy head, the design being a modification of the probe developed for the intraoperative detection of osteoid osteoma (Szypryt et al. 1986). The probe is supplied with a tungsten alloy, parallel-hole collimator of 15 mm length and 5 mm bore and a 1 mm stainless steel cap with an 11 mm diameter epoxy window, effectively acting only as a protective cover and

966 allowing a wide angle of response (wide-angle collimator). The detector is connected by an angled tube to the handle, containing the preamplifier. The counter-timer has variable upper and lower level discriminators and allows timed and manual counting, displayed by an LCD panel, ratemeter and an audible tone. A multichannel analyser may be added for spectral analysis. Energy resolution was determined from a full energy spectrum for ~X~In in air, found as for the NaI(T1) detector. An energy window of 120-300 keV was also selected for this detector from consideration of the spectrum obtained. Spatial resolution was investigated as for the NaI(T1) detector. Detailed isoresponse characteristics were obtained for both parallel-hole and wide-angle collimators with a 120-300 keV energy window and for the parallel-hole collimator with a 220-300 keV window to study the response when input is limited to primary radiation from the 247 keV emission alone. Sensitivity was recorded for both NaI(T1) and CdTe detectors for a 5 ml ~11In source, positioned at 20 mm depth in a block of tissue-equivalent medium. Comparative sensitivity for each detector/collimator/energy window combination was similarly determined in both air and at 10 mm depth in scattering medium. Count rate capability was determined for both detectors to identify dead time losses at activities found in clinical use. Counts were recorded for ~HIn sources of 500 kBq and 70 MBq and any count loss for the higher activity source observed. Early CdTe detectors suffered from polarisation; the progressive loss of sensitivity due to a time-dependent reduction in charge collection or depletion thickness (Bell et al. 1974; Malm and Martini 1974; Siffert et al. 1976), although more recent detectors have largely overcome this (Bojsen 1984; Frederick et al. 1987). To identify any polarisation with the CdTe probe, counts were recorded using a SVCo source in a fixed geometry with a multi-channel analyser set to multiscaler mode. A series of 100 sequential 1 s acquisitions was recorded immediately after switching on the unit, with a further study of 1 rain acquisitions made for the 1st h and for the next 13 h. The NaI(T1) detector was similarly investigated.

P h a n t o m model For a more direct study of detector performance, an abdominal phantom was developed to simulate the radiolabelled antibody uptake found in both tumour and normal body tissue. This model allowed a quantitative assessment of the performance of both detectors, investigating the effect on tumour detection of a number of factors: tumour volume, tumour to normal tissue (background) uptake ratio, counting time, detector energy window and probe collimation. Tumour detection was determined using a significance criterion derived from Poisson statistics, where the threshold of detection is based on the number of standard deviations of the difference between tumour and background mean counts. Six turnout models were constructed, covering a range of volumes for small nodal tumour deposits, and four tumour to background ratios studied, consistent with current human biodistribution data for 111In-labelled MoAbs. The ~lln-filled models were suspended in a tank containing a background l I l I n solution and counts taken both directly over the tumour and with the detector distant from the model.

Phantom design. Few published radioimmunoscintigraphy studies using 1*1In-labelled MoAbs give detailed human biodistribution data. Mean tumour to normal tissue uptake ratios reported for gastrointestinal cancers obtained from the direct assay of biopsy samples range from 2:1 (Mach et al. 1980; Armitage et al. 1984;

Davidson et al. 1989) to 8:1 (Armitage et al. 1986). Tumour to background ratios of 2 : 1, 4:1, 6 : 1 and 8 : 1 were therefore simulated. Absolute tumour antibody uptake has been determined with a range of values reported, from 0.001% injected dose (ID)/gram tissue (Armitage et al. 1984; Halpern et al. 1985; Halpern and Dillman 1987; Carrasquillo et al. 1988) to 0.007%ID/g (Armitage et al. 1985; Davidson et al. 1989). The lowest value, 0.001%ID/g, was adopted as a worst-case study, with normal tissue activity concentration set at a level of 0.0005%ID/g to be consistent with this at the lowest turnout to normal tissue ratio of 2:1. As absolute tumour uptake rises, so should the tumour to background ratio rise, with an essentially constant level of absolute uptake in normal tissue. This yields absolute tumour and normal tissue background antibody uptakes of 1~4 kBq/g and 0.5 kBq/g, with respect to an activity of 100 MBq 11*In-MoAb, the maximum permitted for human administration (Administration of Radioactive Substances Advisory Committee). A 15 1 volume background solution was used to represent the adult abdominal cavity, comprising 0.5 kBq/ml **lInC1 in 0.04 M HC1 to prevent aggregation. Background and tumour solutions were prepared from a solution directly assayed to have a known activity concentration at a noted reference time. Aliquots from the background and tumour solutions were counted for each study to ensure a concentration ratio within + / - 5 % of that specified. Turnout models were constructed for 0.5, 1.0, 1.5, 2.0, 5.0 and 10.0 ml volumes (10 26 mm diameter), filled with active solution to produce the turnout to normal tissue ratios required. The 5 and 10 ml models comprised fluid-filled latex rubber spheres and the smaller models, thin-walled test tubes sealed by a thin layer of wax to give an approximately spherical volume of liquid. Each detector was held perpendicular to and just below the surface of the solution, centred above the model, with the top of each model held 10 mm beneath the probe to simulate the maximum likely thickness of overlying normal tissue. Tumour counts were recorded in this geometry and background counts with the detector moved 10 mm laterally to simulate an irremovable tumour. An energy window of 120-300 keV was used, with the time at each count noted to permit a correction for decay. Counts were recorded for 20 s, as used in clinical studies, and for 100 s, to simulate a significant increase in detector sensitivity with a consequent reduction in the random error associated with the counts obtained. Three 20 and 100 s counts were recorded for tumour and background, for each tumour volume, probe/collimator combination and turnout to background ratio. Mean and standard deviation were calculated for each set of counts and the mean corrected for background radiation and decay corrected to the reference time at assay.

Optimum energy window. The 120-300 keV window selected by considering the energy spectra obtained for the two detectors may not be the optimum choice for the application. A range of windows for each detector and collimator were therefore studied at a turnout to background ratio of 4:1. Counts were recorded simultaneously for a number of windows using a multi-channel analyser with this facility (Tracor Northern 1710). Energy windows of 55-300, 120300, 210-300 and 225-270 keV were tested for the NaI(T1) probe, windows of 25-300, 90-300, 120-300, 160-300, 240-300 and 240260 keV examined for the CdTe probe with wide-angle collimator and windows of 90-300, 120-300, 160-300, 240-300 and 240260 keV tested for the CdTe probe with the parallel-hole collimator.

Statistical analysis. The stochastic (random) error in the data obtained is characterised by the standard deviation, or, of the differ-

967 NaI(TI) ,7-0c m

-- ~ - ~ ~ ~ -

\

1do L ~---O c m p-

~-2

,"--i

p8

k if-2

i..--'3

k

--4

76

::-5

--5 .L

5%

'--6

5%

_7 1%

--8

4 Fig. 4. Isoresponse characteristic for the NaI(T1) detector with pinhole collimator

C/ 5%

/ 5a

b

c

collimator and 120-300 keV window (b) and paralM-hole collimator and 220-300 keV window (c)

Fig. 5a-c. Isoresponse characteristics for the CdTe probe with: wide-angle collimator and 120-300 keV window (a), parallel-hole

ence between the mean tumour and background counts:

~(r-e)=l/r/t+~T/ where T is the mean tumour count, B the mean background count, t the n u m b e r of tumour counts and b the n u m b e r of background counts contributing to the mean tumour and background count, respectively. If the difference between mean tumour and background counts exceeds 3 standard deviations of the difference (the 3 standard deviation confidence level), then there is greater than a 99.7% chance that this is not due to r a n d o m error alone. This is considered to be a statistically significant criterion for the phantom study described, indicating less than a 1% chance that a greater difference is purely random, and was chosen as the threshold for detection of the tumour model.

Results

Physical characteristics of detectors The FWHM spatial resolution found for both detectors is shown in Table 1, indicating that the NaI(T1) detector has a spatial resolution in scattering medium between that of the CdTe probe with wide-angle and parallel-hole collimators. The isoresponse characteristics obtained for the NaI(T1) detector (Fig. 4) and for the CdTe probe (Fig. 5) illustrate their responses in more detail. The NaI(T1) detector has a pear-shaped response (Fig. 4) with the resolution at shallow depth falling off gradually as depth increases. The CdTe detector with the wideangle collimator has a response (Fig. 5a) that is very broad and shallow, while with the parallel-hole collimator it has an elongated response (Fig. 5b) of limited width. The parallel-hole collimator has a noticeably broader and flatter response for an energy window of

Table l. Full width half maximum ( F W H M ) spatial resolution for NaI(T1) and CdTe detectors with depth in tissue-equivalent scattering medium Detector system

NaI(T1) probe: conical-bore collimator CdTe probe: wide-angle collimator CdTe probe : parallel-hole collimator

20 m m depth

50 m m depth

16 mm

38 mm

43 mm

82 mm

12 mm

23 mm

220-300 keV (Fig. 5c) due to the great reduction in scattered radiation detected under these conditions. Energy resolution for the NaI(T1) detector was calculated from the energy spectrum obtained for this detector (Fig. 6a) to be 27.5% for the lower photopeak; this is worse than the FWHM resolution of 16% estimated by extrapolating a curve fitted to the 171 keV peak obtained for the CdTe probe (Fig. 6b). However, both peaks in this spectrum are markedly non-Gaussian in shape, with the 171 keV emission only partially evident. The form of the spectrum is quite different, incorporating a much higher proportion of lower energy radiation. This is consistent with 'hole-tailing', due to incomplete charge collection, and is a feature of CdTe detectors (Frederick et al. 1987). Detector sensitivity, investigated for an 1t tin source at 20 mm depth in tissue-equivalent medium, was 317 cps/MBq for the CdTe probe with wide-angle collimator and 70 cps/MBq with the parallel-hole collimator and 390 cps/MBq for the NaI(T1) detector. A greater

968 Table 2. Comparative sensitivities of the detector systems 03

Energy window

Counts/ second

g

Relative sensitivity

o

l l l-In source in air

CdTe probe parallel-hole collimator 120-300 keV (spectrum) 220-300 keV (photopeak) i

I

I

I

120

171

247

300

keY

c

g

CdTe probe wide-angle collimator 120-300 keV (spectrum) 220-300 keV (photopeak) NaI probe conical-bore collimator 120-300 keV (spectrum) 220-300 keV (photopeak)

4010 741

26% 5%

12900 2060

84% 13%

15300 6 690

100% 44%

l l l-In source in tissue-equivalent scattering medium (10 mm depth)

o

I

120

171

247 keV

Fig. 6a, b. Full energy spectrum for indium-ill in air. a) NaI(T1) detector and b) CdTe probe with parallel-hole collimator

relative sensitivity is seen for the NaI(T1) detector than for the CdTe probe (Table 2), both in air and in tissueequivalent medium, under all conditions except that for the CdTe probe with a wide-angle collimator at 120300 keV with the source in scattering medium. A greater reduction in sensitivity is also found for the CdTe probe with both collimators when the higher p h o t o p e a k only is detected (220-300 keV), as c o m p a r e d with the NaI(T1) detector. Both systems have good count-rate performance for the activities tested, with no count loss observed, excluding the possibility of dead-time losses for either system in clinical use. Polarisation of the CdTe semiconductor detector was investigated by recording the counts obtained for a range of times following switching on of the CdTe probe. A slight increase in sensitivity over the first 10 s was noted (probably associated with an initial ' w a r m - u p ' period), but a second study of I min acquisitions over the 1st and subsequent 13 h showed no progressive loss of sensitivity with time, with all points lying within + / - 1 SD of the mean count, indicating a drift of less than + / - 0 . 4 % . It is concluded that prolonged application of a bias voltage to the CdTe crystal does not result in progressive reduction in sensitivity due to polarisation. No drift in sensitivity was noted for the NaI(T1) detector when similarly investigated. Penetration through the detector shielding was tested with a collimated l t l l n source and indicated leakage less than 5% of the peak on-axis response at all points for the NaI(T1) detector and a m a x i m u m penetration of only 0.3% for the CdTe probe.

CdTe probe parallel-hole collimator 120-300 keV (spectrum) 220-300 keV (photopeak) CdTe probe wide-angle collimator 120-300 keV (spectrum) 220-300 keV (photopeak) NaI probe conical-bore collimator 120-300 keV (spectrum) 220-300 keV (photopeak)

788 164

19% 4%

5 560 653

133% 16%

4180 1750

100% 42%

Table 3. Minimum detectable tumour sizes Probe:

NaI(T1)

Counting time (s) : Ratio of radiolabelled MoAb uptake

20

2:1 4:1 6:1 8 :1

10,0 1,5 1,5 1.0

CdTe (w.a.)

CdTe (p.h.)

100

20

20

100

5.0 0.5 0.5 0.5

none 10.0 5.0 5.0 1.0 1.0 0.5

none 10.0 5.0 1.0

10.0 1.0 1.0 0.5

100

The minimum tumour sizes (expressed in ml) are based on a threshold of detectability of 3 SD. w.a., wide-angle collimator; p.h., parallel-hole collimator; MoAb, monoclonal antibody

Phantom model

Results obtained from the p h a n t o m study are summarised in Table 3. Applying the threshold for statistically significant tumour detection to the data indicates the m i n i m u m detectable t u m o u r volume for each combination of probe and collimator, tumour to background ratio and counting time. The improved tumour detection with a longer counting time can clearly be observed. This five-fold increase in time simulates an equal increase in detector sensitivity and reduces the associated r a n d o m error by a factor of 51/2, with a consequent rise in the statistical significance of the result obtained.

969 Table 4. Comparing tumour to background ratio and statistical

significance of data Ratio

1.8

f

Turnout size (ml)

CdTe (p.h.)

CdTe (w.a.)

NaI(T1)

T/B

cr

T/B

T/B

20 s counting time 2:1 0.5 1.0 1.5 2.0 5.0 10.0

1.05 1.18 1.21 1.11 1.25 1.13

0.51 1.91 2.03 1.23 2.50 1.42

1.00 0.17 0.99 -0.80 0.96 -2.33 0.99 -0.89 1.02 1.37 1.03 2.13

0.98 -0.72 0.97 -0.94 1.01 0.16 1.01 0.37 1.07 2.37 1.12 3.81

1.01 1.02 0.97 1.00 1.04 1.10 0.99 0.98 1.03 1.03 1.10 1.15 1.04 1.05 1.06 1.09 1.10 1.25

0.39 1.13 - 1.98 0.15 2.82 6.19 -0.31 - 1.62 1.82 1.72 6.49 9.56 2.86 3.00 4.09 5.70 6.20 15.55

0.99 1.09 1.10 1.01 1.24 1.28 1.07 1.07 1.11 1.15 1.39 1.56 1.03 1.16 1.24 1.34 1.47 1.59

-0.17 2.88 3.27 0.38 8.00 9.09 2.42 2.29 3.68 5.15 12.19 16.78 1.38 5.42 7.92 10.82 14.78 18.71

0.99 0.99 0.98 1.00 1.02 1,02 0.~'~ I t)l 1111 I Ol l(~(~ 1.(17 0.99 1.02 1.03 1.02 1.09 1.15 1.04 1.03 1.05 1.07 1.12 1.24

--1.11 -- 1.66 --2.84 --0.04 2.16 2.50 - 0.93 1.82 1.15 1.51 7.86 10.54 -2.15 3.24 4.33 3.19

0.99 1.04 1.02 1.04 1.04 1.12 1.07 1.58 1.09 1.06 1.21 1.29 1.06 1.07 1.15 1.18 1.34 1.52 1.08 1.15 1.20 1.32 1.43 1.70

--1.12 2.63 1.30 --0.45 3.32 8.53 5.44 4.39

4:1

6:1

8:1

0.5 1.0 1.5 2.0 5.0 10.0 0.5 1.0 1.5 2.0 5.0 10.0 0.5 1.0 1.5 2.0 5.0 10.0

0.87 -1.57 1.15 1.67 1.21 2.24 1.16 1.81 1.25 2.67 1.41 4,29 1.10 1.15 1.05 0.58 1.05 0.57 1.23 2.58 1.43 4.72 1,50 5.42 1.10 1.19 1.37 3.95 1.16 1.94 1.45 4.80 1.39 4.41 1.86 8.57

100 s counting time 2:1 0.5 1.00 --0.10 1.0 1.06 1.52 1.5 1.06 1.36 2.0 0.98 --0.55 5.0 1.07 1.83 10.0 1.13 3.17 4:1 0.5 1.05 1.29 1.0 1.16 4.01 1.5 1.16 4.04 2.0 1.17 4.27 5.0 1.25 6.28 10.0 1.31 7.64 6:1 0.5 1.09 2.31 1.0 1.18 4.52 1.5 1.07 1.86 2.0 1.24 6.18 5.0 1.49 11.53 i0.0 1.56 13.42 8:1 0.5 1.15 4.04 1,0 1.12 3.28 1,5 1.04 6.29 2.0 1.36 8.74 5.0 1.46 11.35 10.0 1.84 19.08

•

13.10

21.59 5.24 4.35 7.25 9.71 17.77 33.13

a

6.80

4.37 15.44 20.79 4.47 5.75 11.53 13.16 24.66 35.76 6.26 11.16

15.24 23.49 31.28 47.14

T/B is the ratio of tumour to background counts and a, the number of SD separating the tumour and background counts. This is printed in bold type if it exceeds the threshold of 3 SD defined as the lower limit of tumour detectability

8-14 51.2 I

1010 20

I

I

50 100

I

I

I

1000

I~"""-'~'-~1

{

10000

Background count Fig. 7. Relationship between tumour to background ratio and background count at the statistical significance threshold of 3 SD

T h e NaI(T1) p r o b e p e r f o r m s b e t t e r t h a n b o t h C d T e / c o l l i m a t o r c o m b i n a t i o n s at all t u m o u r to b a c k g r o u n d ratios, p a r t i c u l a r l y 2: 1, 4 : 1 a n d 6:1, d e t e c t i n g the 10 ml t u m o u r m o d e l at a r a t i o o f 2:1 a n d the 1 ml m o d e l at 8:1. T h e p e r f o r m a n c e o f the two C d T e c o l l i m a t o r s is v e r y similar, a l t h o u g h the p a r a l l e l - h o l e c o l l i m a t o r d e m o n s t r a t e s m a r g i n a l l y b e t t e r t u m o u r d e t e c t a b i l i t y at r a t i o s o f 2:1 a n d 4: 1, w h e n c o u n t i n g for the e x t e n d e d 100 s c o u n t i n g time. T h e m u l t i p l e e n e r g y w i n d o w s t u d y i n d i c a t e d a wind o w in the s p e c t r a l r e g i o n o f 120-300 k e V to be o p t i m a l for the NaI(T1) d e t e c t o r , w i t h 2 5 - 3 0 0 k e V a n d 9 0 300 k e V w i n d o w s f o u n d m o s t f a v o u r a b l e for the C d T e p r o b e w i t h w i d e - a n g l e a n d p a r a l l e l - h o l e c o l l i m a t o r s , respectively. This signifies t h a t a w i n d o w c o v e r i n g b o t h p h o t o p e a k s o n l y is i n d e e d o p t i m a l for the NaI(T1) detector, b u t t h a t o n e i n c l u d i n g a higher p r o p o r t i o n o f scattered r a d i a t i o n w o u l d h a v e b e e n m o r e a p p r o p r i a t e for the less sensitive C d T e p r o b e . T h e significance v a l u e a n d t u m o u r to b a c k g r o u n d r a t i o o f c o u n t s c a l c u l a t e d for each d a t a p o i n t in the p h a n t o m s t u d y a r e c o m p a r e d in Table 4, w h e r e the r a t i o is as u s e d for the analysis o f p a t i e n t d a t a b y o t h e r w o r k e r s in p r e v i o u s clinical studies. F i g u r e 7 illustrates the t u m o u r to b a c k g r o u n d u p t a k e r a t i o c o n s i s t e n t w i t h the significance t h r e s h o l d 3 S D for a r a n g e o f a b s o l u t e b a c k g r o u n d counts.

Discussion T h e successful d e t e c t i o n o f t u m o u r s in this clinical a p p l i c a t i o n requires t h a t the d e t e c t e d r a d i a t i o n o r i g i n a t e s f r o m the r e g i o n o f clinical i n t e r e s t only, w i t h o u t the introduction of radiation scattered from more distant sources o f u p t a k e . This f a c t o r is d e p e n d e n t b o t h o n the b i o d i s t r i b u t i o n o f the M o A b a n d the p h y s i c a l p r o p e r t i e s o f the labelling r a d i o n u c l i d e , in this case 11~In, a n d o n the design c h a r a c t e r i s t i c s a n d c o l l i m a t i o n o f the r a d i a tion d e t e c t o r used. To d e t e c t small t u m o u r d e p o s i t s lying b e n e a t h a l a y e r o f n o r m a l tissue, the d e t e c t o r s h o u l d

970 have good lateral discrimination at superficial depth, with an appropriately rapid decrease in response with depth to limit detection of more deeply lying sources, where a greater probability exists that uptake may be due to normal structures such as bone marrow or major blood vessels. The results demonstrate that the NaI(T1) detector has a greater sensitivity for 111In than the CdTe probe when collimated, with superior spatial and spectral response characteristics. The phantom study shows the NaI(T1) detector to have a greater tumour detectability and, accordingly, to be a more suitable detector for this clinical application than the CdTe probe. Narrowing the energy window to 220-300 keV for the CdTe parallel-hole collimator improved the pattern of response but with a significant reduction in sensitivity. Results indicate that a wider energy window would improve detectability but also suggest that the performance of the CdTe detector is degraded by the collimators supplied. The two CdTe collimators have designs similarly but oppositely removed from the optimum: the parallelhole collimator has good lateral spatial resolution but a slow drop-off in response with depth and very low sensitivity, and the wide-angle collimator has good sensitivity but negligible directional response. Detectability should improve if a collimator with increased sensitivity and adequate spatial resolution could be designed. Since the small size of the CdTe crystal precluded use of a pinhole collimator, a lead prototype was constructed of 20 mm length, 5 mm exit aperture (to expose fully the CdTe crystal) and 11 mm entrance aperture, to limit the field of view to the maximum solid angle required to encompass the largest tumour volume studied. However, when tested with the phantom model, only a negligible improvement in sensitivity and overall performance over the original collimators was found. The CdTe probe is well designed for surgical use and has good shielding against penetration of radiation from surrounding organs. The probe itself may be sterilised, and electrical safety hazards associated with mains-operated equipment are avoided. The NaI(T1) detector is cumbersome and has a more time-consuming setting-up procedure. Little reduction in the overall size and weight of the detector is possible, and a high-voltage supply is required. NaI(T1) probes using a fibre-optic cable to couple the NaI(T1) crystal to the photomultiplier tube have been constructed to reduce probe size but have resulted in very poor energy resolution and a high failure rate in use (Harvey and Lancaster 1981; Colton and Hardy 1983). Compact detectors using a CsI(T1)-photodiode technology have been used clinically to monitor physiological function (Hunter et al. 1990) but, to date, with poorer energy resolution and lower sensitivity. Consequently, there is still an advantage in using the CdTe probe for clinical studies, but better performance is required to extend the usefulness of the technique. Improved energy resolution or spatial response is difficult to achieve, but detector sensitivity may be increased by enlarging the effective diameter of the CdTe crystal

to the maximum currently available diameter of 12 mm, adding a fourth CdTe wafer or increasing the bias voltage to the crystal to 300-400 V from the 60 V used at present. The phantom study suggests that at a ratio of 2:1, as typically found in recent clinical studies, tumours of 10ml and above may be reliably detected with the NaI(T1) probe at 20 s counting time, but with the CdTe probe only when counting for 100 s. However, a tumour of 1.5 ml volume is detectable at 20 s counting time if the NaI(T1) detector is used with an uptake ratio of 4:1, and a tumour deposit of 1 ml or less may be detected at operation with both detectors if a tumour to background ratio of 8 : 1 can be achieved. These results rely on the biodistribution data used as a basis for the phantom model and are consequently specific to 111In as the labelling radionuclide and dependent upon the MoAb used. They indicate that the technique has the potential to be clinically useful if the specificity of the antibody can be improved sufficiently to obtain the higher uptake ratios discussed above. Deposits small enough to be of normal appearance at operation should then be detectable, and there may be a further use in deciding the margins for resection of recurrent growths. The phantom model assumes that there is a homogeneous distribution of background activity, but imaging studies show the presence of adjacent normal structures with increased uptake which may have a significant effect on tumour detectability. A mean tumour to circulating blood ratio of 2.05:1 and a mean tumour to normal tissue ratio of 2.1:1 were found from our clinical study (Davidson et al. 1989), showing a similar uptake in blood and normal tissue, but region of interest analysis of clinical images obtained in a concurrent study using an 111In-MoAb suggests that uptake in the liver accounts for 20%-30% of the injected dose of radioactivity at 48 h (Davidson 1989). Additionally, imaging studies demonstrate an early and significant uptake of activity in bone marrow which is considerably greater than that in normal tissues or the circulating blood pool. Uptake in the bone marrow of the lumbar spine and pelvic girdle is an important factor when probing the abdominal and pelvic cavities, and radiolabelled antibody in circulating blood soon after administration renders the major abdominal vessels significant sources of radioactivity if early probing is carried out. In this sense, the model provides only an upper bound to detectability, and a further study incorporating anthropomorphic models of the liver and pelvic girdle into the phantom as sources of increased background activity would be required to give quantitative information on the extent to which these factors reduce tumour detection. The comparison of statistical significance value and tumour to background count ratio illustrated in Table 4 highlights the discordance between a simple ratio and the significance criterion, where the latter depends both on the tumour count relative to the background and on the absolute count obtained and is consequently a

971 m o r e sensitive a n d precise measure o f t u m o u r detectability. A t the low c o u n t levels encountered clinically, when the data o b t a i n e d are o f p o o r statistical quality, it is possible to obtain high t u m o u r to b a c k g r o u n d ratios if this f o r m o f analysis is used, giving a potentially false result. Figure 7 indicates the low t u m o u r to b a c k g r o u n d ratios required to p r o d u c e a statistically significant finding at high c o u n t levels. I f a portable c o m p u t e r is interfaced to the intraoperative p r o b e it is possible to evaluate this p a r a m e t e r in real time, allowing rapid feedback o f results at the time o f the investigation. T h o u g h data used for this study are representative o f those currently published, significant further improvements m a y be possible with developments in a n t i b o d y technology. Additionally, use o f iodine-125 as a labelling radionuclide with its longer physical half-life (60 days) allows for a longer interval between administration a n d operation, exploiting any increase in t u m o u r to n o r m a l tissue a n t i b o d y u p t a k e with time (Martin et al. 1988), and its lower g a m m a energy (28 keV) should reduce the c o n t r i b u t i o n o f scattered radiation f r o m s u r r o u n d i n g organs. Technetium-99m is n o w m o r e widely used as a label for r a d i o i m m u n o s c i n t i g r a p h y , following the introduction o f simpler labelling techniques (Schwarz and Steinstrasser 1987), a n d its short, 6 h half-life has allowed the a d m i n i s t r a t i o n o f m u c h higher activities, resulting in significant i m p r o v e m e n t s in image quality and in diagnostic a c c u r a c y ( G r a n o w s k a and Britton 1991). It w o u l d seem reasonable to expect a similar improvem e n t in t u m o u r detectability by the intraoperative detection o f M o A b s labelled with t e c h n e t i u m - 9 9 m w h e n the i m p r o v e d c o u n t rate resulting f r o m the use o f this radionuclide is considered.

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