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Physics intercomparisons for neutron radiation therapy
Inr J. Rodraaon
Oncology
Eld
Phys..
197?. Volume
3. pp. 169-176.
?erpunon
Press.
Prmtcd in the U.S.A.
0 Neutrons-l
PHYSICS INTERCOMPARISONS FOR NEUTRON RADIATION THERAPY i PETER Department
R.
ALMOND,
Ph.D.
of Physics. M. D. Anderson Hospital, Houston. TX 77025, U.S.A.
and J. B. SMATHERS, Bioengineering
Ph.D.
Division, Texas A&M University,
College Station. TX 77840, U.S.A.
For several years, a concerted effort has been going on to ensure that all the groups involved in neutron dosimetry have intercompared their absorbed dose standards. These intercomparisons have relevance to both neutron radiotherapy and neutron radiobiology. An International Neutron Dosimetry Intercomparison (INDI) sponsored by the International Commission on Radiation Units and Measurements, has been completed at the Radiologkal Accelerator Facility at Broolthaven National Laboratory with fourteen groups from six countries, participating. In Europe, the European Neutron Doshnetry Intercomparlson Project (ENDIP) has also been conducting neutron dose intercomparison along sfmflar lines. In the United St&s of America, an informal group of medkal physkkts associated with the ougoing therapy programs have also cart-fed out extensive intercomparison between theii institutions. They have also made measurements at several of the fast neutron therapy projects in Europe as well as partkipating in the INDI and ENDIP projects. Through the U.S.-Japan Co-operation Cancer Research Program intercomparison measurements have been made between the U.S. center and the Japanese centers. The Japanese were also involved in the INDI measurements. It is, therefore, apparent that extensive world-wide intercomparisons have been made and a summary of these measurements will be given. Several conclusions rehttfng to types of dosimeters and precautions that need to be made in neutron do&retry can be drawn from these measurements.
Neutron dosimetry, Neutron Dosiietry
Absorbed dose Intercomparison
standards, International Project, U.S.-Japan
INTRODUCTION At the Particle Radiation Therapy Workshop in Key Biscaye last year, Smith’ reported on various national and international dosimetry intercomparisons. He dealt primarily with the work carried out by the International Neutron Dosimetry Intercomparison (INDI) sponsored by the ICRU,) the Neutron Intercomparisons in the United Kingdom carried out under the auspices of the British Committee on Radiation Units and Measurement (BCRU)5 and the Dosimetry Intercomparisons between Fast Neutron Radiotherapy Facilities in the United States and Great Britain conducted by the Neutron Dosimetry Physics Group (NDPG).6 He also noted that a group under the sponsorship of the European Organization for Research on Treatment of Cancer (EORTC)
Neutron Dosiietry Intercomparison, Cooperative Cancer Research Program.
European
was participating in a Neutron Dosimetry Intercomparisons Project (ENDIP) and that Intercomparisons were also going to be carried out under the U.S.-Japan Cooperative Cancer Research Program (CCRP) sponsored by the Japan Society for the Promotion of Science and the U.S. National Cancer Institute. Since that time, the ENDIP group has made a preliminary report of their work,’ the CCRP Intercomparisons were begun and the NDPG Intercomparisons were extended. Table 1 lists the various institutions participating in these studies. Twenty-eight institutions from nine different countries are involved and because of the various overlaps between the groups, all centers have directly or indirectly intercompared their measurements.
*This work supported by U.S. Public Health Service Research Grant CA 12542 from the National Cancer Institute.
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170
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Table I.
Table 1 (Contd)
INDI
M. D. Anderson Hospital and Texas A & M University (MDAH-TAMU) U.S. Naval Research Laboratory Washington, D.C. (NRL) MRC Cyclotron Unit Hammersmith, London (MRC) Radiobiological Institute of the Organization for Health Research Rijswijk (TNO) Geseltschaft fiir Strahlen-und Unweltforschung Neuherberg (GSFN) Commissariat a I’Energie Atomique Forteray-aux Roses (CEW Oak Ridge National Laboratory Oak Ridge, Tenn. (ORNL) Radiological Research Laboratories Brookhaven National Laboratory
U.S.A.
U.S.A.
U.K.
The Netherlands
West Germany
France
U.S.A.
U.S.A.
(BNL) Christie Hospital & Holt Radium Institute, Manchester
U.K.
(MAN) National Institute of Radiological Science, Chiba (NIRS) Geseltschaft fi.ir Strahlen-und Unweltforshung, Frankfurt/Main (GSFF) Armed Forces Radiobiology Research Institute Bethesda
Japan
West Germany
U.S.A.
( AFRRI)
Erprobungsstelle 53der Buncheswehr 3042 Munster Argonne National Laboratory Argonne
West Germany U.S.A.
Manchester MRC-Hammersmith Hospital London HAM-Radiobiologische Universitats, Klinik, Hamburg GRO-University of Groningen, Groningen LOUV-University of Louvain MDAH-M. D. Anderson Hospital & TAMU-Texas A 8c M University KFZ-Heidelberg BCRU MAN-Manchester GLA-Glasgow MRC-MRC Hammersmith OXFORD-Churchill Hospital Oxford NPDG (MDAH-TAMU)-M. D. Anderson Hospital & Texas A & M University (NRL)-Naval Research Laboratory (U of W&University of Washington, Seattle (MRC)-MRC Hammersmith (MAN)--Manchester (LOUV)-Laboratorum du Cyclotron Louvain-La Neuve CCRP MADH-TAMU NRL u of w NIRS IMS-Institute of Medical Sciences, Univ. of Tokyo, Tokyo
U.K. U.K. Germany The Netherlands Belgium U.S.A. Germany
U.K. U.K. U.K. U.K.
U.S.A. U.S.A. U.S.A. U.K. U.K. Belgium
U.S.A. U.S.A. U.S.A. Japan
Japan
Footnote. This table was made up from information available at the time of writing this paper. Some institutions may have been left off and other institutions may not be correctly identified. If this is the case, the authors apologize. Any errors are unintentional.
ENDIP
GSF-Neuherberg TNO-Rijswijk CNEN-Roma WWD-Munster EUR-Euratom Ispra NRPB-Harwell GSF-Frankfurt CENG-Grenoble AERE-Harwell IAEA-Iaea Vienna CENF-Centre d’Etides Nucleaires Forteray-aux Roses GLA-Belvidere Hospital Glasgow AVL-Antoni Van Leeuwenhoch Hospital, Amsterdam MAN-Christie Hospital & Holt Radium Institute
West Germany The Netherlands Italy West Germany Italy U.K. Germany France U.K. Austria France U.K. The Netherlands
COMPARISON
OF THE VARIOUS
STUDIES
Although all these studies have much in common, there are some basic differences which should be pointed out. INDI had the following stated purpose *‘. . . to compare the results obtained by van’ous individuals andlor groups in performing absolute fast neutron dosimetry using similar or diflerent techniques in situations approximating those generally encountered in radiotherapy and radiobiology. Dosimetry for contaminant y-rays will also be performed. A long range goal will be to identify the most accurate method(s) of performing absolute fast neutron dosimetry . . .“.
The study, therefore, was primarily to compare dosimetry methods and to identify the most accurate
Physics
intercomparisons
for neutron
radiation
therapy
purpose of calibrating the ion chambers. The ENDIP study was very similar to the INDI study. Table 3 lists the neutron sources available and
a 13’Cs y-ray source was also provided. These intercomparisons were carried out at two locationsthe GSF Neuherberg and TN0 Rijswik in 1975. Both studies asked for dose in ICRU muscle6 and measurements were done both in air in depth in water. The importance of these studies cannot be overestimated. Because the measurements were made at specific locations, all participants had to bring their systems to these locations. The intercomparisons are, therefore, a comparison of the instrumentation and methods used. Because of this, significant recommendations could be made concerning the best type of instrumentation and methods to be used in neutron dosimetry. The British study covered only existing British procedures in 1973 in the measurement of neutron absorbed dose. Although, it compared the dosimetry systems in use at the three centers involved, it also radiation
Reaction
Energy spread (2%)
‘H(d, n )‘He ‘H(d, n)3He ‘H(p, n)3He ‘H(p, n)3He fission
15.4 5.5 2.1 0.67 “‘Cf spectrum
4 7 5 20 -
INDI
A variety of dosimeters
was used to determine
and Kerma rates for INDI
Max. total tissue Kerma rate in air 30cm from source (rad h-‘)
Approx. y-ray tissue Kerma rate (% of total)
40 80 20 5 6t
5 5 5 5 40t
tFor a -2 mg *‘*Cf source.
Table 3. Neutron
producing
target reactions
used for the ENDIP Neutron
Part. energy (MeV)
Reaction T(p. n )‘He 7Yp. nj3He T(d, n)‘He D(d, n$He Note:
D =
‘H, T = 3H.
1.50 2.90 0.40 2.30
171
RESULTS Obviously with this number of intercomparisons having been carried out, it is only possibie to give a very brief summary of the various results.
quantities
Neutron energy (MeV)
P. R. ALMOND and J. B. SMATHERS
went further and made measurements of the treatment dose at the two centers who had treated patients with collimated beams. This is an important difference from the INDI and ENDIP studies since it carried the intercomparisons a step further and, therefore, involved additional factors. The NDPG and the CCRP started out specifically to compare the neutron beams actually used for therapy. This has meant that the dosimetry systems used had to be taken to each institution doing neutron therapy. Where possible, reciprocal visits have been made between institutions. Further, the type of information obtained has been extended. Although, Kerma values in air and absorbed doses in phantom have been compared along with the n/r ratio’s, information about the clinical beam has also been obtained such as surface dose, depth of dose maximum, depth doses and transverse doses, etc. Also, because these intercomparisons were directly related to clinical trials, most measurements were made in TE liquid rather than water. The measurements have been made primarily on cyclotron produced neutrons with fairly high dose rates (Table 4).
method. The study was carried out at the Radiological Research Accelerator Facility at Brookhaven National Laboratory in 1973. Table 2 lists the nominal radiation quantities and Kerma rates and it can be seen that, although the aim was to approximate situations encountered in radiotherapy, these conditions were not really met since the majority of neutron radiotherapy is currently done with cyclotron produced neutrons at considerably higher dose rates. In this study, IOCi 13’Cs y-ray sources was used for
Table 2. Nominal
0
intercomparison energy (MeV)
Target
min.
max.
2.0 mg/cm’TiT 2.0 2.0 4.0 mg/cm* TiD
0.46 2.01 14.4 4.95
0.68 2.12 15.7 5.55
Mean or weighted mean 0.57 2.07 15.1
5.25
the
172
Radiation Oncology 0 Biology 0 Physics
Table
MDAH-TAMU
Energy of D’ (MeV) Beryllium target (thickness,
mm)
Mean neutron energy (MeV) Tissue Kerma in air 10x 10 cm field (rad/min) at stated SSD
situations.
There
were
much
larger
NRL
50 thick (8.5) 21
35 thick (4.7) 15
60 at 140 cm
66 at 125 cm
total or neutron Kermas and absorbed doses. These included homogeneous tissue-equivalent (TE) ionization chambers, polyethylene chambers filled with ethylene gas (CZH~), a proportional counter of the same materials, a chamber with styrene-equivalent (CH) walls and filled with acetylene, a precision long counter, and silicon diodes. Twelve of the 14 groups used TE chambers. The major uncertainties in computing the desired Kermas and absorbed doses were: (1) The effective W, the energy required to produce an ion pair in the chamber gas, for neutrons; (2) The basic calibration of the instrument; and (3) The neutron sensitivity of the photon dosimeters. For both Kerma and absorbed dose measurements, the CzH4 chambers gave results consistently lower than those from other dosimeters. The CH chamber gave consistently higher results than did other dosimeters. The value of Kerma and absorbed dose agreed to within 25% for the majority of determinations from TE ionization chambers. Most of the neutron values reported differ from the mean values obtained with ionization chambers by 5% or less. Nevertheless, there were many larger differences, several exceeding 15%. A variety of dosimeters were used to determine the photon Kermas and absorbed doses, including energycompensated Geiger-Muller counters, magnesium and aluminum ionization chambers filled with argon, graphite chambers filled with carbon dioxide, thermoluminescent materials, and photographic film. There did not appear to be an identifiable response pattern for the different types of photon dosimeters in most
4.
differences
in the reported values for photons than for neutrons; about one-third of the values in air differed from the mean by more than 50%. There were also large differences at depth in the phantom. ENDIP The ENDIP results were basically the same. Out of the 17 groups reporting results, 14 used TE chambers,
u of w 21.5 intermediate (1.5) 8
MRC 16 intermediate (0.8) 7.6
34 at 150 cm
43 at 116cm
two used plastic chambers (CH) filled with acetylene and one used a polythene-ethylene chamber gas system but no consistent difference could be found between the different systems. Various systems were also used to determine the photon Kerma including neutron insensitive chambers (C-CO?, Al-Ar, Mg-Ar, etc.), G-M counters, film, TLD. The greatest variations in the results were found for 15.1 MeV neutrons where out of 11 institutions reporting results, 5 had variations from the mean of less than 5%, 3 between 5% and lo%, and 3 greater than 10% with an indication that the variation in the total Kerma were less than those in the neutron Kerma only. In this study, participants quoted systematic uncertainties of 7-S% on the neutron and total Kerma arising from uncertainties in the knowledge of the basic constants, e.g. w, stopping power ratios and Kerma ratios. However, the choice of effective measurement point and the corrections for saturation in the ion chambers may also contribute to the uncertainties. United Kingdom
intercomparisons
Intercomparisons were conducted on the D-T generator at Manchester independently by both Hammersmith and Glasgow teams. Glasgow and Hammersmith dosimetry were intercompared on the Hammersmith cyclotron. At Hammersmith and in Manchester, clinical dosimetry is performed with the CzH4 chamber, and in Glasgow with the CH chamber. Agreement within experimental error (-+3%) was found between all hospitals when the C2Hs chambers were used, although no such agreement existed when the preferred methods of dosimetry of each hospital were intercompared. Good agreement was found between Manchester and Glasgow where TE chambers were used. In Manchester and Glasgow, they observed that the dose measured by the CrH4 system is about 5% lower than that measured by the CH chambers. This was believed to be caused by the layer of graphite on the inside of the CzH4 chamber. Relative to the dose given by the Hammersmith treatment monitor, the Glasgow mean dose was greater by (8 * 6)% with the CH and TE chambers. At
Physics
intercomparisons
for neutron
radiation
therapy
Glasgow, using a secondary method-Kerma conversion of fluence measurements made with calibrated fission chambers-they deduced the dose was
(102 7)% greater than the Hammersmith monitor dose, thus supporting the ionization-chamber measurements. One of the major problems was the estimation of the neutron response of photon dosimeters. A national reference standard for the measurement of neutron absorbed dose was proposed to promote uniformity and continuity in the dose measurements at the various centers. Because of accuracy, reproducibility, and ease of operation, twin cavity chambers were proposed as the most promising dosimetry system. Such a system might be a TE chamber with TE gas in combination with a carbon chamber filled with CO*. Significantly, the Hammersmith rad appeared to be &IO% too low in these intercomparisons. This was later confirmed by Bewley’ from measurements with a TE calorimeter and a correction of 8% was introduced on 1 January 1975. Hammersmith also purchased two TE ionization chambers for neutron dosimetry; measurements with these chambers have confirmed the Glasgow intercomparison and calorimetry results. NDPG
intercomparison
The results of these intercomparisons were as follows: The institutions in the United States agree within 2% in their total dose measurements. In their comparisons of the standard y-ray calibrations of the ionization chambers on 13’Cs and ‘?Zo irradiators, the agreement was within 3%. Relative to the dose given by the Hammersmith treatment monitor, the dose measured by NDPG physicists was greater by 11% when measuring tissue
0
P. R. ALMOND and J. B. SMATHERS
Kerma in air, and greater by 14% when measuring tissue dose in a water phantom. The differences in the above measurements can be accounted for by adding to the Hammersmith doses the 8% dosimetry error in the neutron dose described in the last section, and also by adding the photon dose which Hammersmith excludes (3% in air, 6% at the depth of dose maximum, and 9% at IO-cm depth in phantom). Physicists in the United States and in Great Britain agree within 1% of the standard y-ray calibrations of their ionization chambers. The measurements were done with 8-MeV X-rays. Table 5 gives the latest NDPG results conducted in the Autumn of 1975. The use of different parameters of the Kerma ratio, the stopping power ratio and the Wn/ We ratio accounts for the 4-5% difference with MAN. Therefore. all these tissue Kerma in air measurements and photon calibrations agree to within 1%. The measurements at depth, conducted at Hammersmith, however exhibit greater differences and these are still under study but may be related to uncertainties in back scatter factors used by Hammersmith. CCRP inter-comparisons
Table 6 gives the preliminary results of these measurements made by both the Japanese in the U.S. and vice versa. Again, the agreement is good-but with uncertainties in the measurements made at depth. CONCLUSIONS Some very general conclusions can be stated from all these intercomparisons. Each group has drawn their own conclusions but it is remarkable how similar they are: (1) No gross discrepancies were found (i.e. a factor
Table 5. Intercomparison results: ratios of measurements with respect to MDAH-TAMU measurements during the Autumn of 1975 MAN MDAH-TAMU
MRC MDAH-TAMU
Location Beam
MAN 15MeV d-+T
16MeV d+Be
Tissue Kerma in air Dose at depth
0.95t
Participants
MRC
1.01 0.97 @ 1 cm O.% @ 5 cm 0.97 @ 10 cm
Photon calibration Whoice
NRL MDAH-TAMU NRL 35MeV
d+Be
account
for 4% of the 5% difference
Louv MDAH-TAMU Louv 50MeV
d+Be
1.00 1.018 @ 2cm 1.02 @ 1Ocm 1.00 (‘37cs)
of parameters
173
(see text).
Photon calibration
Dose at depth
0.97 @ 5 cm 0.96 @ IOcm 0.97 @ I5 cm O.% @ 20 cm 0.99 (“Co)
1.01
TAMU 30MeV d+Be
Location Beam
Tissue Kerma in air
NIRS MDAH-TAMU
Participants
t Be
0.996 @ IOcm
I.006
NRL 35 MeV d
TAMU 16MeV d+Be
I.01
NIRS NRL
NIRS MDAH-TAMU
0.974 @ 2cm 0.%3 @ IOcm
0.95 0.94 0.94 0.93
@ 5 cm 0 IOcm @IScm 8 20 cm 0.99
0.97
NIRS 30MeV d+Be
u of w 21.5MeV d+Be
0.975
NIRS MDAH-TAMU
U.S. Japan-measurements
NIRS u of w
Table 6. intercomparison results: of the CCRP
NIRS 30MeV d+Be 0.985
NIRS 15MeV d+Be
0.97 0 5 cm O.% @ IOcm 0.94 el5cm
0.99
NIRS u of w
NIRS MDAH-TAMU
made in 1976
NIRS 15 MeV d+ Be 1.013
NIRS u of w
9 Y 5. ,:
if! E 0 F 0
0
P
2 %
s
Physics
intercomparisons
for neutron
radiation
of 2 from the mean) in the Kerma measurements in air or the absorbed dose in a phantom. (2) Agreement to better than 5% in neutron Kerma and absorbed dose was found in the majority of cases. (3) Fairly large variations were found in the y-ray dose. Since this is a small fraction (approximately 5%) of the neutron dose and the neutrons have a higher RBE, the variations are not serious. The variations are mainly due to the large uncertainties in the
neutron response of the detectors used. (4) The majority of investigators used ion chambers as the basic means of obtaining the neutron doses with the tissue equivalent chambers being the most popular. However, where considerably different detectors have been used, silicon diode, activation foil, proportional counters similar results were obtained. (5) The predominately used TE ion chambers showed variation in the results amongst the various investigators. The variations can generally be traced to differences in the basic parameters used, e.g. W, stopping power ratio and Kerma ratios. (6) Some variations may be due to differences in the composition of the tissue equivalent plastic and in the difference in the tissue equivalent gas. (7) Other factors affecting the neutron calibration are the y-ray calibrations, saturation characteristics, wall thickness, chamber displacement factors, etc.
In
the
DISCUSSION overall scope of neutron
dose
in-
Table 7. Comparison MDAH-TAMU 50
Energy of D’ (MeV) Beryllium
target (thickness,
mm)
Mean neutron energy (MeV) Collimation for field shaping SSD (cm) Field size definition (Isodose-light field; IO x 10 cm field at SSD) Beam flattening Depth of d,., (g/cm*) Deuteron beam current (PA) Tissue Kerma in air (10 x IO cm field. rad/min at SSD) Tissue dose at d,,, (10 x 10 cm field. rad/min at SSD) Gamma component in air (10x 1Ocm field at SSD) Gamma component at d,., (10 x 10 cm field at SSD) Depth of 50% dose (10 x 10 cm fieid. g/cm’) Density of phantom fluid (g/cm’)
Thick (8.5) 21 WEP+B+Fe (70 cm) 140
therapy
0
P. R. AI MOND
175
and J. B. SMATHERS
tercomparisons several points should be kept in mind: (1) A wide variety of neutron beams are being used or proposed as sources for radiotherapy. Mono energetic neutron produced by the D-T reaction and neutron with a wide energy spread produced by deuterons upon Be targets with the deuteron energy varying from 16 to 50 MeV and proposals to use higher energy deuterons on Be and also protons on Be. These facts not only complicate the dosimetry intercomparisons but also the radiobiological and radiotherapeutic intercomparisons. (2) These intercomparisons are important. however. in determining the best type of dosimetry systems to use and for forming the basis for neutron dose
protocols. (3) Kerma and dose comparisons, although useful, are limited in value. The determination of absorbed dose to the tumor is important, as is the beam characteristics, so that the dose to normal tissue can also be compared (i.e. surface and build up dose, depth dose, etc.). Table 7 gives some of this information and Fig. 1 shows a typical comparison of build up data. (4) For radiobiological intercomparison, two types of dosimetry are required. If a standard biological system is being taken from institution to institution, a uniform dosimetry system that measures the dose to the biological system should accompany the radiobiology. Assuming the dosimetry to be constant, variations in the radiobiology results should reveal true radiobiological differences in the beams. If a biological system is used that is insensitive to neutron measurements carried out energy, radiobiological of beam characteristics NRL 35 Thick (4.7) 15 Benelex (65 cm) 125
u of w 21.5 Intermediate (1.5) 8 WEP+B (75 cm) 150
MRC 16 Intermediate (0.8) 7.6 Wood (68 cm) 116
60% Yes 1.07 7
90% Proposed 0.55 10
50% No 0.30 30
50% No 0.23 80
56
60
30
40
60
66
34
43
3%
1%
4%
3%
5%
2%
5%
6%
13.8 1.065
12.8 1.092
10.2 1.10
8.8 1.10
Radiation
176
Oncology
0
.
Biology 0 Physics o
cl---*----._______
~____~_____l.__--------*-----
__--
0.
*.
~~“Mwsurments
at TAMVEC (50 F)
0 Louvaln existtng data (50) . /
, 2
0
1
1 4
1
TAMVEC measurements at Louvain (50) I 6
1
/ 8
/
I IO
/ I2
Depth mm TE p4astlc
Fig. 1. Build up curves measured for neutron produced by 50 MeV deuterons on Be. The dashed line is taken on the MDAH-TAMU beam which is filtered by 3 cm of polyethylene to produce a flat beam. The open circles were measured by University of Louvain and the closed circles were measured by MDAHTAMU on the University of Louvain machine. Although the nominal energies of the beams are the same, it is apparent that the filtered beam has a different build up curve. However, the measurement techniques by the two groups appears to be consistent, but real differences in skin reaction might be expected-even
though the dosimetry comparisons were very close.
with the local dosimetry should reflect any difference in the dosimetry. If, however, a neutron energy sensitive biological system is used and the experiment done with local dosimetry very little can be said about the results. (5) The overall clinical response will be a function
of both the dosimetry and the radiobiological characteristics of the beam. The intercomparison to date should, along with the radiobiological results, provide most of the information needed to compare the radiotherapeutic results.
REFERENCES Bewley, D.K.: Private communication, 1975. Broerse, J.J., Burgess, G., Coppola, M.: Preliminary results of the European Neutron Dosimetry Intercomparisons Project (ENDIP). Private communication, 1976. Caswell, R.S., Goodman, L.J., Colvett, R.D.: International Intercomparison of Neutron Dosimetry. Radiot. Res. 532-546, 1975. International Measurements
Irradiation.
Commission on Radiation Report, lob, 1%4. Physical
National
Bureau of Standards
Units
and
Aspects of (U.S.) Hand-
book 85 4. 5. Martin, ‘J.H., Axton, E.J., Bewley, D.K., Dennis, J.A.. Greene,
D., Halnan,
K.E.,
Lawson,
R.C.:
Dosimetry
practice in neutron radiotherapy centers in Great Britain, London, England, Nat1 Phys. Lab. Rep., PSI, Jan. 1974. 6. Smith, A.R., Almond, P.R., Smathers, J.B., Otte, V.A., Attix, F.H., Theus, R.B., Wooton, P., Bichsel, H., Eenmaa, J., Williams, D., Bewley, D.K., Parnell, C.J.: Dosimetry intercomparisons between fast neutron radiotherapy facilities. Med. Phys. 2: 195-200, 1975. 7. Smith, A.R.: Zntercomparisons of particlegenerators in the United States, The United Kingdom, and International Proceedings of an International Workshop, Particle American College of Radiation Therapy. Philadelphia. Radiology, 1976, pp. 6048.
Oncology
Eld
Phys..
197?. Volume
3. pp. 169-176.
?erpunon
Press.
Prmtcd in the U.S.A.
0 Neutrons-l
PHYSICS INTERCOMPARISONS FOR NEUTRON RADIATION THERAPY i PETER Department
R.
ALMOND,
Ph.D.
of Physics. M. D. Anderson Hospital, Houston. TX 77025, U.S.A.
and J. B. SMATHERS, Bioengineering
Ph.D.
Division, Texas A&M University,
College Station. TX 77840, U.S.A.
For several years, a concerted effort has been going on to ensure that all the groups involved in neutron dosimetry have intercompared their absorbed dose standards. These intercomparisons have relevance to both neutron radiotherapy and neutron radiobiology. An International Neutron Dosimetry Intercomparison (INDI) sponsored by the International Commission on Radiation Units and Measurements, has been completed at the Radiologkal Accelerator Facility at Broolthaven National Laboratory with fourteen groups from six countries, participating. In Europe, the European Neutron Doshnetry Intercomparlson Project (ENDIP) has also been conducting neutron dose intercomparison along sfmflar lines. In the United St&s of America, an informal group of medkal physkkts associated with the ougoing therapy programs have also cart-fed out extensive intercomparison between theii institutions. They have also made measurements at several of the fast neutron therapy projects in Europe as well as partkipating in the INDI and ENDIP projects. Through the U.S.-Japan Co-operation Cancer Research Program intercomparison measurements have been made between the U.S. center and the Japanese centers. The Japanese were also involved in the INDI measurements. It is, therefore, apparent that extensive world-wide intercomparisons have been made and a summary of these measurements will be given. Several conclusions rehttfng to types of dosimeters and precautions that need to be made in neutron do&retry can be drawn from these measurements.
Neutron dosimetry, Neutron Dosiietry
Absorbed dose Intercomparison
standards, International Project, U.S.-Japan
INTRODUCTION At the Particle Radiation Therapy Workshop in Key Biscaye last year, Smith’ reported on various national and international dosimetry intercomparisons. He dealt primarily with the work carried out by the International Neutron Dosimetry Intercomparison (INDI) sponsored by the ICRU,) the Neutron Intercomparisons in the United Kingdom carried out under the auspices of the British Committee on Radiation Units and Measurement (BCRU)5 and the Dosimetry Intercomparisons between Fast Neutron Radiotherapy Facilities in the United States and Great Britain conducted by the Neutron Dosimetry Physics Group (NDPG).6 He also noted that a group under the sponsorship of the European Organization for Research on Treatment of Cancer (EORTC)
Neutron Dosiietry Intercomparison, Cooperative Cancer Research Program.
European
was participating in a Neutron Dosimetry Intercomparisons Project (ENDIP) and that Intercomparisons were also going to be carried out under the U.S.-Japan Cooperative Cancer Research Program (CCRP) sponsored by the Japan Society for the Promotion of Science and the U.S. National Cancer Institute. Since that time, the ENDIP group has made a preliminary report of their work,’ the CCRP Intercomparisons were begun and the NDPG Intercomparisons were extended. Table 1 lists the various institutions participating in these studies. Twenty-eight institutions from nine different countries are involved and because of the various overlaps between the groups, all centers have directly or indirectly intercompared their measurements.
*This work supported by U.S. Public Health Service Research Grant CA 12542 from the National Cancer Institute.
Reprint requests 169
to: P. R. Almond. Ph.D
170
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Oncology
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Physics
Table I.
Table 1 (Contd)
INDI
M. D. Anderson Hospital and Texas A & M University (MDAH-TAMU) U.S. Naval Research Laboratory Washington, D.C. (NRL) MRC Cyclotron Unit Hammersmith, London (MRC) Radiobiological Institute of the Organization for Health Research Rijswijk (TNO) Geseltschaft fiir Strahlen-und Unweltforschung Neuherberg (GSFN) Commissariat a I’Energie Atomique Forteray-aux Roses (CEW Oak Ridge National Laboratory Oak Ridge, Tenn. (ORNL) Radiological Research Laboratories Brookhaven National Laboratory
U.S.A.
U.S.A.
U.K.
The Netherlands
West Germany
France
U.S.A.
U.S.A.
(BNL) Christie Hospital & Holt Radium Institute, Manchester
U.K.
(MAN) National Institute of Radiological Science, Chiba (NIRS) Geseltschaft fi.ir Strahlen-und Unweltforshung, Frankfurt/Main (GSFF) Armed Forces Radiobiology Research Institute Bethesda
Japan
West Germany
U.S.A.
( AFRRI)
Erprobungsstelle 53der Buncheswehr 3042 Munster Argonne National Laboratory Argonne
West Germany U.S.A.
Manchester MRC-Hammersmith Hospital London HAM-Radiobiologische Universitats, Klinik, Hamburg GRO-University of Groningen, Groningen LOUV-University of Louvain MDAH-M. D. Anderson Hospital & TAMU-Texas A 8c M University KFZ-Heidelberg BCRU MAN-Manchester GLA-Glasgow MRC-MRC Hammersmith OXFORD-Churchill Hospital Oxford NPDG (MDAH-TAMU)-M. D. Anderson Hospital & Texas A & M University (NRL)-Naval Research Laboratory (U of W&University of Washington, Seattle (MRC)-MRC Hammersmith (MAN)--Manchester (LOUV)-Laboratorum du Cyclotron Louvain-La Neuve CCRP MADH-TAMU NRL u of w NIRS IMS-Institute of Medical Sciences, Univ. of Tokyo, Tokyo
U.K. U.K. Germany The Netherlands Belgium U.S.A. Germany
U.K. U.K. U.K. U.K.
U.S.A. U.S.A. U.S.A. U.K. U.K. Belgium
U.S.A. U.S.A. U.S.A. Japan
Japan
Footnote. This table was made up from information available at the time of writing this paper. Some institutions may have been left off and other institutions may not be correctly identified. If this is the case, the authors apologize. Any errors are unintentional.
ENDIP
GSF-Neuherberg TNO-Rijswijk CNEN-Roma WWD-Munster EUR-Euratom Ispra NRPB-Harwell GSF-Frankfurt CENG-Grenoble AERE-Harwell IAEA-Iaea Vienna CENF-Centre d’Etides Nucleaires Forteray-aux Roses GLA-Belvidere Hospital Glasgow AVL-Antoni Van Leeuwenhoch Hospital, Amsterdam MAN-Christie Hospital & Holt Radium Institute
West Germany The Netherlands Italy West Germany Italy U.K. Germany France U.K. Austria France U.K. The Netherlands
COMPARISON
OF THE VARIOUS
STUDIES
Although all these studies have much in common, there are some basic differences which should be pointed out. INDI had the following stated purpose *‘. . . to compare the results obtained by van’ous individuals andlor groups in performing absolute fast neutron dosimetry using similar or diflerent techniques in situations approximating those generally encountered in radiotherapy and radiobiology. Dosimetry for contaminant y-rays will also be performed. A long range goal will be to identify the most accurate method(s) of performing absolute fast neutron dosimetry . . .“.
The study, therefore, was primarily to compare dosimetry methods and to identify the most accurate
Physics
intercomparisons
for neutron
radiation
therapy
purpose of calibrating the ion chambers. The ENDIP study was very similar to the INDI study. Table 3 lists the neutron sources available and
a 13’Cs y-ray source was also provided. These intercomparisons were carried out at two locationsthe GSF Neuherberg and TN0 Rijswik in 1975. Both studies asked for dose in ICRU muscle6 and measurements were done both in air in depth in water. The importance of these studies cannot be overestimated. Because the measurements were made at specific locations, all participants had to bring their systems to these locations. The intercomparisons are, therefore, a comparison of the instrumentation and methods used. Because of this, significant recommendations could be made concerning the best type of instrumentation and methods to be used in neutron dosimetry. The British study covered only existing British procedures in 1973 in the measurement of neutron absorbed dose. Although, it compared the dosimetry systems in use at the three centers involved, it also radiation
Reaction
Energy spread (2%)
‘H(d, n )‘He ‘H(d, n)3He ‘H(p, n)3He ‘H(p, n)3He fission
15.4 5.5 2.1 0.67 “‘Cf spectrum
4 7 5 20 -
INDI
A variety of dosimeters
was used to determine
and Kerma rates for INDI
Max. total tissue Kerma rate in air 30cm from source (rad h-‘)
Approx. y-ray tissue Kerma rate (% of total)
40 80 20 5 6t
5 5 5 5 40t
tFor a -2 mg *‘*Cf source.
Table 3. Neutron
producing
target reactions
used for the ENDIP Neutron
Part. energy (MeV)
Reaction T(p. n )‘He 7Yp. nj3He T(d, n)‘He D(d, n$He Note:
D =
‘H, T = 3H.
1.50 2.90 0.40 2.30
171
RESULTS Obviously with this number of intercomparisons having been carried out, it is only possibie to give a very brief summary of the various results.
quantities
Neutron energy (MeV)
P. R. ALMOND and J. B. SMATHERS
went further and made measurements of the treatment dose at the two centers who had treated patients with collimated beams. This is an important difference from the INDI and ENDIP studies since it carried the intercomparisons a step further and, therefore, involved additional factors. The NDPG and the CCRP started out specifically to compare the neutron beams actually used for therapy. This has meant that the dosimetry systems used had to be taken to each institution doing neutron therapy. Where possible, reciprocal visits have been made between institutions. Further, the type of information obtained has been extended. Although, Kerma values in air and absorbed doses in phantom have been compared along with the n/r ratio’s, information about the clinical beam has also been obtained such as surface dose, depth of dose maximum, depth doses and transverse doses, etc. Also, because these intercomparisons were directly related to clinical trials, most measurements were made in TE liquid rather than water. The measurements have been made primarily on cyclotron produced neutrons with fairly high dose rates (Table 4).
method. The study was carried out at the Radiological Research Accelerator Facility at Brookhaven National Laboratory in 1973. Table 2 lists the nominal radiation quantities and Kerma rates and it can be seen that, although the aim was to approximate situations encountered in radiotherapy, these conditions were not really met since the majority of neutron radiotherapy is currently done with cyclotron produced neutrons at considerably higher dose rates. In this study, IOCi 13’Cs y-ray sources was used for
Table 2. Nominal
0
intercomparison energy (MeV)
Target
min.
max.
2.0 mg/cm’TiT 2.0 2.0 4.0 mg/cm* TiD
0.46 2.01 14.4 4.95
0.68 2.12 15.7 5.55
Mean or weighted mean 0.57 2.07 15.1
5.25
the
172
Radiation Oncology 0 Biology 0 Physics
Table
MDAH-TAMU
Energy of D’ (MeV) Beryllium target (thickness,
mm)
Mean neutron energy (MeV) Tissue Kerma in air 10x 10 cm field (rad/min) at stated SSD
situations.
There
were
much
larger
NRL
50 thick (8.5) 21
35 thick (4.7) 15
60 at 140 cm
66 at 125 cm
total or neutron Kermas and absorbed doses. These included homogeneous tissue-equivalent (TE) ionization chambers, polyethylene chambers filled with ethylene gas (CZH~), a proportional counter of the same materials, a chamber with styrene-equivalent (CH) walls and filled with acetylene, a precision long counter, and silicon diodes. Twelve of the 14 groups used TE chambers. The major uncertainties in computing the desired Kermas and absorbed doses were: (1) The effective W, the energy required to produce an ion pair in the chamber gas, for neutrons; (2) The basic calibration of the instrument; and (3) The neutron sensitivity of the photon dosimeters. For both Kerma and absorbed dose measurements, the CzH4 chambers gave results consistently lower than those from other dosimeters. The CH chamber gave consistently higher results than did other dosimeters. The value of Kerma and absorbed dose agreed to within 25% for the majority of determinations from TE ionization chambers. Most of the neutron values reported differ from the mean values obtained with ionization chambers by 5% or less. Nevertheless, there were many larger differences, several exceeding 15%. A variety of dosimeters were used to determine the photon Kermas and absorbed doses, including energycompensated Geiger-Muller counters, magnesium and aluminum ionization chambers filled with argon, graphite chambers filled with carbon dioxide, thermoluminescent materials, and photographic film. There did not appear to be an identifiable response pattern for the different types of photon dosimeters in most
4.
differences
in the reported values for photons than for neutrons; about one-third of the values in air differed from the mean by more than 50%. There were also large differences at depth in the phantom. ENDIP The ENDIP results were basically the same. Out of the 17 groups reporting results, 14 used TE chambers,
u of w 21.5 intermediate (1.5) 8
MRC 16 intermediate (0.8) 7.6
34 at 150 cm
43 at 116cm
two used plastic chambers (CH) filled with acetylene and one used a polythene-ethylene chamber gas system but no consistent difference could be found between the different systems. Various systems were also used to determine the photon Kerma including neutron insensitive chambers (C-CO?, Al-Ar, Mg-Ar, etc.), G-M counters, film, TLD. The greatest variations in the results were found for 15.1 MeV neutrons where out of 11 institutions reporting results, 5 had variations from the mean of less than 5%, 3 between 5% and lo%, and 3 greater than 10% with an indication that the variation in the total Kerma were less than those in the neutron Kerma only. In this study, participants quoted systematic uncertainties of 7-S% on the neutron and total Kerma arising from uncertainties in the knowledge of the basic constants, e.g. w, stopping power ratios and Kerma ratios. However, the choice of effective measurement point and the corrections for saturation in the ion chambers may also contribute to the uncertainties. United Kingdom
intercomparisons
Intercomparisons were conducted on the D-T generator at Manchester independently by both Hammersmith and Glasgow teams. Glasgow and Hammersmith dosimetry were intercompared on the Hammersmith cyclotron. At Hammersmith and in Manchester, clinical dosimetry is performed with the CzH4 chamber, and in Glasgow with the CH chamber. Agreement within experimental error (-+3%) was found between all hospitals when the C2Hs chambers were used, although no such agreement existed when the preferred methods of dosimetry of each hospital were intercompared. Good agreement was found between Manchester and Glasgow where TE chambers were used. In Manchester and Glasgow, they observed that the dose measured by the CrH4 system is about 5% lower than that measured by the CH chambers. This was believed to be caused by the layer of graphite on the inside of the CzH4 chamber. Relative to the dose given by the Hammersmith treatment monitor, the Glasgow mean dose was greater by (8 * 6)% with the CH and TE chambers. At
Physics
intercomparisons
for neutron
radiation
therapy
Glasgow, using a secondary method-Kerma conversion of fluence measurements made with calibrated fission chambers-they deduced the dose was
(102 7)% greater than the Hammersmith monitor dose, thus supporting the ionization-chamber measurements. One of the major problems was the estimation of the neutron response of photon dosimeters. A national reference standard for the measurement of neutron absorbed dose was proposed to promote uniformity and continuity in the dose measurements at the various centers. Because of accuracy, reproducibility, and ease of operation, twin cavity chambers were proposed as the most promising dosimetry system. Such a system might be a TE chamber with TE gas in combination with a carbon chamber filled with CO*. Significantly, the Hammersmith rad appeared to be &IO% too low in these intercomparisons. This was later confirmed by Bewley’ from measurements with a TE calorimeter and a correction of 8% was introduced on 1 January 1975. Hammersmith also purchased two TE ionization chambers for neutron dosimetry; measurements with these chambers have confirmed the Glasgow intercomparison and calorimetry results. NDPG
intercomparison
The results of these intercomparisons were as follows: The institutions in the United States agree within 2% in their total dose measurements. In their comparisons of the standard y-ray calibrations of the ionization chambers on 13’Cs and ‘?Zo irradiators, the agreement was within 3%. Relative to the dose given by the Hammersmith treatment monitor, the dose measured by NDPG physicists was greater by 11% when measuring tissue
0
P. R. ALMOND and J. B. SMATHERS
Kerma in air, and greater by 14% when measuring tissue dose in a water phantom. The differences in the above measurements can be accounted for by adding to the Hammersmith doses the 8% dosimetry error in the neutron dose described in the last section, and also by adding the photon dose which Hammersmith excludes (3% in air, 6% at the depth of dose maximum, and 9% at IO-cm depth in phantom). Physicists in the United States and in Great Britain agree within 1% of the standard y-ray calibrations of their ionization chambers. The measurements were done with 8-MeV X-rays. Table 5 gives the latest NDPG results conducted in the Autumn of 1975. The use of different parameters of the Kerma ratio, the stopping power ratio and the Wn/ We ratio accounts for the 4-5% difference with MAN. Therefore. all these tissue Kerma in air measurements and photon calibrations agree to within 1%. The measurements at depth, conducted at Hammersmith, however exhibit greater differences and these are still under study but may be related to uncertainties in back scatter factors used by Hammersmith. CCRP inter-comparisons
Table 6 gives the preliminary results of these measurements made by both the Japanese in the U.S. and vice versa. Again, the agreement is good-but with uncertainties in the measurements made at depth. CONCLUSIONS Some very general conclusions can be stated from all these intercomparisons. Each group has drawn their own conclusions but it is remarkable how similar they are: (1) No gross discrepancies were found (i.e. a factor
Table 5. Intercomparison results: ratios of measurements with respect to MDAH-TAMU measurements during the Autumn of 1975 MAN MDAH-TAMU
MRC MDAH-TAMU
Location Beam
MAN 15MeV d-+T
16MeV d+Be
Tissue Kerma in air Dose at depth
0.95t
Participants
MRC
1.01 0.97 @ 1 cm O.% @ 5 cm 0.97 @ 10 cm
Photon calibration Whoice
NRL MDAH-TAMU NRL 35MeV
d+Be
account
for 4% of the 5% difference
Louv MDAH-TAMU Louv 50MeV
d+Be
1.00 1.018 @ 2cm 1.02 @ 1Ocm 1.00 (‘37cs)
of parameters
173
(see text).
Photon calibration
Dose at depth
0.97 @ 5 cm 0.96 @ IOcm 0.97 @ I5 cm O.% @ 20 cm 0.99 (“Co)
1.01
TAMU 30MeV d+Be
Location Beam
Tissue Kerma in air
NIRS MDAH-TAMU
Participants
t Be
0.996 @ IOcm
I.006
NRL 35 MeV d
TAMU 16MeV d+Be
I.01
NIRS NRL
NIRS MDAH-TAMU
0.974 @ 2cm 0.%3 @ IOcm
0.95 0.94 0.94 0.93
@ 5 cm 0 IOcm @IScm 8 20 cm 0.99
0.97
NIRS 30MeV d+Be
u of w 21.5MeV d+Be
0.975
NIRS MDAH-TAMU
U.S. Japan-measurements
NIRS u of w
Table 6. intercomparison results: of the CCRP
NIRS 30MeV d+Be 0.985
NIRS 15MeV d+Be
0.97 0 5 cm O.% @ IOcm 0.94 el5cm
0.99
NIRS u of w
NIRS MDAH-TAMU
made in 1976
NIRS 15 MeV d+ Be 1.013
NIRS u of w
9 Y 5. ,:
if! E 0 F 0
0
P
2 %
s
Physics
intercomparisons
for neutron
radiation
of 2 from the mean) in the Kerma measurements in air or the absorbed dose in a phantom. (2) Agreement to better than 5% in neutron Kerma and absorbed dose was found in the majority of cases. (3) Fairly large variations were found in the y-ray dose. Since this is a small fraction (approximately 5%) of the neutron dose and the neutrons have a higher RBE, the variations are not serious. The variations are mainly due to the large uncertainties in the
neutron response of the detectors used. (4) The majority of investigators used ion chambers as the basic means of obtaining the neutron doses with the tissue equivalent chambers being the most popular. However, where considerably different detectors have been used, silicon diode, activation foil, proportional counters similar results were obtained. (5) The predominately used TE ion chambers showed variation in the results amongst the various investigators. The variations can generally be traced to differences in the basic parameters used, e.g. W, stopping power ratio and Kerma ratios. (6) Some variations may be due to differences in the composition of the tissue equivalent plastic and in the difference in the tissue equivalent gas. (7) Other factors affecting the neutron calibration are the y-ray calibrations, saturation characteristics, wall thickness, chamber displacement factors, etc.
In
the
DISCUSSION overall scope of neutron
dose
in-
Table 7. Comparison MDAH-TAMU 50
Energy of D’ (MeV) Beryllium
target (thickness,
mm)
Mean neutron energy (MeV) Collimation for field shaping SSD (cm) Field size definition (Isodose-light field; IO x 10 cm field at SSD) Beam flattening Depth of d,., (g/cm*) Deuteron beam current (PA) Tissue Kerma in air (10 x IO cm field. rad/min at SSD) Tissue dose at d,,, (10 x 10 cm field. rad/min at SSD) Gamma component in air (10x 1Ocm field at SSD) Gamma component at d,., (10 x 10 cm field at SSD) Depth of 50% dose (10 x 10 cm fieid. g/cm’) Density of phantom fluid (g/cm’)
Thick (8.5) 21 WEP+B+Fe (70 cm) 140
therapy
0
P. R. AI MOND
175
and J. B. SMATHERS
tercomparisons several points should be kept in mind: (1) A wide variety of neutron beams are being used or proposed as sources for radiotherapy. Mono energetic neutron produced by the D-T reaction and neutron with a wide energy spread produced by deuterons upon Be targets with the deuteron energy varying from 16 to 50 MeV and proposals to use higher energy deuterons on Be and also protons on Be. These facts not only complicate the dosimetry intercomparisons but also the radiobiological and radiotherapeutic intercomparisons. (2) These intercomparisons are important. however. in determining the best type of dosimetry systems to use and for forming the basis for neutron dose
protocols. (3) Kerma and dose comparisons, although useful, are limited in value. The determination of absorbed dose to the tumor is important, as is the beam characteristics, so that the dose to normal tissue can also be compared (i.e. surface and build up dose, depth dose, etc.). Table 7 gives some of this information and Fig. 1 shows a typical comparison of build up data. (4) For radiobiological intercomparison, two types of dosimetry are required. If a standard biological system is being taken from institution to institution, a uniform dosimetry system that measures the dose to the biological system should accompany the radiobiology. Assuming the dosimetry to be constant, variations in the radiobiology results should reveal true radiobiological differences in the beams. If a biological system is used that is insensitive to neutron measurements carried out energy, radiobiological of beam characteristics NRL 35 Thick (4.7) 15 Benelex (65 cm) 125
u of w 21.5 Intermediate (1.5) 8 WEP+B (75 cm) 150
MRC 16 Intermediate (0.8) 7.6 Wood (68 cm) 116
60% Yes 1.07 7
90% Proposed 0.55 10
50% No 0.30 30
50% No 0.23 80
56
60
30
40
60
66
34
43
3%
1%
4%
3%
5%
2%
5%
6%
13.8 1.065
12.8 1.092
10.2 1.10
8.8 1.10
Radiation
176
Oncology
0
.
Biology 0 Physics o
cl---*----._______
~____~_____l.__--------*-----
__--
0.
*.
~~“Mwsurments
at TAMVEC (50 F)
0 Louvaln existtng data (50) . /
, 2
0
1
1 4
1
TAMVEC measurements at Louvain (50) I 6
1
/ 8
/
I IO
/ I2
Depth mm TE p4astlc
Fig. 1. Build up curves measured for neutron produced by 50 MeV deuterons on Be. The dashed line is taken on the MDAH-TAMU beam which is filtered by 3 cm of polyethylene to produce a flat beam. The open circles were measured by University of Louvain and the closed circles were measured by MDAHTAMU on the University of Louvain machine. Although the nominal energies of the beams are the same, it is apparent that the filtered beam has a different build up curve. However, the measurement techniques by the two groups appears to be consistent, but real differences in skin reaction might be expected-even
though the dosimetry comparisons were very close.
with the local dosimetry should reflect any difference in the dosimetry. If, however, a neutron energy sensitive biological system is used and the experiment done with local dosimetry very little can be said about the results. (5) The overall clinical response will be a function
of both the dosimetry and the radiobiological characteristics of the beam. The intercomparison to date should, along with the radiobiological results, provide most of the information needed to compare the radiotherapeutic results.
REFERENCES Bewley, D.K.: Private communication, 1975. Broerse, J.J., Burgess, G., Coppola, M.: Preliminary results of the European Neutron Dosimetry Intercomparisons Project (ENDIP). Private communication, 1976. Caswell, R.S., Goodman, L.J., Colvett, R.D.: International Intercomparison of Neutron Dosimetry. Radiot. Res. 532-546, 1975. International Measurements
Irradiation.
Commission on Radiation Report, lob, 1%4. Physical
National
Bureau of Standards
Units
and
Aspects of (U.S.) Hand-
book 85 4. 5. Martin, ‘J.H., Axton, E.J., Bewley, D.K., Dennis, J.A.. Greene,
D., Halnan,
K.E.,
Lawson,
R.C.:
Dosimetry
practice in neutron radiotherapy centers in Great Britain, London, England, Nat1 Phys. Lab. Rep., PSI, Jan. 1974. 6. Smith, A.R., Almond, P.R., Smathers, J.B., Otte, V.A., Attix, F.H., Theus, R.B., Wooton, P., Bichsel, H., Eenmaa, J., Williams, D., Bewley, D.K., Parnell, C.J.: Dosimetry intercomparisons between fast neutron radiotherapy facilities. Med. Phys. 2: 195-200, 1975. 7. Smith, A.R.: Zntercomparisons of particlegenerators in the United States, The United Kingdom, and International Proceedings of an International Workshop, Particle American College of Radiation Therapy. Philadelphia. Radiology, 1976, pp. 6048.