Baarli, J

Sullivan, A-H

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH 1. INTRODUCTION DI/HP/165 Health Physics Group 25 May, 1973 DOSIMETRY PROBLEMS OF NEGATIVE n MESONS J. Baarli and A.H. Sullivan CERN, Geneva Switzerland Negative pions are of interest for radiotherapy on account of their promising depth-dose distribution and the possible high REE of the charged secondaries produced in the nuclear inter-actions occurring at the end of their range. The intensities of existing pion beams have so far excluded their application to radiation therapy but have permitted limited radiobiological studies and allowed some investigation into dose measurement methods. CERN has a modest programme of radiobiological investiga-tions which include the effect of negative pions on mice (spermatogonia survival) and Vicia Faba roots (10 days growth). The dosimetry necessary for these investigations has shown the problems that exist in trying to estimate absorbed dose to the biological specimens in the radiation field of a stopped negative pion beam. Invited paper to be presented at XIIIth International Congress of Radiology Madrid, 15-20 October, 1973 - 2 -2. PROPERTIES OF PION BEAMS The properties of pions and the kinetics of their inter-actions have been extensively reviewed and many computations made in an attempt to assess the influence of various parameters on the applicability of pions to radiation therapyl- 5), Computa-tions of the charged particle types and spectra produced in pion interactions have be(;n made using an intranuclear cascade model 6 ), which givesan indication of the secondaries that may be produced; these are listed in Table 1 for carbon. As can be seen from this table, over 80% of the energy of charged secondaries is expected to be carried off by protons and alpha particles with mean ener-gies of about 20 and 15 MeV. The results of absorbed dose measurements when a beam of nega-tive pions is stopped in tissue-equivalent plastic is shown in Fig. 1. The main peak in this depth-dose curve is due to pion inter-actions together w~th the Bragg peak of the slowing down charged pions. The smaller peak after the main peak is due to stopping muons (of the same initial momentum as the pions). Electron contamination of the beam can be seen as the dose beyond the stopped muon peak. The form of ti1is curve depends on the beam optics used, the momentum spread of the pions and the amount of contamination. The depth-dose curve is also & function of the absorber used. The influence of momentum spread and beam diameter on the shape of the depth·-dose curve has been investigated theoretically?). These calculations show that the smaller the momentum spread, the narrower the stopped pion peak and the greater the contribution of the pion interaction secondaries to the peak dose. Experience .shows that the parameters that best describe the depth-dose curves are: beam half-width, peak depth, ra ti.a peak/entrance dose, peak width at so% height. - 3 -The proportion of dose from nuclear interactions can inferred from Fig. 2, which shows the depth~dose distributions from negative and positive pions of the same momentum. In the latter the peak dose is due to pion slowing down with a small contribution from pion decay (approximately 7 MeV lo from the decay muon and electron). It is also interest to note that the largest difference between these two curves and hence the nuclear interaction dose occurs at the back e of the nega~ tive pion peak. Measurements in the stopped pion region indicate that at the peak dose, 45% of the energy deposited in tissue-equivalent plastic originates from pion interactions, 40% is due to pion ionization, and the rest is contamination8 ). 3 PROBLEMS OF DOSIMETRY Computations and measurements indicate that the spectra of secondary charged particles produced in pion interactions depend on the nucleus with which the interaction takes Hence a dosimeter must be of a material where the nuclear interaction products as well as the stopping power for charged particles is known, in order that the necessary corrections can be made to determine the dose in tissue from the dosimeter read The most direct method of dosimetry would be to use a tissue-equivalent ionization chamber. Several factors have to be taken into account in the design of an ionization chamber for measure ment of the dose from stopped negative pions. Tissue-equivalent plastics used for ionization chambers contain an excess of carbon in place of oxygen in tissue. This difference in composition or "non tissue e nee n will affect the response of the dosemeter to stopped negative pions. The probability of pion capture by a nucleus is proportional to its charge 9). A capture by hydrogen is quickly transferred to a 6l nucleus. Using the computed data of Guthrie et al. / and the r - 4 -experimental data obtained using emulsions by Fowler and Mayes9 ) for the secondary particle energies per capture in various ele-ments, it is possible to estimate the corrections necessary. The estimated energy deposition per stopped pion in dosimetric materials and tissue is given in Table 2. Although the computed secondary particle spectra give an overall picture of a pion interaction it is doubtful whether they are sufficiently accurate to assess small differences in the energy liberated between different capturing elements when these differences are only a small fraction of the 140 MeV involved in the interaction. Both the computed and measured energy deposition indicate that a correction is necessary which is estimated to be of the order of 5-10%. Typical dose-rate variations with depth and isodose con-tours found with a symmetrically focussed beam are shown in Fig. 3. 'The position of a 1 cm radius sphere (which is the order of volume over which a reasonably uniform dose can be determined in a stopped pion peak) is also indicated in the figure. If this solid is replaced by a gas volume, as may be the case when ionization chambers are used, then the radiation field will be perturbed. The proportion of charged particle energy that is deposited within a given distance from the site of an interaction in water is shown in Fig. 4 lO). As can be seen, 50% of the energy deposited is at distances greater than 0.7 mm (700 microns) from the site of pion capture. As the stopped pion region is relatively small and the secondaries are isotropically emitted, energy will be transported through the gas away from the position of maximum interaction rate. Hence, unless the gas volume is small compared to the dimensions of the peak, the dose measured will not necessarily be the same as the average in the solid that the gas volume has replaced. The magnitude of the correction necessary with a parallel-plate ionization chamber depends on the ratio of beam diameter to electrode spacing and diameter of the collector electrode, as well as to some extent on gas pressure. It is calculated that the problem is largely avoided if the width of the pion beam is greater than 10 times the plate spacing. - 5 -Another consideration with ionization chambers is the varia-tions that occur in energy loss per ion pair produced in the gas for the different charged particles emitted from pion inter-actions. Estimated corrections to the t.J value for the different secondaries are given in Table 1. These ·are estimated on the basis of an ionization defect model. The mean value is only about 2% higher than that for electrons. This correction is small as the majority of energy is deposited by protons and alpha particles of relatively high initial energy. 4. DISCUSSION A series of measurements of depth-dose curves with pion beams in water or tissue-equivalent plastic have been made at ~ ' . 8 .11) ' ' h d t' CERN Ior raaiobiology experiments ' · • A typicaL oeam a ne following parameters: pion energy peak dose-rate beam half-width peak depth width of stopped peak (at 80% max. dose) % dose from contamination in peak = 90 MeV :::: 2.8 rad/h = 5 cm 22.2 g/cm 2 = 2. 5 g/ cm2 The dosimeter used was a parallel-plate tissue-equivalent chamber which is.shown diagrammatically in Fig. 5. The collector diameter (2 cm) is chosen to correspond to the usable cross-section of the pion peak as illustrated in Fig. 3. The plate spacing is 2.3 cm, making the sensitive volume about 9 cm3 • This volume is necessary on account of the requirement to measure the low dose-rates in the CERN pion beams. The design of the dosimeter used is not i.deal and a much smaller sensitive volume would be desirable. A dosimeter of parallel-plate design seems the best configuration as the depth of the sensitive volume in the stopping material is then well - 6 -defined. A chamber with variable plate spacing so that extrapola-tion to zero gas volume can be made would be ideal. The electrodes should be of tissue-equivalent plastic and at least 1 cm thick to ensure that the whole pion stopping region is in the material of the chamber. The corrections necessary with such a chamber to take account of the composition of the chamber and uncertainties in the (;} values would then be less than 10%. Use of other dosi-meters, such as solid-state detectors or plastic scintillators, would require considerable detailed investigation into their response relative to tissue. Radiobiological measurements indicate that the maximum "effect rate" in a stopped pion peak may not coincide with the peak absorbed dose, which is consistent with the fact that the nuclear interaction concentration is higher on the back edge of the stopped pion peak12 ). Hence it may be questionable whether absorbed dose alone is sufficient for optimizing therapeutic exposure conditions. Another parameter which would be of interest is the spatial distribution and location of the maximum pion stopping rate. This parameter together with absorbed dose-rate may be necessary in order to adjust the position of any tumour for a treatment using a stopped negative pion beam. - 7 -REFERENCES 1. P.H. Fowler and D.H. Perkins, Nature 189, 524 (1961). 2. M.R. Raju and C. Richman, Current Topics in Radiation Research 8, 159 (1972). 3. S. Curtis and M.R. Raju, Rad. Res. l!, 239 (1968). 4. T.W. Armstrong and K.C. Chandler, Rad.Res. 52, 247 (1972). 5. J. Turner et al., Rad. Res • ..21_, p. 229 (1972). 6. M.P Guthrie, R.G. Alsmiller and H.W. Bertini, Nucl. Instr. and Methods.£.§_, 29 (1968). 7. J. Turner et al., Proc. Int. Congr. on Protection against Accelerator and Space Radiation, CERN 71-16, Vol. 1, p. 231 (1971). 8. A.H. Sullivan and J. Baarli, Phys.Med.Biol. 12, 435 (1968). 9. P.H. Fowler and V.M. Mayes, Proc.Phys.Soc • .21_, 377 (1967). 10. J. Dutrannois, R.N. "Hamm, J.E. Turner and H.A. Wright, Phys. Med. Biol. 17, 765 (1972). 11. J. Baarli, A.H. Sullivan, M. Bianchi and M. Quintiliani, Proc. Int. Congr. on Protection against Accelerator and Space Radiation, CERN 71-16, p. 133 (1971). 12. J. Baarli and M. Bianchi, Int. J. Radiat. Biol. g_g_, 183 (1972). - 8 -FIGURE CAPTIONS Fig. 1. Depth-dose curve for negative pions in tissue-equivalent plastic. Fig. 2. Comparison of negative and positive pion depth-dose distributions indicating the location of dose from nuclear interactions. Fig. 3. Variation of dose with depth and dose contours at the peak dose in tissue-equivalent plastic. Fig. 4. Per cent of energy deposited within a given distance from the site of a pion interaction. Fig. 5. Cross-section of the parallel-plate tissue-equivalent ionization chamber used for pion dosimetry. - 9 -Table 1 Charged products from pion capture (in carbon) Energy per Average Energy per Particle interaction energy of ion pair particle % w /wo MeV of total MeV Proton 20 47 20 1.00 Deutron 2.8 6.6 11 1.02 Triton 1.0 2.3 13 1.02 3He 0.3 0.7 13 1.02 Alpha 17 40 15 1.03 Li 1. 22 2.9 6 1.1 Be 0 .16 0.38 3 1. 3 B 0.13 0.3 2 1.5 Table 2 Energy deposition in dosimetric materials based on computed and measured energy distribution of charged particles Local energy deposition Material per stopped pion Calculated Measured MeV MeV Tissue 37.3 27.3 T.E.Plastic 41.9 28.5 T.E.Gas 40.0 28.3 I 1 l ) / #/ I ---0---.-cf' ~t==--0--------<,.---0 L() N l BJ esop · 1etJ 0 ("') Li) N 0 N 3.l'o'~ 3$00 3All\f13~ 0 + t= I I t:= LO ...... .c. - a. a,, "O 0 ...-Lf> Li) ci 1· x )( fe.9 )( ""I I fl) 0·8 ~ ..... )C < I fl) 0... 0.7 0 !fl (I) ""'I 0·6 l2J ...... <I) 3 2 0 ·--.+--.--,--+--r-+-~~~ 4 3 2 Fig· 3 \ )( \ a) dose rate van with depth n peak 2 3 g/cm2 b) dose rate contours across beam at peak cm 90 0 80 w I- 20 MeVp -CJ) 0 * Cl. 70 w 0 50 Me VII( > ! 99% at SOmm (!) a: w 60 z w w _, u 50 I-a: <t Cl. 0 40 1oor w 1-f CJ a: <t :I: u 30 LL 0 ~ 20 10 1 2 3 4 DISTANCE FROM INTERACTION SITE (mm water) FIG .4 ~-i. ~· .. '=!---~. ALUMINIUM T.E. PLASTIC BEA f IG. 5 cotu:croR ----1 01 ···-__] DOSEMETER 

Dosimetry problems of negative $]pi$ -mesons

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Dosimetry problems of negative ]pi]pi]pi-mesons

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Dosimetry problems of negative $]pi$ -mesons