A STUDY ON PROPERTIES OF WATER SUBSTITUTE SOLID PHANTOM USING EGS CODE

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1 Proceedngs of the Tenth EGS4 Users' eetng n Japan, KEK Proceedngs , p A STUDY ON PROPERTIES OF WATER SUBSTITUTE SOLID PHANTO USING EGS CODE H. Satoh a, T. Tomaru b, T. Fujsak c, S. Abe c, A. yojoyama a and K. Fukuda b a Department of Radologcal Scences, Tokyo etropoltan Unversty of Health Scences Tokyo, , Japan b Chyoda Technol Co., Tokyo, , Japan c Department of Radologcal Scences, Ibarak Prefectural Unversty of Health Scences,Ibarak, Japan e-mal: satoh@metro-hs.ac.jp Abstract To reduce the uncertanty n the calbraton of radaton beams, absorbed dose to water for hgh energy electrons s recommended as the standards and reference absorbed dose by AAP Report no.51, IAEA Techncal Reports no.398 and JSP Standard dosmetry for radotherapy In these recommendatons, water s defned as the reference medum, however, the water substtute sold phantoms are dscouraged. Nevertheless, when accurate chamber postonng n water s not possble, or when no waterproof chamber s avalable, ther use s permtted at beam qualtes R 50 < 4 g/cm 2 (E 0 < 10 ev). For the electron dosmetry usng sold phantom, a depth-scalng factor s used for the converson of depth n sold phantoms to depth n water, and a fluence-scalng factor s used for the converson of onzaton chamber readng n astc phantom to readng n water. In ths work, the propertes, especally depth-scalng factors c and fluence-scalng factors h of several commercally avalable water substtute sold phantoms were determned usng EGS onte Carlo smulaton. Futhermore, the electron dosmetry usng these scalng method was evaluated. As a result, t s obvously that dose-dstrbuton n sold phantom can be converted to approprate dose-dstrbuton n water by means of IAEA depth-scalng. 1. Introducton To reduce the uncertanty n the calbraton of radaton beams, absorbed dose to water for hgh energy photons and electrons s recommended as the standards and reference absorbed dose by AAP Report no.51 1), IAEA Techncal Reports no.398 (TRS-398) 2) and JSP Standard Dosmetry for Radotherapy 01 (JSP01) 3). In these recommendatons, water s defned as the reference medum, however, the water substtute sold phantoms (sold phantoms) are dscouraged because they have the largest dscrepances n the determnatons of absorbed dose. However, almost users n hosptals are confusng because accurate chamber postonng n water s not easy, no waterproof chamber s avalable and t takes a consderable tme that water proof chambers become popular. Therefore sold phantom use s permtted at beam qualtes R 50 < 4 g/cm 2 (E 0 < 10 ev) for the electron dosmetry n the TRS-389 and JSP01. Dose-dstrbuton n sold phantom can be converted to approprate dose-dstrbuton n water by means of depth-scalng. To convert a depth n sold phantom to a depth n water, several depth-scalng methods have been proposed. In the ICRU Report 35, the lnear contnuous-slowng-down approxmaton (csda) range rato of water to sold phantom was ntroduced 4). The csda range accounts for contnuous collson and radatve energy losses only. After that t has been cleared that multe scatterng could apprecably affect penetraton depths of electrons, the new 55

2 depth-scalng methods usng depth-scalng factor C 5) (n the IAEA TRS-381) 6) and c (n the IAEA TRS-398) 2) have been proposed. Both C and c are the rato of the average depth of electron penetraton n water and astc, nevertheless depth for C s defned n unt of cm and depth for c s expressed n g cm -2. In addton to depth-scalng, the readng of onzaton chamber Q, n the sold phantom must be scaled to the approprate readng Q n water by fluence-scalng factor h. To the best of our knowledge, these two factors have been determned n a few study and factors of only specfc phantoms are publshed n the IAEA Reports 2). In ths work, the depth-scalng factors and fluence-scalng factors of several commercally avalable sold phantoms were determned usng EGS onte Carlo smulaton, and the electron dosmetry usng these factors was evaluated. 2. aterals and ethod 2.1 Fundamental physcal propertes In ths work, WT1 (GAEX RI, Wsconsn, USA), RI-457 (GAEX RI, Wsconsn, USA), Plastc Water (Nuclear Assocate, New York, USA), Vrtual Water (ed-tech, Iowa, USA), WE211 7) (Kyoto Kagaku, Kyoto, Japan),, Polymethyl ethacrylate (PA) and xdp, whch as commercally avalable materal, were evaluated. The elemental composton, mass fracton, nomnal densty and mean atomc number are summarzed n Table 1. The mean atomc number Z s used for mxtures and/or compounds when comparson of the scalng parameter, and defned as Z = p p Z 2 A Z A (1) where p s the mass fracton, Z s the atomc number, and A s the molar mass of element The mass stoppng powers and densty correcton factors of sold phantoms were determned accordng to ICRU Report 37 8, 9), and cross secton data were prepared usng PEGS preprocessor of EGS code system 10). 4). 2.2 Depth-scalng factor: c Dose-dstrbuton n sold phantom can be converted to approprate dose-dstrbuton n water by means of depth-scalng. easurement made at a depth z (g cm -2 ) n a sold phantom, approprate depth n water z w (g cm -2 ) s gven by z = z c (2) w where c s a depth-scalng factor. The c s the rato of the average depth of electron penetraton n water and sold phantom, defned as water zav ρwater c = (3) zav ρ 56

3 water z av av where and z s an average penetraton depth (cm) n water and sold phantom, and ρ and ρ s densty (g cm -3 ) of water and sold phantom materal, respectvely. To calculate z av, orgnal user code on EGSnrc verson2 14) was coded newly. onoenergetc electron pencl beam of energes from 1 to 30 ev have been assumed to mpnge normally on fnte slab of water and the other materals. The transport of prmary electrons has been followed down to the cutoff energy at 10 kev, penetraton depths z of each hstory were samed and z av was calculated. As an exame of smulaton, Fgure 1 shows geometry of smulaton and coordnates where prmary electrons lost ther knetc energy and came to standstll. water 2.3 Fluence-scalng factor: h To convert a readng of onzaton chamber n the sold phantom to an approprate readng n water, the fluence-scalng factor h has been proposed n the TRS-389 2). The readng of onzaton chamber Q, n the sold phantom must be scaled to the approprate readng Q n water usng the next equaton, = h (4) Q Q, where h s a fluence-scalng factor. Namely, when Q, s a readng of onzaton chamber at z ref, n the sold phantom and Q s a readng at z ref n water, h s defned as Q h = (5) Q, To the best of our knowledge, fluence-scalng factors for varous materals have been determned n a few expermental works 11-13). In ths work, absorbed dose dstrbuton was calculated usng EGSnrc and DOSXYZnrc onte Carlo smulaton 14), then the h s were determned by next equaton. In the dentcal rradaton condton, when absorbed dose to water s D water and absorbed dose to sold phantom s D, h s gven by D s Q water h = = (6) Q, D ρ, water where (s/ρ), water s mass collson stoppng-power rato of sold phantom to water. 3. Results 3.1 ass collson stoppng power rato Fgure 2 shows mass collson stoppng power ratos of sold phantom to water as a functon of electron energy. As compared wth other sold phantoms, xdp has a hgher mass collson stoppng power rato, to for electron energy of 1 to 100 ev. 3.2 Depth-scalng factor: c Fgure 3 shows Depth-scalng factor c as a functon of electron energy. c of Plastcwater s for electron energy rage from 1 to 30 ev, namely, ndependent of electron energy. xdp and, whch has a lower mean atomc number than water, obvously depend on electron energy. For exame, c of s for 1 ev and for 30 ev, respectvely. However, ths depth-scalng method s proposed at beam qualtes R 50 < 4 57

4 g/cm 2 (E 0 < 10 ev), and avalable lowest energy of accelerator s taken nto consderaton, mean c of 6 to 10 ev were determned. The mean c of several materals are tabulated n Table 2. Although c s mean value, dfference from mean c to c as a functon of electron energy s small wthn 0.3% at energy range 6 10 ev. The c of ths work gave good agreement wth the c of TRS Fluence-scalng factor: h Fgure 4 shows the rato of absorbed dose at reference depth n water to that n sold phantom. The uncertanty of absorbed dose rato may be estmated as %. The fluence-scalng factors were derved from these absorbed dose ratos D water /D and above-mentoned (s/ρ), water usng equaton (6). Fgure 5 shows fluence -scalng factor h as a functon of electron energy. Although h slghtly depend on electron energy, as the same reasons of depth-scalng factor, h are determned as a mean value for electron energy range of 6 to 10 ev. The mean h (6-10 ev) of several materals are tabulated n Table 3. The h of Plastcwater and RI457 gave good agreement wth that of TRS-389, however, the other materals have a sgnfcant dfference. 4. Dscusson Percentage depth dose dstrbutons n water have been compared wth dstrbuton n sold phantom wth and wthout scalng. As some results, Fgure 6 shows percentage depth dose dstrbutons n water and. It can be seen that depth scaled dstrbuton n usng c s n good agreement wth that n water, although, mnor devatons can be observed near the surface and at the end of the electron range. It s dffcult to determne the fluence-scalng factor h expermentally because of dffculty n accurate chamber postonng and charge storage effect etc. Therefore, h were derved from absorbed dose ratos D water /D whch obtaned from onte Carlo smulaton and (s/ρ), water n ths work. The h of was descrbed n detal by Thwates 11). At 7.5 ev of nomnal energy, (for NE farmer chamber graphte wall), (for NE farmer chamber nylon wall), (for NE farmer chamber A-150 wall) and (for PTW ntra-cavtary) have been reported as h of. It s obvous that h depend on chamber wall materal. For that reason, theoretcal equaton whch takes account of chamber wall have been requred to determne h. 5. Conclusons The propertes, especally depth-scalng factors c and fluence-scalng factors h of several commercally avalable water substtute sold phantoms were determned usng EGS onte Carlo smulaton and the electron dosmetry usng these scalng methods was evaluated. As a result, the c of ths work gave good agreement wth the c of TRS-389. And t s obvously that depth n sold phantom s converted to approprate depth n water by means of depth-scalng usng c. The h of Plastcwater and RI457 gave good agreement wth the h of TRS-389, however, the other materals have a sgnfcant dfference between h of ths work and that of TRS-389. Acknowledgment Ths work was supported by a specfed grant-n-ad of Tokyo etropoltan Unversty of Health Scences. References 1) P. R. Almond, P. J. Bggs, W. F. Hanson et al.: AAP s TG-51 protocol for clncal reference dosmetry of hgh-energy photon and electron beams, ed. Phys. 26(9), (1999). 2) IAEA: Absorbed dose determnaton n external beam radotherapy, -An nternatonal code of practce for dosmetry 58

5 based on standards of absorbed dose to water-, Techncal Reports seres No. 398, IAEA, Venna (2000). 3) JSP: Standard dosmetry for Radotherapy, Tsusho-Sangyo Kenkyu-Sha, Tokyo (2002). 4) ICRU: Radaton dosmetry. Electron beams wth energy between 1 and 50 ev, ICRU Report 35 (1984). 5) J.. Fernandez-Varea, P. Andreo and T. Tabata: Detour factors n water and astc phantoms and ther use for range and depth scalng n electron-beam dosmetry, Phys. ed. Bol. 41, (1996). 6) IAEA: The use of ane parallel onzaton chambers n hgh energy electron and photon beams An nternatonal code of practce for dosmetry-, Techncal Reports seres No. 381, IAEA, Venna (1997). 7) T. Hraoka: Dosmetry -Phantom-, Journal of Japan Assocaton of Radologcal Physcst Sup. 8, 1-27 (1978). 8) ICRU: Stoppng powers for electrons and postrons, ICRU Report 37 (1984). 9) 10) W. R. Nelson, H. Hrayama, D. W. O. Rogers: The EGS4 code system, SLAC Report SLAC-265 (1985). 11) D. I. Thwates: easurements of onzaton n water, polystyrene and a sold water phantom materal for electron beams, Phys. ed. Bol. 30, (1985). 12) V.. Tello et al.: How water equvalent are water-equvalent sold materals for output calbraton of photon and electron beams?, ed. Pys. 22, (1995). 13) A. Nsbet et al.: An evaluaton of epoxy resn phantom materals for electron dosmetry, Phys. ed. Bol. 43, (1998). 14) I. Kawrakow and D.W.O. Rogers: The EGSnrc Code System, onte Carlo Smulaton of Electron and Photon Transport, NRCC Report PIRS-701 (2001). 59

6 Table 1 Elemental composton, mass facton, nomnal densty and average atomc number of water and water substtute sold phantoms. composton and mass fracton Z A water WT1 RI457 Plastc Vrtual W WE211 PA xdp H C N O F g Cl Ca T Br densty g/cm mean Z x electrons y z water or sold phantom Fgure 1 Geometry of z av smulaton and coordnates whch prmary electrons came to standstll 60

7 (s/ρ), water xdp Plastcwater WE211 WT1 RI457 Vrtualwater PA electron energy [ev] Fg. 2 ass collson stoppng power raton (s/ρ), water as a functon of electron energy. c : (z av ρ) w, PlastcW WT1 xdp VrutalW WE211 PA RI electron energy [ev] Fgure 3 shows Depth-scalng factor c as a functon of electron energy. 61

8 Table 2 ean depth-scalng factors, c for sold water substtute materals (E 0 = 6 to10 ev) ateral xdp PA Plastc W WE211 Vrtual W WT1 RI457 Ths work TRS D water / D WT1 Vrtualwater xdp E 0 : electron energy [ev] D water / D PA WE211 RI457 Plastcwater E 0 : electron energy [ev] Fgure 4 Rato of absorbed dose at reference depth n water to that n sold phantom D water /D. 62

9 1.060 h : fluence-scalng factor xdp WE211 RI457 Plastcwater E 0 : electron energy [ev] h : fluence-scalng factor PA WT1 Vrtualwater E 0 : electron energy [ev] Fgure 5 fluence-scalng factors, h as a functon of electron energy. Table 3 ean fluence-scalng factors, h for sold water substtute materals (E 0 = 6 to10 ev) xdp PA Plastcwater WE211 Vrtual W WT1 RI457 Ths work TRS

10 100.0 E 0 = 6 ev Percentage depth dose [%] n water wthout correcton wth c correcton Depth n phantom [g cm -2 ] E 0 = 10 ev Percentage depth dose [%] n water wthout correcton wth c correcton Depth n phantom [g cm -2 ] Fg. 6 Comparson of percentage depth dose curve between n pure water, n wthout correcton and wth c correcton. 64