Gamma Putty shielding effect in megavoltage photon beam 1 [602341]

Gamma Putty shielding effect in megavoltage photon beam 1
Abstract2
Aim:Traditionally, lead and Cerrobend have been employed for field shaping in radiation 3
therapy. Lately ,another shielding material calledGamma Putty has emerged. The objective of 4
thisreportis toexamineitsdosimetric and shielding characteristics inmegavoltage photon beam . 5
Methods: All measurements were carried out ina dual energy linac. Data were collected using a 6
calibrated ionization chamber. Percenttransmission , linear attenuati on, and field size dependence 7
were measured for open square fields ( 4x4cm2to 10x 10cm2) defined by collimator jaws and 8
for different Gamma Putty thickness es (t=0, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5 cm) at6 and 18MV 9
photon beams .Themeasurements we re performed both in air usingappropriate acrylic buildup 10
capandin solid water .Results:TheGamma Putty tray factor increased steadilywith field size 11
for both 6 and 1 8MV.It was characterized by h alf value thickness of 2.513±0.101, 2.855±0.024 12
cm for 6 and 18 MV, respectively. The reduction in surface dose was about 6%, 14.5%, 22%, 13
36.37%, and 54%, for 6 MV, 2.75 %, 9.36 %, 16.25 %, 28.95 %, and 44.47 % for 18 MV for14
Gamma Putty thicknesses of 3, 5, 10, 15, 20, and 25 mm. Conclusion: The result of Gamma 15
Putty shielding on the photon beam output increases with thickness, beam ener gy, and field size. 16
Therefore, c linical use of Gamma Putty tray factors should be tailored for all thicknesses, beam 17
energies, and field sizes . 18
Keywords: Gamma Putty, tray factor, l inear attenuation, surface dose. 19
20
21
22
23
24
25
26
27
28
29
30
31
32

Introduction33
Thepurposeof radiotherapy treatment is to deliver dose to the tumorandcurtaildoseto the 34
surrounding normal tissue by sometimes using beam modifiers .Based on several reports, Lead 35
andcerrobend blocks have been commonly andpredominantly used1, 2.Several authors ha ve 36
described awiderange use of block applications .Purdy et al3used a gonadal shield based 37
Cerrobend in the pelvic irradiation of males to reduce the gonadal dose to about1.5 –2.5% of the 38
given dose .Sohn et al4employed a mobile shield to decrease the scatter dose to the contralateral 39
breast from the linear accelerator by a factor of 3 to 4. In the head and neck treatment r adiation, 40
stentswereutilizedto protect healthy tissu es5, 6whereas in the mantle instance, MLC leaves 41
were moved in or out to get the required shape in some cases7. 42
Nevertheless , theseshielding blocks cannot alwaysconform exactly to the patient 43
anatomyandregion of interest .They experienced variable thickness consistency and are hard to 44
plan and align effectively on the patient. To maximize a good shielding material, some properties 45
are desirable . They included high atomic number and attenuation coefficient so as to absorb a 46
radiation, and a low melti ng point. The shielding material must be malleable for better 47
conformity, inexpensive, and easily disposable for environment concerns .Recently, a material 48
calledGamma Putty that has been used in the industrial setting for different purposes such as 49
shielding cable tray penetrations around pipes and r adiographic film masking to prevent radiation 50
scatterhasbeen wooing as an alternative for leadand cerrobend for many reasons .ThePuttyis 51
lead free, malleable ,reusable, and can be thinned, thickened, res haped tofollowpatient anatomy . 52
As soon as , the wanted shield is made and wrapped in plastic, it can be used for the whole 53
treatment schedule. Furthermore ,when treating small lesions electron cutouts are often used to 54
shape the beam to the tumor andsparing normal tissue surrounding the tumor. Th e drawback of 55
using acutout is that the field size is smaller and to some extent inadequate for treatment, 56
resulting in underdosage of lateral tissues. Several important dosimetric parameters for 57
controlling the dose at extended distances, such as the percent depth dose, flatness, penumbra, 58
and uniformity, are greatly affected. In addition, a slight offset of the field can also result in a 59
large dose displacement from the region of interest . These facts have been emphasized by several 60
studies1, 2, 3, 4.Again, from dosimetric perspective lead or cerrobend arebelieved to be a better 61
skin collimator. However, Gamma Putty as askin collimator ismore easily fabricated and 62
required no specific tools than lead or Cerrobend. The goal of this study is to evaluate the 63
dosimetric and shielding properties of Gamma Putty inmegavoltage radiation therapy. 64
65
Materials and Methods66
TheGamma Putty (shieldw erx, Rio Rancho, New Mexico, US A) shielding blocks were 67
produced fromIron poly putty (LDPE) loaded with 90%ofBismuthanda high hydrogen content 68
gear to slow fast neutrons to thermal neutr ons with a density of 3 .8g/cc. In this study, t he Gamma 69
Putty bloc ks were in circular shapes with different thicknesses . They were mounted on the 70
blocking tray at 67.2 cm distance from source target oftheTrilogy ((Varian Medical systems, 71
Palo Alto, USA) delivering both 6 and18MV photon beams . 72
73
Gamma Putty tray factors (GPTF ) 74

A photon attenuation characteristic of Gamma Putty was assessed for6 and 18 MV 75
megavoltage photon beams usingvariable field sizes ( 4×4, 5×5, 6×6, 8×8, 9×9, 10×10) and 76
normalized to a 10×10 cm2field size. A PTW (PTW Freiburg, Germany) chamber was placed in 77
air to evaluat e GPTF. The charge was collected by the ioniz ation chamber and was measured 78
with an electrometer Model 206 (CNMC, Nashville, TN, USA) using a bias potential of to -300 79
V across the chamber. All measurements were performed with the detector set isocentrically and 80
perpendicular tocentral axis of the beam. An Acrylic build up caps with adequate thickness large 81
enoughto provide maximum dose at the chamber wasused for 6 an d 18MV photon beams. The 82
radiation transmission factor of Gamma Putty (GPTF) isdefined here as the ratio of the charge 83
measured with and without Gamma Putty in the beam, for the same number of monitor units: 84

fczDfczDcGPTF
openGP
,,,, (1) 85
Where c is the collimator opening, depth ( z) and distance from the source ( f). 86
Percent ionization depth measurement87
88
In order to evaluate change of depth dose distribution by the Gamma Putty , percentage 89
ionization depths were measured for several Gamma Putty thicknesses (0.3, 0.5, 1.0, 1.5, 2.0, 2.5 90
cm). Thus, analyzing the dosimetric impact of beam hardening, softening, scattering, and 91
electron co ntamination. The measurements were performed in solid water Model 458 (CNMC, 92
Nashville, TN, USA). The field size was 10 ×10 cm2, and thesource to surface distance (SSD) 93
wasset at 100 cm. 100monitors unit (MU) were given each time . 94
95
Linear attenuation co efficientdetermination 96
97
The linear attenuation coefficients value was measured at6 and 18 MV photon beams via 98
a Farmer typeionization chamber . These were performed variable field sizes(4×4, 5×5, 6×6, 99
8×8, 9×9, 10×10) moderated by Gamma Putty withthickness of (0.3, 0.5, 1.0, 1.5, 2.0, 2.5 cm) 100
placed on accessory tray and performed in air with appropriate buildup cap to reduce the impact 101
of phantom scatter . 102
103
Results and Discussion104
105
Gamma Putty Blocks106
In this study, the effect of Gamma Putty shielding block(Figure 1 (a ),(b))onrelative 107
dose in various field sizes and thicknesses were analyzed .Six blocks of variable thickness es 108
(0.3, 0.5, 1.0, 1.5, 2.0, 2.5 cm) were utilized to assess the dose distribution in various field sizes 109
shielded by the Gamma Putty.The GPTF as a function of field size is illustrated in Figure 1a and 110
Figure 1b for 18 and 6 MV photon beam, respectively. GPTF was generated based on111
measurements achieved in air using Farmer type ionization chambers placed on the beam central112
axisat the reference depth of d maxin acrylic build up caps that provided c harged particle 113
equilibrium. The reference depths for 6 and 18 MV photon beams are 1.3 and 3.5 cm114
respectively.115

116
Figures2(a),(b)showthe GPTF as a function of field size for variab le Gamma Putty 117
block thickness for both 6 and 18 MV . The figures revealed that GPTFincreases steadily with 118
increasing field size and decreasing Gamma Putty thickness for both 6 and 18 MV photons119
beam.This is due to increased scatter contribution, for al lGamma Putty thicknesses at two 120
energies. However, t here is a minor GPTF changes as a function of field size for the same 121
Gamma Putty thickness, which is systematically higher for 18MV. The changes at 18 MV 122
increasewith field size and a50.6% difference was noticeable for a 25 mm G amma Putty block . 123
However, b ehind a block of 3 mm thickness, the difference was evaluated at 12.55%.Thetrends 124
could bedescribed by linear regression fits that depend edonGamma Putty thickness as 125
illustrated in Table 1. Theobserved effect wasperhaps due to the secondary electron 126
contamination of the photon beam especially at 18 MV. Therewasa strong relationship betwee n 127
field size and GPTF as Gamma P utty thickness increases. For instance, for 25 mm, R2=0.997 and 128
0.963 for 6 and 18 MV, respectively. In fact,dose received by a point in air shielded by Gamma 129
Putty blocks include dcontribution from the primary photon, scattered photons produced, and 130
electron contamination derived from the Gamma Putty. This effect is enhanced b y increasing 131
field size, and energy (18 MV). Also ,90 % of the Gamma Putty is loaded with Bismuth 132
component of atomic number Z=83. Both attenuation and scattering of the photon beam by the133
Gamma Putty occurred due to pair production.134
Figure3(a)displayedthe measured relative ionization depth dose curves in solidwater 135
phantomat18 and6 MVphoton beam, respectively for open beam. Similarly, Figures3(b),(c) 136
illustrated the changes inrelativepercentionization depth dose insolid water for various Gamma 137
Puttythicknesses .This will be translated in notable variation in 18 MV compared to 6 MV. For 138
6 MV, the curve slopes positions aredeeper than the dmax,anddecreased because of the beam 139
hardening effect as Gamma Putty thickness increases.For instance, for 25 mm Gamma Putty 140
thickness, the difference in the relative percent ionization dose at a depth of 10 cm photons was 141
55.6%, and 58.55% for 6and18MV photons beams, respectively .Furthermore, a small 142
separation of the curves is noticeable at depths higher than 10 cm, denoting induced Gamma 143
Putty beam hardening. These curves are comparable to the open beam field for both 6 and 18 144
MV.Nevertheless, Figure3(b)does notrevealany effect at depth of Gamma Putty contaminant 145
electrons. Some reports echoed the same results using Cerrobend compensators on a 6 MV 146
photon beam and suggested that compensators donot significantly impact the percent depth dose 147
characteristics8, 9. On the other hand, Figure 3(c)revealedthat at 18 MV the Gamma Putty 148
produceda beam softening at depth that risesas Gamma Putty thickness increase d. This is due 149
to pair production from the GammaPutty. The resulting percent ionization decreasesat15cm 150
depthandreaches 50% for the 25mm block thickness. Conversely at 6 MV, G amma Putty 151
affectsthe dose in the build -up region indicating the presence of Gamma Putty scattered photons 152
or contaminant electrons in the beam.153
154
Evaluation of the attenuation coefficient of Gamma Putty 155
156
The effects of radiation beam attenuation for 6 an d 18MVphoton beam areillustratedin 157
Table2. The half value thickness (HVT) and Tenth value thickness (TVT) are determined based 158
on an exponential fitting. An example is displayed in figure s4(a),(b)for 10×10 cm2field size 159
withagood linear regress ion coefficient for 6 and 18 MV photon beam, respectively. This is 160
translated by a coefficient of variation of 4.0% and 0.8% for 6 and 18 MV, respectively. The 161

difference observed betweenHVTsuggested a strong beam energy hardening dependent.The 162
evaluation of linear attenuation coefficient in this study is simil ar to that of Du Plessis et al10. 163
164
Evaluation Off-axis relative dose 165
166
Off-axis relative percent ionization dose profiles areshown infigure5(a),(b)for 6 and 167
18 MV photon beam, respectively. Onlysmall infield differences, 2% in some in stancesare 168
accounted for . This difference in out of field photon dose was higherat18 MV.The increase in 169
percentage skin dose due to the Gamma Putty is greater at CAX with an estimate reduction of 170
5–10% from theCAXout to the field edge. This may be due to the lateral scatter of electron 171
contamination from the Gamma Putty that supplies a larger amount of dose at the CAX with172
respect to the off axis areas. This result is comparable for both 6 and 18 MV but mo re 173
significantly with 25 mm Gamma Putty thickness at 18 MV. The majority of dose deposited at174
the surface directly under the blocks has been generated by electron contamination. It was also175
reported that dose from the blo ck tray and air to be the main sou rce of skin dose. This effect was 176
dominant in larger fiel d sizes and high energies. A ir between source and skin generates 177
secondary electrons and these electrons absorbed or scattered in air depend ed on beam 178
divergence and some of them c ouldreach the pati ent’s skin. The impact will become more 179
prevalent as SSD increased and the number of electrons that reach the patient’s skin decreased11. 180
However, air has more impact than electron contamination.181
Surface dose182
Surface dose is usually defined as percentdepth doseat0.5mm depth, with 183
normalization to d max.Similarly to a block tray, Gamma Putty increases skin dose. The effect is 184
stronger with increasing field size and decreasing Gamma Putty thickness .Skin dose values with 185
theGamma Putty block were higher than those with an open field by comparing GPTF of Figure 186
6(a)and figure s6(b),(c). Theeffect was dominant in larger field sizes astheGamma Putty 187
eliminates electrons from upstream and generates new secondary electrons by itself12.These 188
resultsin higher number of electrons createdthaneliminated .In addition , secondary electrons 189
initiatedat theGamma Putty can reach the patient, thus increasing the skin dose more 190
significantly. The effect is exacerbated from the upper and lower windows in addition to t he 191
impact of SSD, field size, and energy. Figures6(b),(c) shows the measured surface dose as a 192
function of G PTFtaken at 5 mm depth. The surface dose decreases with Gamma Putty thickness 193
due to the beam hardening effect. The reduction is about 6%,14.5%, 22%, 36.37%, and 54%, for 194
6 MV, 2.75 %, 9.36 %, 16.25 %, 28.95 %, and 44.47 % for 18 MV withGamma Putty 195
thicknesses of 3, 5, 10, 15, 20, and 25 mm, respectively. This demonstrates theimpact of beam 196
hardening on the surface dose. All these factorsenumerated above such ascontaminant electron s 197
generated by the collimator, interactions of photons with Gam ma Putty, self -absorption of 198
Gamma Putty played a major role in dose summation. However, at 10 cm depth interestingly 199
enough,GPTFdoes notget closer to unity .Hence, for small field size, photons and electrons are 200
scattered out of the field when they pass though the Gamma Putty . Beyond d max, the factor nearly 201
remains constant.202
Conclusion 203
204

Inthisreport,Gamma Putty shielding effect on themegavoltage photonbeam output has 205
beenanalyzedfor18 and 6 MV . Theresultshowed that several parameters are responsible for 206
fluctuation of dose received by a point in air blocked by a Gamma Putty. They included: field 207
size,thickness,and beam energy .The impact of these parameters isheightened byelectrons 208
contamination w ithincreasingfield sizesandbeamenergy.In addition, the measurement 209
showed that GPTF values decreased with increasing field size, depth, and thickness as beam210
hardening and mor e scatter for larger field sizes contributed to dose at the given depth . The study 211
revealed that the doseoutside the field are governed by the skin collimation stopping the 212
electrons in the incident beam due to the presence of the Gamma Putty shielding and increased 213
lateral scatter of the photon beam as the Gamm a Putty thickness is increased. Furthermore, the 214
study suggested that c ontaminant electrons to be a major factor of dose outside the field at 215
shallow depths. The magnitude and extent increase dwithbeam energy, even more in the 216
presence of beam modifiers. In general, the primary dose rate at shallow depths in the phantom217
may actually increase at distance s away from the central axis due flattening filter effects on the 218
radiation beam13, 14. 219
220
Referenc es. 221
222
1. Matsumoto Y, Umezu Y, Fujibuchi T, Noguch i Y, Fukunaga J, Kimura T. Verification of the 223
protective effect of a testicular shield in postoperative radiotherapy for seminoma. Nihon224
Hoshasen Gijutsu Gakkai Zasshi. 2014; 70(9):883–887 225
2. Craciunescu OI , Steffey BA, Kelsey CR, Larrier NA, Paarz -Largay CJ, Prosnitz RG. Renal 226
shielding and dosimetry for patients with severe systemic sclerosis receiving227
immunoablation with total body irradiation in the scleroderma: cyclophosphamide or228
transplantation trial . Int J Radiat Oncol 2011;79(4):1248–1255 229
3. J.A. Purdy, R.D. Stiteler, G.P. Glasgow, W.B. Mill. Gonadal shield Radiology. 1975;117-226 230
4. Sohn JW, Macklis R, Suh JH,Kupelian P .A mobile shield to reduce scatter radiation to the 231
contralateral breast during radiotherapy for breast cancer: preclinical results. Int J Radiat Oncol 232
Biol Phys. 1999 Mar 15 ; 43(5):1037-41. 233
5. Toljanic JA, Saunders VW Jr. Radiation therapy and management of the irradiated patient. J234
Prosthet Dent 1984 ; 52:852-8 235
6. Mantri SS, Bhasin AS, Shankaran G, Gupta P . Scope of prosthodontic services for patients 236
with head and neck cancer. Indian J Cancer 2012 ; 49:39-45. 237
7.Ramachandran Prabhakar R ,Kunhi Parambath P Haresh ,Pappiah S Sridhar ,Macharla A 238
Laviraj,Pramod K Julka ,Goura K Rath . Execution of mantle field with multileaf collimator: A 239
simple approach. Journal of cancer research and therapeutics 2008;Vol: 4Issue: 1Page: 18- 240
20241
8. Xu T, Shikhaliev P M, Al-Ghazi M and Molloi S. Reshapable physical modulator for intens ity 242
modulated radiation therapy. Med Phys 2002;292222–9 243
9.Jiang S B and Ayyangar K M. On compensator design for photon beam intensity -modulated 244
conformal therapy Med Phys 1998 ;25668–75. 245
10. Du Plessis FCP, Willemse CA. Monte Carlo calculation of effective attenuation coefficient 246
for various compensator materials. Med Phys 2003;30:2537–2543 247
11. Khan FM. The physics of radiation therapy, Williams and Wilkins, Baltimore , 1994, pp: 323- 248
32.249

12. Kim S, Liu CR, Zhu TC, Palta JR. Photon beam skin dose analyses for different clinical 250
setups. Med Phys 1998; 25: 860-6. 251
13. Thomas SJ, Thomas RL. A beam generation algorithm for linear accelerators w ith 252
independent collimators. Phys Med Biol. 1990 ; 35(3):8. 253
254
14. Kwa W, Kornelsen RO, Harrison RW, el -Khatib E. Dosimetry for asymmetric x -ray fields. 255
Med Phys. 1994 Oct ; 21(10):1599 -604. 256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294

295
296
297
Figures298
299
300
Figure1: (a)VariableGamma Putty thickness inaplastic wrap. 301
302
303
304
Figure1: (b)Measurement with Gamma Putty on the linac using a solid water and ion chamber . 305
306
307

308
309
Figure2: (a)Gamma Putty Tray factor for different thickness at 18 MV 310
311
312
313
Figure2: (b)Gamma Putty Tray factor for different thickness at 6 MV 314
315
316
317

318
319
Figure3: (a)Ionization depth dose curve without Gamma Putty thickness with 6 and 18 MV 320
photon beam at 10 ×10 cm2field, measured using a Farmer type chamber in solid water phantom. 321
322
323
324
325
Figure3: (b)Ionization depth dose curve for variable Gamma Putty thicknesses block with 6 MV 326
photon beam at 10 ×10 cm2field, measured using a Farmer type chamber in soli d water phantom. 327
328
329
330
331

332
333
Figure3: (c)Ionization depth dose curve for variable Gamma Putty thicknesses block with 18 334
MV photon beam at 10 ×10 cm2field, measured using a Farmer type chamber in solid water 335
phantom. 336
337
338
339
Figure4: (a)Attenuation of photon beam intensity with the variation of Gamma Putty block 340
thickness for 6MV341
342
343

344
345
Figure4: (b)Attenuation of photon beam intensity with the variation of Gamma Putty block 346
thickness for 18 MV photons. 347
348
349
350
351
352
353
354
Figure5: (a)Off-axis relative dose profile at a depth of 0.5 cm for a 10×10 cm2field size at 6 355
MV and 100 cm SSD356
357

358
359
360
Figure5: (b)Off-axis relative dose profile at a depth of 0.5 cm for a 10×10 cm2field size at 18 361
MV and 100 cm SSD362
363
364
365
Figure6: (a)Surface dose as a GPTF at 0 mm depth for 18 and 6 MV beam. GPTF is shown as a 366
function of the Gamma Putty thickness.367
368
369

370
371
Figure6: (b)Surface dose as a GPTF at 5 mm depth for 18 and 6 MV beam. GPTF is shown as a 372
function of the Gamma Putty thickness.373
374
375
376
377
Figure6: (b)Surface dose as a GPTF at 10 cm depth for 18 and 6 MV beam. GPTF is shown as a 378
function of the Gamma Putty thickness.379
380
381
382
383

Tables.384
Table1:Linear regression for Gamma at variable thicknesses and field sizes 385
Thickness (mm) 6 MV18 MV
3 0.4040.126
5 0.6340.704
10 0.9030.446
15 0.9880.960
20 0.9500.951
25 0.9970.963
386
Table 2:Half Value and Tenth Value thickness for Gamma Putty blocks 387
Field size Energies
6 MV 18 MV
HVT TVT HVT TVT
10X10 2.501 8.312 2.840 9.436
9X9 2.739 9.101 2.899 9.634
8X8 2.484 8.253 2.875 9.554
7×7 2.475 8.223 2.851 9.473
6X6 2.466 8.194 2.840 9.434
5X5 2.466 8.194 2.840 9.434
4X4 2.448 8.136 2.828 9.396
Mean ± std 2.513±0.101 8.350±0.337 2.855±0.024 9.487±0.081
C.V (%) 4.00 4.00 0.80 0.80
388
389