A problem in deep x-ray therapy is to deliver to a deep seated malignant tumor, a sufficiently large dose of x-rays to affect the malignancy without permanent injury to the skin and intervening tissue. To accomplish this it is necessary to obtain a very penetrating beam of x-rays

Smith, Scott W., Jr.

English

University of Northern Iowa

Proceedings of the Iowa Academy of ScienceVolume 32 | Annual Issue Article 731925Economical Filtration in Deep X-ray TherapyScott W. Smith Jr.Coe CollegeCopyright © Copyright 1925 by the Iowa Academy of Science, Inc.Follow this and additional works at: https://scholarworks.uni.edu/piasThis Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion inProceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contactscholarworks@uni.edu.Recommended CitationSmith, Scott W. Jr. (1925) "Economical Filtration in Deep X-ray Therapy," Proceedings of the Iowa Academy of Science, 32(1), 371-375.Available at: https://scholarworks.uni.edu/pias/vol32/iss1/73ECONOMICAL FILTRATION IN DEEP X-RAY THERAPY S co1'1' W. S :-.n1'H, ]R. A problem in  deep x-ray therapy is to deliver to a cleep seated malignant tumor, a sufficiently large dose of x-rays to affect the malignancy without permanent injury to the skin ancl intervening t issue. To accomplish this it is  necessary to obtain a very pene­trating beam of x-rays .  Radiation f rom a Cool idge tube is decidedly heterogeneous as to wave length, although as the v oltage is raised, the proportion of short penetrating wave lengths is increased. However, even with the high voltages now in use ( as high as 240,000 volts ) much undesirable so ft  radiation remains in the beam. In an effort to reduce the proport ion of  so ft rays,  filters are placed in the path of the beam. The fluestion now arises as to the best material for the filter, and the correct thickness of the selected material that should be used . The practice o i  using a millimeter or more of copper as  a filter for x-rays produced at 200 Kv. has become rather common. I t  is our belief  that such a filter thickness is excessive and with its  use much time i s  wasted . To i nvestigate this problem, readings were made at the surface and at a depth of  10 cm. in a water phantom, by means o f  an ionization chamber and a gold leaf electroscope, of the time re­quired for the shadow of the golcl leaf to fall ten scale divisions in the microscope, when various thicknesses of several different materials were used as filters. From Fig. 1 it  will be seen that the time required to discharge the electroscope ten units when the ionization chamber is at the surface of the water. increases with each aclclitional layer o f  filter material . c'\ simi lar graph might be constructed showing the increase in time when the ioniza­tion chamber is at the 10 cm. depth in the phantom. I f  for any particular filter thickness, the surface time be divicled by the depth time, the percentage of the beam reaching a depth of 10 cm. is obtained. Fig. 2 shows these percentages plotted against their corresponding fi lter thicknesses . It wil l  b e  seen at once from this graph that with the addition 1Smith: Economical Filtration in Deep X-ray TherapyPublished by UNI ScholarWorks, 1925FIELD :1 0><. 10  ,m.l A I I -S D. 150 c.m. 1 K. v. 144) l M. A. 3 1 : I I I F I G. I I I I l I I I I I I � I I 1 I I I I I I I I I I 1 co  P PER I FILTER -:THICKNE��-IN-rms, "' ,f .2 . 3  .s .b 1 2. .3 4 5 ' b  ' .7 7 INCl1fASE-ll't-SURfAU-TIME-t>Uf-TO- lNCR[A5 1N6- FILTER-'TlilCKNl'. SS Fig, 1 .  Sp ecimen graph showing the time in seconds required to discharge the ele c· troscope with the ionization ch amb er at the surface of the water phantom, plotted against filter thick ness expressed in millimeters, and also expressed as the thicknesses 3 L.J 28 i.b <( � 2G z w V 24 a'. !W 0.. 22 /6 required to i ncrease the surface time in equal multiples. FIELD :10� 10 cm. AS. D I 50 c.m K.V 14P M.A. 3: I I I I I I I I I I I I F I G  JI I I I I I I I I I I I I t I I I I I I t I : co P P E RtfllTER �TH ICKN ES�-IN-MM.s.1 : 14..�'--�-'-· '1--1:���· 2+-...,s.�-·�s��..--���-.!�·s.._��-!+-6���-±-�-' 2 3 6 lNCREASE- IN-SURfAC(-TlME-DU[-TO -INCREASINlr-FILTER- THICKNESS Fig. 2. Percentage of x-rays reach i n g  a depth of 10 cm. in water, pl otted aga inst filter thickness required to increas e the s111rfacc tlme i n  equal multiples o f  th e time required by the unfiltered beam to d ischarge the electroscope. 2Proceedings of the Iowa Academy of Science, Vol. 32 [1925], No. 1, Art. 73https://scholarworks.uni.edu/pias/vol32/iss1/73W 2 lb <( r- 2 z lU v 24 0: u.J 0.. 22 :r \- zo Cl. UJ 0 /8 '" I Fl[LD 1 10� 10 cm. I A..S.D. 1 50c.m. K..V. 14P M.A. 3 i  I I I I I I I Fl&. m 14 J 2 5 6 7 IN<'.R.EASE-IN-SUREACE-flnf-DUE-TO- lNCRE�SlNG-fJLTfR - THICKNESS Fig. 3 .  Comparative depth-dose percctJ1 ages o f  x- rays filtered through copper and alum� inum, plotted against tl· e ir  absorption values expressed by the thickness required to increase the surface time i n equal multiples.  lL.l 0 28 � z Zb w u ci'. 24 Li..l 0.. 22 I f- 20 CL lL..l i::i /8 ' I FI ELD p0 . 10 c.tl1 A.S. D. I 50 U'Y\. K. V 1..i :o M.A. 3 1 I I 16 ""51 z 3 4 5 � 7 INCREASE- IN-SURfACE.-TIME-DVf-10- ll'tCREAS IN('.,- FILTER-THlCK N[SS Fig. 4. Comparative depth-dose percentages with Ya rious filter materials, pl otted aga i n st their  absorption values. 3Smith: Economical Filtration in Deep X-ray TherapyPublished by UNI ScholarWorks, 1925f IEL D l 10. 10 cm. : I I AS.D. I 50c.m. 1 I l<..V2@ i M. A. 31 I I I I I I I I I I I I FIG-. JZ" n.i, 2 5 lNC REASE-IN-S VRfAC[- TIMf-DUE-TO-INCREhSIN&- flLTER- THICKNESS f ig. 5. Comparative depth-dose percentages of x-rays produced at 2 0'0 Kv. ,  filtered through copper, z inc,  and aluminum. F I G. . 1!I f, f I FIE L D  1 0 d0 c.m. : vj A . 'S. D. 50cm. '!:: .9 M.A. 3 'i: z .8 Cl lu .b I-: Ls IL.. 5 Ct'. � UJ . 3  4 Cl. n.. 0 .2.  u . I 2 0 140 1 0 WO 200 K I LO V O LT S  Fig. 6 .  Thickness o f  copper required a t  yarions volta.ges t o  increase the t ime o f  the surface-dose 2, 4, S ,  and 7 times, or to absorb 5 0 ,  7 5 ,  80, and 85 percent o f  the un­filtered beam of x-rays. 4Proceedings of the Iowa Academy of Science, Vol. 32 [1925], No. 1, Art. 73https://scholarworks.uni.edu/pias/vol32/iss1/73D E E P  X-RAY T H E R A P Y  3 7 5  of the first few thicknesses o f  filter, the penetration of  the  beam is greatly increased, but t hat after a certain amount has been added a further increase in filter thickness does not bring about a very great increase in penetration. while the decrease i n  i ntensity of the beam goes on steadily. There is. then, a point beyond which increase in  fi l ter thickness may no longer be consider­ed as profitable because the s l ight increase in penetration is out­balanced by the increase in  time required to deliver a certain close o f  radiation. This point we have termed the " l imit o f  economical filtration ."  I t  may b e  considered as  attained when sufficient fi lter has been adclecl to increase the surface time to five times the value with no filter. In order to determine the best material to use as a filter, a study was macle of various filter materials including zinc, brass, copper , iron, glass, a luminum, and tin. Fig. 3 shows the method o f  com­paring the efficiency of the fi lters. \i\'ith both copper ancl alumi­m1111 the penetration i s  increased as the filter thickness is  increast;<l, but with equal absorption of the beam, the beam filtered through copper is slightly more penetrating than that filtered through aluminum . There fore copper may be considered a more efficient fi lter material than aluminum . Fig. 4 shows a comparison o f  the various materials investigated, and shows that copper, zinc, and brass are good filter materials, while tin is a poor filter material . Fig. S shows the thickness o f  copper filter at va rious voltages required to increase the surface time in equal multiples. From this graph it is  seen that as the voltage is increased the filter thick­ness required to fi lter ont a certain percentage of  the beam is l ikewise increased. Therefore the limit of  economical filtration should be expressed in terms o f  absorption rather than filter thickness. My thanks i s  clue Dr. A. \V. Erskine of  Cedar Rapids, Ia. ,  for his guidance and help in  this and work previously reported in the Proceedings o f  the Iowa Academy of Science. CoE CoLLEGF., CEDAR RAPIDS, Iow A. 5Smith: Economical Filtration in Deep X-ray TherapyPublished by UNI ScholarWorks, 1925

Economical Filtration in Deep X-ray Therapy

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Economical Filtration in Deep X-ray Therapy

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