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Description of a computer controlled therapy machine
the complete range of exit doses (24-200 rads) encountered in clinical radiation therapy based on experiences at three institutions: The University of Chicago, Mallinckrodt Institute, and M.D. Anderson Hospital and Tumor Institute. This presentation will provide information such that most radiation therapy facilities will be able to set up and use amethod for verification of the patient radiation treatment. (152)
COBALT 60 TELETHERAPY SURVEY il. Thompson*, H. Wyckoff**, C. Soares*** *Bureau of Radiological Health, Food and Drug Administration, Rochville, Maryland, **International Commission on Radiation Units and Measurements, Washington, D.C., ***National Bureau of Standards, Department of Commerce, Githersburg, Maryland.
During the past three years the National Bureau of Standards and the Bureau of Radiological Health have been conducting a survey of cobalt-60 teletherapy facilities to determine their accuracy in exposing a phantom to a prescribed dose. All cobalt-60 facilities in the U.S. were invited to participate in this program. Those that agreed were provided a questionnaire regarding dose calculations and facility information, and a set of thermoluminescent dosimeters with instructions for their exposure. As of May 1977, some 700 units were surveyed out of the approximately 1,000 which are licensed to administer therapy in the U.S. Preliminary data indicate that about two-thirds of the respondents were able to calculate the specified 300 rad dose within 5 rads; while 8 percent reported values which varied from the specified dose by more than 15 rads. The mean exposure determined for the set of dosimeters differed from the 300 rad value by less than 5 percent for 83 percent of units; 4 percent of the units produced an exposure which differed by more than 10 percent of the target value. Analyses of exposures and dose calculations will be presented for the various characteristics of the facilities, e.g. type of institution, bed size, location and type of administration. (153)
DESCRIPTION OF A COMPUTER CONTROLLED THERAPY MACHINE Bengt Bjarngard*, Peter Kijewski*, Charles Pashby** *Joint Center for Radiation Therapy, Department of Radiation Therapy, Harvard Medical School, Boston, Massachusetts **Siemens A.G., Walnut Creek, California
We have initiated a project to study the feasibility of optimizing dose distributions by using a computer to control the therapy machine during irradiation of the patient. A Siemens Mevatron XII, modified to allow computer control of some dqzen therapy machine parameters including gantry angle, couch position, dose rate, and collimator jaw position has been installed. The presentation will describe the special features of this therapy machine that are necessary for computer control. 142
A closed loop servo system is used to control position as well as velocity of the motorized parameters of the therapy machine. The associated Communication control software is implemented on a 32K core minicomputer. between computer and therapy machine is in ASCII format via serial lines. The performance of the system as pertains to tracking accuracy will be described. An important design consideration of the system is safe operation in A detailed discussion of system features relevant a clinical environment. to system safteiy and ease of operation by the technologist will be presented. (154)
AN EMPIRICAL STUDY OF ISOCENTRIC DEPTH DOSE DATA FOR WEDGED PHOTON BEAMS
10 MeV
Wing-Chee Lam, Ph.D., David G. Grant, M.A., Bengt A. Lindskoug, Ph.D. Radiation Oncology Physics, The Johns Hopkins Oncology Center, Baltimore, Maryland
High enlergy photon beams produced by a clinical linear accelerator have a complex continuous spectrum. Added wedge filters alter the spectrum and also modify beam scatter depending upon the filter material and thickness. As a result, differences in critical beam parameters, such as percent depthdose, will exist between wedged and open fields. Conventional open field depthdose data have been measured and reported in the literature. Corresponding wedged fields are usually assumed to have the same depth-dose fall-off. Depthdose data of a 10 MeV photon beam for various field sizes have been measured for both open and wedged fields on a Varian Clinac-18. Standard brass wedges supplied by the Varian Associates were used. The results clearly demonstrate that differences in depth-dose between wedged and open fields could amount to 7%. The table presents isocentric percentage depth-dose for a 10 cm x 10 cm field for open and for wedged fields. The isocentric depth dose is defined as the ratio of dose at isocenter at depth "d" to the dose at isocenter at depth "dmax" an'd where the field size is defined at isocenter. ISOCENTRIC PERCENTAGE DEPTH DOSE FOR 10 x 10 FIELD Open
0
0
Field
WZge
WZge
45O Wedge
1.00
1.00
1.00
1.00
0
WZge 1.00
5.0
.957
.956
.957
.961
,959
9.0
,861
.862
,861
.869
.866
13.0
.762
.765
.767
.774
,774
17.0
.669
.675
.677
.684
.686
22.0
.572
,577
.579
.585
.590
27.0
.485
.489
.492
.498
.504
143
COBALT 60 TELETHERAPY SURVEY il. Thompson*, H. Wyckoff**, C. Soares*** *Bureau of Radiological Health, Food and Drug Administration, Rochville, Maryland, **International Commission on Radiation Units and Measurements, Washington, D.C., ***National Bureau of Standards, Department of Commerce, Githersburg, Maryland.
During the past three years the National Bureau of Standards and the Bureau of Radiological Health have been conducting a survey of cobalt-60 teletherapy facilities to determine their accuracy in exposing a phantom to a prescribed dose. All cobalt-60 facilities in the U.S. were invited to participate in this program. Those that agreed were provided a questionnaire regarding dose calculations and facility information, and a set of thermoluminescent dosimeters with instructions for their exposure. As of May 1977, some 700 units were surveyed out of the approximately 1,000 which are licensed to administer therapy in the U.S. Preliminary data indicate that about two-thirds of the respondents were able to calculate the specified 300 rad dose within 5 rads; while 8 percent reported values which varied from the specified dose by more than 15 rads. The mean exposure determined for the set of dosimeters differed from the 300 rad value by less than 5 percent for 83 percent of units; 4 percent of the units produced an exposure which differed by more than 10 percent of the target value. Analyses of exposures and dose calculations will be presented for the various characteristics of the facilities, e.g. type of institution, bed size, location and type of administration. (153)
DESCRIPTION OF A COMPUTER CONTROLLED THERAPY MACHINE Bengt Bjarngard*, Peter Kijewski*, Charles Pashby** *Joint Center for Radiation Therapy, Department of Radiation Therapy, Harvard Medical School, Boston, Massachusetts **Siemens A.G., Walnut Creek, California
We have initiated a project to study the feasibility of optimizing dose distributions by using a computer to control the therapy machine during irradiation of the patient. A Siemens Mevatron XII, modified to allow computer control of some dqzen therapy machine parameters including gantry angle, couch position, dose rate, and collimator jaw position has been installed. The presentation will describe the special features of this therapy machine that are necessary for computer control. 142
A closed loop servo system is used to control position as well as velocity of the motorized parameters of the therapy machine. The associated Communication control software is implemented on a 32K core minicomputer. between computer and therapy machine is in ASCII format via serial lines. The performance of the system as pertains to tracking accuracy will be described. An important design consideration of the system is safe operation in A detailed discussion of system features relevant a clinical environment. to system safteiy and ease of operation by the technologist will be presented. (154)
AN EMPIRICAL STUDY OF ISOCENTRIC DEPTH DOSE DATA FOR WEDGED PHOTON BEAMS
10 MeV
Wing-Chee Lam, Ph.D., David G. Grant, M.A., Bengt A. Lindskoug, Ph.D. Radiation Oncology Physics, The Johns Hopkins Oncology Center, Baltimore, Maryland
High enlergy photon beams produced by a clinical linear accelerator have a complex continuous spectrum. Added wedge filters alter the spectrum and also modify beam scatter depending upon the filter material and thickness. As a result, differences in critical beam parameters, such as percent depthdose, will exist between wedged and open fields. Conventional open field depthdose data have been measured and reported in the literature. Corresponding wedged fields are usually assumed to have the same depth-dose fall-off. Depthdose data of a 10 MeV photon beam for various field sizes have been measured for both open and wedged fields on a Varian Clinac-18. Standard brass wedges supplied by the Varian Associates were used. The results clearly demonstrate that differences in depth-dose between wedged and open fields could amount to 7%. The table presents isocentric percentage depth-dose for a 10 cm x 10 cm field for open and for wedged fields. The isocentric depth dose is defined as the ratio of dose at isocenter at depth "d" to the dose at isocenter at depth "dmax" an'd where the field size is defined at isocenter. ISOCENTRIC PERCENTAGE DEPTH DOSE FOR 10 x 10 FIELD Open
0
0
Field
WZge
WZge
45O Wedge
1.00
1.00
1.00
1.00
0
WZge 1.00
5.0
.957
.956
.957
.961
,959
9.0
,861
.862
,861
.869
.866
13.0
.762
.765
.767
.774
,774
17.0
.669
.675
.677
.684
.686
22.0
.572
,577
.579
.585
.590
27.0
.485
.489
.492
.498
.504
143