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The computation of dosage in interstitial and intracavitary radiation therapy
J. &on.
Dis. 1966, Vol. 19, pp. 519-522. Pergamon
Press Ltd. Printed in Great Britain
THE COMPUTATION OF DOSAGE IN INTERSTITIAL AND INTRACAVITARY RADIATION THERAPY* ROBERTJ. SHALEKand MARILYNS-rovALLt Department
of Physics, The University
of Texas, M. D. Anderson Houston, Texas
Hospital and Tumor Institute,
WITHIN two years after its discovery in 1898, radium was employed for the treatment of malignant disease. In 1913, the survival of a patient treated seven years previously for carcinoma of the cervix was reported [l]. As in other forms of radiation therapy, techniques of treatment evolved by trial and error. For lack of better quantitation, the unit of milligram-hours as a measure of radiation treatment was employed. This unit for the description of radiation treatment gives the product of the amount of radioactive material and the time of treatment but tells nothing of the geometrical arrangement of the radiation sources or of the resulting distributions of absorbed radiation in the structures under treatment. Nonetheless, it was a useful unit which continues today to have some applicability as a rough indicator of requisite dose and patient tolerance. In the early 1930’s, a transition was made to calculations which were based on the energy absorbed in the tissue rather than the source strength or the treatment time. However, because of the complexity of the dose distributions and calculations, systems were devised which permitted the calculation from tables of a single average dose or the dose to one or a few points. Not until the availability of automatic computers has it been feasible to calculate the radiation distributions in detail for individual cases. INTERSTITIAL
IMPLANTATIONS
Of several systems which were invented for quantification of radium therapy, the QUIMBYsystem [2] and the PATERSON-PARKER system [3] continue in clinical use. The Quimby system supplied the calculation procedure for existing treatment methods which utilized uniform sources at equal spacings. The Paterson-Parker method prescribed geometries of sources of two strengths, one twice the other, so that the dose delivered at 0.5 cm from a plane or the dose on lines through a volume were uniform, within limits. These methods, which permit the calculation of a single dose rate representing the complex radiation distribution around an array of needles, have served for 30 years as successful guides to interstitial treatment. The dose so calculated is used to determine the time required for the implant to deliver a prescribed dose. Computer methods for implantations of radioactive seeds or needles permit the description of the full radiation distributions resulting from a particular geometry of sources [4-lo]. Since the placement of sources in the patient is never exactly as planned, *Supported in part by grant C-6294 from the National Cancer Institute, U.S. Public Health Service. ton leave to the International Atomic Energy Agency, Vienna, Austria. 519
520
ROBERT J. SHALEK and MARILYN STOVALL
the calculation of the dose distributions are most useful when they are based upon the array of sources as they exist in the individual patient. In Fig. 1, the radiographs of a radium needle implant and the resulting calculation of the dose distribution in a particular plane through the implant are shown. The geometrical relationship between the sources was determined from the radiographs. The lines are isodose lines, that is, lines connecting points of equal dose; in the method shown, the isodose lines are plotted automatically by an incremental plotter in planes that may be selected arbitrarily [I 1, 121. Frequently, more than one plane will be requested for dose calculation in an individual treatment. For calculations of this type, a fairly large computer of the CDC 1604 or the IBM 709 class is required. In the input to the computer, the length, activity, specific gamma ray emission, thickness of filter and tissue absorption are variables which are specified in the input data. Thus, the program may be utilized for seeds or linear sources of any radioactive material of any length. The radiation distributions are calculated as soon as possible after the implantation; the time of treatment or the alteration of treatment by the removal of source units at various times is determined from the distribution. An important consideration is the clinical usefulness of the detailed information which the computer develops for individual cases. At this time, there is no information contrasting cure rates from implants calculated by tabular methods and those calculated by computer methods. However, in a retrospective study, previously treated cases of cancer of the tongue and floor of the mouth which had been calculated by tabular methods were recalculated by computer [13]. Of the 50 cases which were chosen because of local recurrence or necrosis, more than two-thirds of the complications could be explained on the basis of local underdose or overdose as revealed by the isodose distributions calculated by computer. Thus, it seems likely that the added dosimetric information supplied by the computer calculations for interstitial implantations will be effective in upgrading the quality of treatment. INTRACAVITARY THERAPY The treatment of carcinoma of the uterine cervix by radium was one of the earliest uses for radium in the treatment of cancer and, indeed, the treatment of this disease with local intracavitary radiation continues to be a major method of therapy. This method of treatment has the advantage that a high radiation dose can be delivered to the tumor while a relatively low dose is delivered to normal tissues. In addition, the placement of radiation sources is accomplished easily with little trauma to the patient. The milligram-hour description of gynecological radiation treatment has been utilized by many physicians. Another method of dose control has been the calculation of the dose to two points which are located at specific distances from the radium system [14]. More recently, a manual method for the calculation of a single isodose curve in a single plane defined by the radium system has been described [15]. With computer methods, families of isodose curves in arbitrary planes can be calculated (Fig. 2). From such dosimetric information, an estimate can be made of the sufficiency of the treatment in the tumor volume, of the dose delivered to other areas such as the nodes on the pelvic wall or the intestine, and of the precision of the matching of supplementary radiation delivered by external beam techniques. An interesting possibility of optimizing the dose by the selection of sources is feasible with afterloading applicators [16, 171. These applicators, without sources in
FIG. 1. An interstitial radium implant of the floor of the mouth. The implant is composed of ten Indian-club needles, I .8 mg radium each, 4.5 cm active length, 0.65 mm platinum filtration. The anteroposterior and lateral radiographs together with the isodose curves in a plane biThe numbers refer to I-ads per hour. secting the needles are shown. A 1 mm grid is in the background.
FIG. 2. An intracavitary implant of Fletcher cervical applicators. The applicators are loaded with 15 i 10-t IO+6 mg radium tubes in the uterine The The sources are 1*5 cm active length with I ‘0 mm platinum filtration. tandem and 15 l-15 mg radium tubes in the vaginal colpostats. inieroposterior and lateral radiographs together with the isodose curves in the plane pass through the internal OSand the centers of the colpostats. The numbers refer to rads per hour. A 1 mm grid is in the background.
The Computation
of Dosage
in Interstitial and Intracavitary Radiation Therapy
521
place, are placed in the patient in the operating room; the sources are loaded later through the hollow handles of the applicators. From radiographs of the applicators in treatment position, the calculation of the radiation distribution from loadings of several possible source strengths prior to the insertion of the sources into the applicators would permit the selection of the best combination of sources. In this way, a new flexibility of treatment is possible in which the sources are suited to the disease and to the geometry of the applicators. It is too soon to evaluate whether the improved dosimetric information from computer calculations will have the effect of increasing cure rates and reducing the incidence of complications. The choice of the size of computer to be used for such calculations is an important economic consideration. A small size computer of the IBM 1620 or CDC 160A class has been used successfully for the computation of dose distributions around gynecological radiation sources [18, 191. The output from these machines is less elaborate than that from the larger machines and thus may require somewhat more time for interpretation. Another possibility is the communication to a large computer at a distance by remote input-output devices or by telephone data input with mail return. The latter method of telephone input and mail return has been used successfully for several years for interstitial implantations where the treatment times range from 5 to 7 days. This method would likely not be as useful for gynecological treatments which usually require 3 days or less of irradiation time. The cost of the calculations at commercial rates is lo-25 dollars for each patient. Thus, the cost is in the range of minor diagnostic procedures and is a small fraction of other costs during radiation treatment. SUMMARY At this time, it seems likely that interstitial and intracavitary radiation therapy will be improved by the availability of isodose radiation distributions for individual treatments early in the treatment, or before the sources are applied with afterloading applicators. The possibility of utilizing computers at a distance has been demonstrated so that the methods described can be used at treatment centers that may not have computers available at the site. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
ABBE, R.: The QUIMBY, E. H.:
use of radium in malignant disease, Lancet 2, 524, 1913. The grouping of radium tubes in packs or plaques to produce the desired distribution of radiation, Am. J. Roe&g. 27, 18, 1932. PATERSON,R. and PARKER, H. M.: A dosage system for gamma ray therapy, Br. J. Radial. 7, 592, 1934. NELSON, R. F. and MEURK, M. L.: The use of automatic computing machines for implant dosimetry, Radiology 70,90, 1958. SHALEK, R. J. and STOVALL, M.: The calculation of isodose distributions in interstitial implantations by computer, Radiology 76, 119, 1961. STOVALL, M. and SHALEK, R. J.: A study of the explicit distribution of radiation in interstitial implantations. I. A method of calculation with an automatic digital computer, Radiology 78, 950, 1962. LAUGHLIN, J. S., SILER, W. M., HOLODNY, E. I. and RITTER,F. W.: A dose description system for interstitial radiation therapy; seed implants, Am. J. Roentg. 89,470, 1963. mit der elektronischen Rechenmaschine BUSCH, M.: Ober die Berechnung der Dosisverteilung bei interstitieller Anwendung von radioaktiven Seeds, Struhlentherapie 121, 549, 1963. HOPE, C. S., LAURIE, J., ORR, J. S. and WALTERS, J. H.: The computation of dose distribution in cervix radium treatment, Physics Med. Biol. 9, 345, 1964.
522
ROBERTJ. SHALEKand MARILYNSTOVALL
10. POWERS,W. E., BOGARDUS,C. R., WHITE, W. and GALLAGHER,T.: Computer estimation of radiation dose in interstitial and intracavitary imptants, Radiology 85, 135, 1965. 11. STOVALL.M. and SHALEK,R. J. : Automatic calculation of isodose curves from implants of radioactive sources: I. Physical and clinical aspects. (In preparation). 12. BATTEN,G. W. : Automatic calculation of isodose curves from implants of radioactive sources: II. Analytical aspects. (In preparation). 13. FLETCHER,G. H. and STOVALL,M. : A study of the explicit distribution of radiation in interstitial implantations: II. Correlation with clinical results in squamous-cell carcinomas of the anterior two-thirds of tongue and floor of mouth, Radiology 78,766, 1962. 14. TOD, M. C. and MEREDITH,W. J.: A dosage system for use in treatment of cancer of uterine cervix, Br. J. Radiol. 11, 809, 1938. 15. FLETCHER,G. H., WALL, J. A., BLOEDORN,F. G., SHALEK, R. J. and WOO~N, P.: Direct measurements and isodose calculations in radium therapy of carcinoma of the cervix, Radiology 61, 885, 1953. 16. HENSCHKE,V. K., HILARIS, B. S. and MAHAN, G. D.: Afterloading in interstitial and intercavitary radiation therapy, Am. J. Roentg. 90, 386, 1963. 17. SUIT, H. D., MOORE,E. B., FLETCHER,G. H. and WORSNOP,R.: Modification of Fletcher ovoid system for afterloading, using standard-sized radium tubes, Radiology 81, 126, 1963. MEURK, M. L.: The use of a computer to calculate 18. ADAMS, G. D. and surrounding distributed gynaecological radium sources, Physics Med. Biol. 9, 533, 1964. 19. ADAMS,R. M., PETERSON,M. D. and COLLINS,V. P. : Clinically useful calculations of the dose distribution from multiple radiation sources, Radiology 85, 361, 1965.
Dis. 1966, Vol. 19, pp. 519-522. Pergamon
Press Ltd. Printed in Great Britain
THE COMPUTATION OF DOSAGE IN INTERSTITIAL AND INTRACAVITARY RADIATION THERAPY* ROBERTJ. SHALEKand MARILYNS-rovALLt Department
of Physics, The University
of Texas, M. D. Anderson Houston, Texas
Hospital and Tumor Institute,
WITHIN two years after its discovery in 1898, radium was employed for the treatment of malignant disease. In 1913, the survival of a patient treated seven years previously for carcinoma of the cervix was reported [l]. As in other forms of radiation therapy, techniques of treatment evolved by trial and error. For lack of better quantitation, the unit of milligram-hours as a measure of radiation treatment was employed. This unit for the description of radiation treatment gives the product of the amount of radioactive material and the time of treatment but tells nothing of the geometrical arrangement of the radiation sources or of the resulting distributions of absorbed radiation in the structures under treatment. Nonetheless, it was a useful unit which continues today to have some applicability as a rough indicator of requisite dose and patient tolerance. In the early 1930’s, a transition was made to calculations which were based on the energy absorbed in the tissue rather than the source strength or the treatment time. However, because of the complexity of the dose distributions and calculations, systems were devised which permitted the calculation from tables of a single average dose or the dose to one or a few points. Not until the availability of automatic computers has it been feasible to calculate the radiation distributions in detail for individual cases. INTERSTITIAL
IMPLANTATIONS
Of several systems which were invented for quantification of radium therapy, the QUIMBYsystem [2] and the PATERSON-PARKER system [3] continue in clinical use. The Quimby system supplied the calculation procedure for existing treatment methods which utilized uniform sources at equal spacings. The Paterson-Parker method prescribed geometries of sources of two strengths, one twice the other, so that the dose delivered at 0.5 cm from a plane or the dose on lines through a volume were uniform, within limits. These methods, which permit the calculation of a single dose rate representing the complex radiation distribution around an array of needles, have served for 30 years as successful guides to interstitial treatment. The dose so calculated is used to determine the time required for the implant to deliver a prescribed dose. Computer methods for implantations of radioactive seeds or needles permit the description of the full radiation distributions resulting from a particular geometry of sources [4-lo]. Since the placement of sources in the patient is never exactly as planned, *Supported in part by grant C-6294 from the National Cancer Institute, U.S. Public Health Service. ton leave to the International Atomic Energy Agency, Vienna, Austria. 519
520
ROBERT J. SHALEK and MARILYN STOVALL
the calculation of the dose distributions are most useful when they are based upon the array of sources as they exist in the individual patient. In Fig. 1, the radiographs of a radium needle implant and the resulting calculation of the dose distribution in a particular plane through the implant are shown. The geometrical relationship between the sources was determined from the radiographs. The lines are isodose lines, that is, lines connecting points of equal dose; in the method shown, the isodose lines are plotted automatically by an incremental plotter in planes that may be selected arbitrarily [I 1, 121. Frequently, more than one plane will be requested for dose calculation in an individual treatment. For calculations of this type, a fairly large computer of the CDC 1604 or the IBM 709 class is required. In the input to the computer, the length, activity, specific gamma ray emission, thickness of filter and tissue absorption are variables which are specified in the input data. Thus, the program may be utilized for seeds or linear sources of any radioactive material of any length. The radiation distributions are calculated as soon as possible after the implantation; the time of treatment or the alteration of treatment by the removal of source units at various times is determined from the distribution. An important consideration is the clinical usefulness of the detailed information which the computer develops for individual cases. At this time, there is no information contrasting cure rates from implants calculated by tabular methods and those calculated by computer methods. However, in a retrospective study, previously treated cases of cancer of the tongue and floor of the mouth which had been calculated by tabular methods were recalculated by computer [13]. Of the 50 cases which were chosen because of local recurrence or necrosis, more than two-thirds of the complications could be explained on the basis of local underdose or overdose as revealed by the isodose distributions calculated by computer. Thus, it seems likely that the added dosimetric information supplied by the computer calculations for interstitial implantations will be effective in upgrading the quality of treatment. INTRACAVITARY THERAPY The treatment of carcinoma of the uterine cervix by radium was one of the earliest uses for radium in the treatment of cancer and, indeed, the treatment of this disease with local intracavitary radiation continues to be a major method of therapy. This method of treatment has the advantage that a high radiation dose can be delivered to the tumor while a relatively low dose is delivered to normal tissues. In addition, the placement of radiation sources is accomplished easily with little trauma to the patient. The milligram-hour description of gynecological radiation treatment has been utilized by many physicians. Another method of dose control has been the calculation of the dose to two points which are located at specific distances from the radium system [14]. More recently, a manual method for the calculation of a single isodose curve in a single plane defined by the radium system has been described [15]. With computer methods, families of isodose curves in arbitrary planes can be calculated (Fig. 2). From such dosimetric information, an estimate can be made of the sufficiency of the treatment in the tumor volume, of the dose delivered to other areas such as the nodes on the pelvic wall or the intestine, and of the precision of the matching of supplementary radiation delivered by external beam techniques. An interesting possibility of optimizing the dose by the selection of sources is feasible with afterloading applicators [16, 171. These applicators, without sources in
FIG. 1. An interstitial radium implant of the floor of the mouth. The implant is composed of ten Indian-club needles, I .8 mg radium each, 4.5 cm active length, 0.65 mm platinum filtration. The anteroposterior and lateral radiographs together with the isodose curves in a plane biThe numbers refer to I-ads per hour. secting the needles are shown. A 1 mm grid is in the background.
FIG. 2. An intracavitary implant of Fletcher cervical applicators. The applicators are loaded with 15 i 10-t IO+6 mg radium tubes in the uterine The The sources are 1*5 cm active length with I ‘0 mm platinum filtration. tandem and 15 l-15 mg radium tubes in the vaginal colpostats. inieroposterior and lateral radiographs together with the isodose curves in the plane pass through the internal OSand the centers of the colpostats. The numbers refer to rads per hour. A 1 mm grid is in the background.
The Computation
of Dosage
in Interstitial and Intracavitary Radiation Therapy
521
place, are placed in the patient in the operating room; the sources are loaded later through the hollow handles of the applicators. From radiographs of the applicators in treatment position, the calculation of the radiation distribution from loadings of several possible source strengths prior to the insertion of the sources into the applicators would permit the selection of the best combination of sources. In this way, a new flexibility of treatment is possible in which the sources are suited to the disease and to the geometry of the applicators. It is too soon to evaluate whether the improved dosimetric information from computer calculations will have the effect of increasing cure rates and reducing the incidence of complications. The choice of the size of computer to be used for such calculations is an important economic consideration. A small size computer of the IBM 1620 or CDC 160A class has been used successfully for the computation of dose distributions around gynecological radiation sources [18, 191. The output from these machines is less elaborate than that from the larger machines and thus may require somewhat more time for interpretation. Another possibility is the communication to a large computer at a distance by remote input-output devices or by telephone data input with mail return. The latter method of telephone input and mail return has been used successfully for several years for interstitial implantations where the treatment times range from 5 to 7 days. This method would likely not be as useful for gynecological treatments which usually require 3 days or less of irradiation time. The cost of the calculations at commercial rates is lo-25 dollars for each patient. Thus, the cost is in the range of minor diagnostic procedures and is a small fraction of other costs during radiation treatment. SUMMARY At this time, it seems likely that interstitial and intracavitary radiation therapy will be improved by the availability of isodose radiation distributions for individual treatments early in the treatment, or before the sources are applied with afterloading applicators. The possibility of utilizing computers at a distance has been demonstrated so that the methods described can be used at treatment centers that may not have computers available at the site. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
ABBE, R.: The QUIMBY, E. H.:
use of radium in malignant disease, Lancet 2, 524, 1913. The grouping of radium tubes in packs or plaques to produce the desired distribution of radiation, Am. J. Roe&g. 27, 18, 1932. PATERSON,R. and PARKER, H. M.: A dosage system for gamma ray therapy, Br. J. Radial. 7, 592, 1934. NELSON, R. F. and MEURK, M. L.: The use of automatic computing machines for implant dosimetry, Radiology 70,90, 1958. SHALEK, R. J. and STOVALL, M.: The calculation of isodose distributions in interstitial implantations by computer, Radiology 76, 119, 1961. STOVALL, M. and SHALEK, R. J.: A study of the explicit distribution of radiation in interstitial implantations. I. A method of calculation with an automatic digital computer, Radiology 78, 950, 1962. LAUGHLIN, J. S., SILER, W. M., HOLODNY, E. I. and RITTER,F. W.: A dose description system for interstitial radiation therapy; seed implants, Am. J. Roentg. 89,470, 1963. mit der elektronischen Rechenmaschine BUSCH, M.: Ober die Berechnung der Dosisverteilung bei interstitieller Anwendung von radioaktiven Seeds, Struhlentherapie 121, 549, 1963. HOPE, C. S., LAURIE, J., ORR, J. S. and WALTERS, J. H.: The computation of dose distribution in cervix radium treatment, Physics Med. Biol. 9, 345, 1964.
522
ROBERTJ. SHALEKand MARILYNSTOVALL
10. POWERS,W. E., BOGARDUS,C. R., WHITE, W. and GALLAGHER,T.: Computer estimation of radiation dose in interstitial and intracavitary imptants, Radiology 85, 135, 1965. 11. STOVALL.M. and SHALEK,R. J. : Automatic calculation of isodose curves from implants of radioactive sources: I. Physical and clinical aspects. (In preparation). 12. BATTEN,G. W. : Automatic calculation of isodose curves from implants of radioactive sources: II. Analytical aspects. (In preparation). 13. FLETCHER,G. H. and STOVALL,M. : A study of the explicit distribution of radiation in interstitial implantations: II. Correlation with clinical results in squamous-cell carcinomas of the anterior two-thirds of tongue and floor of mouth, Radiology 78,766, 1962. 14. TOD, M. C. and MEREDITH,W. J.: A dosage system for use in treatment of cancer of uterine cervix, Br. J. Radiol. 11, 809, 1938. 15. FLETCHER,G. H., WALL, J. A., BLOEDORN,F. G., SHALEK, R. J. and WOO~N, P.: Direct measurements and isodose calculations in radium therapy of carcinoma of the cervix, Radiology 61, 885, 1953. 16. HENSCHKE,V. K., HILARIS, B. S. and MAHAN, G. D.: Afterloading in interstitial and intercavitary radiation therapy, Am. J. Roentg. 90, 386, 1963. 17. SUIT, H. D., MOORE,E. B., FLETCHER,G. H. and WORSNOP,R.: Modification of Fletcher ovoid system for afterloading, using standard-sized radium tubes, Radiology 81, 126, 1963. MEURK, M. L.: The use of a computer to calculate 18. ADAMS, G. D. and surrounding distributed gynaecological radium sources, Physics Med. Biol. 9, 533, 1964. 19. ADAMS,R. M., PETERSON,M. D. and COLLINS,V. P. : Clinically useful calculations of the dose distribution from multiple radiation sources, Radiology 85, 361, 1965.