VOL. io8, No. 2 EARLY CARCINOMA OF THE VOCAL CORDS* KILOVOLTAGE RESULTS; TELECOBALT 60 DOSIMETRY (A PRELIMINARY REPORT) By B. H. I EI)ER, M.D.,t J. W. SCHAEI LEIN, M.SC., and J. J. STEIN, M.l). LOS ANGELES, CALIFORNIA U NTIL I. KILOVOLTAGE RESULTS May 1965 the primary treatment for early carcinoma of the vocal cords at the Long Beach Veterans Administration Hospital was kilovoltage radiation therapy. In all, 120 cases of laryngeal carcinoma have received primary radiation therapy since 1953. Of these, 32 uncomplicated, consecutive cases of infiltrative squamous cell carcinoma, limited to one or to both true vocal cords, with no impairment of mobility, received primary kilovoltage therapy. These cases would be staged by the American Joint Committee system as Stage i or as Stage ii with mobility unimpaired. These 32 patients were treated either on a 220 kv. constant potential Westinghouse unit or a 300 kv. G.E. Maxitron unit from January 1953 to May 1965. The half-value layer (HVL) was either 2 or 4 mm. Cu. The skin-source distance (SSD) was 50 cm. Opposing 5 cm. circles or cm. X cm. squares were centered on the true cords by means of open ended cones, with the patient sitting upright for the most part. No setup was made without the direct participation of a radiation therapist or a resident physician in radiation therapy. Tissue exposures varied from an estimated 5,00o r to 6,500 r delivered in 5 to 7 weeks. Three of these 32 cases could have been considered indeterminate (2 died of intercurrent disease with no evidence of recurrent laryngeal cancer and i was lost to follow-up before a 4 year period had elapsed). However, these 3 cases have been included as failures in this report. Twenty-six cases have remained alive and well without evidence of recurrence for 4 to i years, but 3 developed recurrent disease. All 3 had total larvngectomies: i is alive and well for 8 years; i died apparently free of laryngeal disease after 6 years; and the third (lied after I year (without frank evidence of residual disease). The gross 4 year survival rate free of disease for kilovoltage radiation therapy, plus laryngectomv for residual disease, is 28/32 or 88 per cent (Table i). (If one were inclined to eliminate the 3 indeterminate cases, the corrected survival rate could be given as 28/29 or 97 per cent for this group, and 26/29 or 90 per cent for the group without laryngectomy for residual disease.) II.DOSIMETRIC DATA Following acquisition of telecobalt 6o equipment we became interested in some of the dosimetry problems involved, particularly with laryngeal cancer. One concern was the distinction between the nominal and the dosimetric field size. Since there is a difference in size of fields measured at the o% and at the 90% isodose lines for some teletherapy equipment, we proceeded to collect some dosimetric data of our own. The data were collected under the following conditions: for field homogeneity, single beam studies in plastic blocks at 0.5 * Presented at the Fifty-first Annual Meeting of the American Radium Society, Philadelphia, Pennsylvania, April 27-30, 1969. Partially supported by the American Cancer Society Grant No. T-a99D. t Chief, Radiotherapy Section, Veterans Administration Hospital, Long Beach, California and Clinical Professor (Radiology), University of California at Los Angeles, School of Medicine. Chief, Radiological Physics Section, Veterans Administration Hospital, Long Beach, California and Clinical Instructor, Department of Radiological Sciences, University of California at Irvine, School of Medicine. Professor of Radiology and Chief, Division of Radiation Therapy, University of California at Los Angeles, School of Medicine. 269
270 B. H. Feder, J. \V. Schaeflein and J. J. Stein FEBRUARY, 1970 0 0 FIG. 1. Nominal cm.x5 cm. fields (at the surface); measured at 4.0 ciii. depth. (1) Kilovoltage, 1.7 mm. Cu half-value layer (HVL); 50 cm. source-skin distance (SSI)); open ended cone. (B) Telecobalt 60; O cm. SSD. cm. and cm. depth; for coaxial opposing beams, studies in an Alderson-Rando phantom; and in vivo studies in patients under treatment. Dose distributions were also drawn from patient contours and isodose charts furnished by the cobalt 6o unit manufacturer. Only pertinent dosimetric data will be presented here; a complete presentation will be published later. We shall not dwell on the subject of wedge filters or angled beams at this time. TABLE 1 UNCOMPLICATED CARCINOMA LIMITED TO TRU E CORDS; MOBILITY UNIMPAIRED; RESULTS OF PRIMARY KILO- VOLTAGE THERAPY; SURVIVAL FOR 4 TO 15 YEARS Radiation Therapy Alone (32 cases)* ALIVE WITHOUT DISEASE Laryngectomy for Recurrencef Total 26 2 28 (88%) * Includes 3 indeterminates (a died of intercurrent disease, i lost to follow-up). t 3 recurrences (2/3 cured by total laryngectomy). Most of the data were collected with thermoluminescent dosimeters (extruded lithium fluoride rods of i mm. diameter X6 mm. length). Some fields and distributions were measured simultaneously with Kodak Translite film and thermoluminescent dosimeters (TLD). For this study, comparisons were made of the following fields: a 5 cm.x5 cm. square at 50 cm. SSD, 2 mm. Cu HVL; a nominal 5 cm.xs cm. square (at the o% isodose line) at 8o cm. SSD, cobalt 6o unit and a nominal 7 cm.x7 cm. square (at the o% isodose line) at 8o cm. SSD on the cobalt 6o unit. The cobalt 6o teletherapy unit is equipped with a 2.0 cm. diameter source (originally 5,000 curies), a Picker collimator Model No. 3347D (with a source to penumbra trimmer distance of approximately 43 cm.), and a source head, Picker Catalog No. 9oD. The source to axis of rotation distance (SAD) is 8o cm. Figures I, 2 and 3 show the measured field homogeneity patterns for nominal 5 cm. X 5 cm. and 7 cm. X7 cm. fields, both at the surface and at a 4 cm. depth. It can
VOL. 108, No. 2 Early Carcinoma of Vocal Cords 271 0 0 FIG. 2. Nominal 5 cm.x5 cm. fields (at the surface). (4) Kilovoltage, 3.7 mm. Cu HVL; 50 cm. SSD; open ended cone; measured at the surface. (B) Telecobalt 6o; 8o cm. SSD; measured at 0.5 cm. depth. be seen that at 4 cm. depth in a cuboidal plastic phantom, with the kilovoltage roentgen-ray beam (Fig. Izi), the distance between the 90% and the o% isodose lines is only about 0.5 cm., whereas with the 0 0 telecobalt 6o beam (Fig. IB), the distance between the 90% and 50% lines is i cm. or more. This distance measures about 0.75 cm. on the surface (Fig. 2B). Thus a surface nominal 5 cm.xc cm. field measures less FIG. 3. Nominal 7 cm.x7 cm. fields (at the surface). Telecobalt 6o; 8o cm. SSD; measured at (4) 0.5 cm. depth and (B) 4.0 cm. depth.
272 B. H. Feder, J. \V. Schaetlein and j. J. Stein FEBRUARY, 1970 tic.. The assembled.alderson- Rando phantom. than. cm. X. cm. at the 90% isodose lines, 4 cm. below the surface of the plastic phantoni on our equipment. Another phase of the study was con- (lucted with the Alderson-Rando phantom. Figures 4, #{231} and 6 show the assembled phantom, the superior surface of slab (approximate level of the true cords), an(l a FIG. 5. The superior surface of slab j of the phantom, approximate level of the true cords. roentgenogram of the phantom loaded with metal plugs to show location of dosimeters. Figures 7, 8 and 9 show the resulting dose distributions through slab when the phantom was irradiated with bilateral opposing fields. These (lose distributions, all normalized to a central point between the true cords, show that the kilovoltage #{231} cm. X s cm. nominal field (Fig. 7) delivers 8o% to 90% in the area of the posterior commissure, while the telecobalt, 5 cm. X 5 cm. nominal (co%) field (Fig. 8) delivers only 6c% to the same area. Figure shows that the telecobalt, 7 cm. X7 cm. nominal (50%) field delivers 90 to 95% to the same posterior commissure area. (These findings suggest that the use of ordinary opposing nominal s cm. X s cm. telecobat 6o fields may result in undertreatment at the posterior commissure area.) 13G. 6. A lateral roentgenogram of the phantom showing among others, slab (representative of the true cord level) loaded with metal plugs. Multiple measurements of exposure were made at points indicated by the plugs.
\OL. io8, No. 2 Early Carcinoma of Vocal Cords 273 FIG.. Dose distribution through slab : opposing 4 cm.x5 cm. fields; kilovoltage, 1.7 mm. Cu HVL; ocm. SSI); open ended cone. FIG. 8. Dose distribution through slab : opposing 5 cm.x5 cm. fields; telecobalt 6o; 8o cm. SSI).
274 B. H. Feder, J. \V. Schaeflein and J. J. Stein FEBRUARY, 1970 One example of another phase of the study is presented in Figure 10, which shows a representative in vivo measurement. The tissue equivalent LiF dosimeters, interspersed with metallic markers for visualization, are present in an intratracheal catheter. The roen tgenogram, with exposure measured in a plane perpendicular to the plane of the phantom, depicts in vivo dosimetric (letermination in the area of the posterior commissure that is consistent with the results of the phantom studies. III. DISCUSSION Irradiation alone results in cure of 8o to 90 per cent of Stage I (T1N0M,) cancer of the vocal cords according to most authors.l.i47.s.l1li14.0 Our results with kilovoltage irradiation are in agreement with those authors. Our telecobalt 6o data do not as vet lend themselves to an evaluation of cure rate. Tissue exposures reported may vary from as much as 7,500 r delivered in weeks to as little as 5,000 r spread over as much as 8 weeks. This represents an acceptable spectrum, admittedly subject to many variables. Specification of absorbed dose has been well standardized in recent years, and is a small problem, comparatively speaking. The specification of field size, however, presents considerable confusion. The field size widths reported vary from 3 cm. to 7 cm., but the convention employed to designate the field size for cobalt 6o teletherapy is not always clear. Dosimetric field sizes may be expressed as the width of the o% isodose curve measured in air. The- are often expressed as the width of the o% isodose curve measured at a 0.5 cm. depth in a unit density phantom, which really differs little from the measurement in air. The widths of the 8o% or the 90% isodose curves have also been used. The ICRU geometric convention (lefines the field edge by a line from the center of the front face of the source, passing through the corners of the diaphragm system (i.e., the geometric o% line). Because the margin of the field is not 13G. 9. l)ose distribution through slab : opposing 7 cm.x7 cm. fields; telecobalt 6o; 8o cm. SSD.
VOL. 108, No. Early Carcinoma of Vocal Cords 275 sharp and clearly defined, specification of field size should be according to some generally accepted standard convention, as suggested by the ICRU in 1962. Unfortunately neither the ICRU nor any other such convention is universally employed. On the other hand, the kilovoltage roentgen-ray beam can, for all practical purposes, have a clearly defined, sharp edge. This is the result of the relatively small focal spot and the effectiveness of a few millimeters of lead as a beam shaper. Specification of field size for kilovoltage therapy is thus no problem. Such designation is simply based on the area receiving primary irradiation. However, cobalt 6o teletherapy has characteristics which contribute to a large penumbra and indistinct field edge. These characteristics are: () the relatively large source diameter; (2) the considerable thickness of absorber necessary to limit the beam; and () the distance required between the end of collimating FIG. TO. A lateral roentgenogram of a patient s neck. The intratracheal catheter running just in front of the posterior commissure contains Lithium Fluoride (LiF) dosimeters, interspersed with metallic markers for visualization; a lead marker on the skin indicates the center of the field. Kilovoltage beam, 5 cm.x5 cm.; open ended cone; 1.7 mm. Cu HVL; 50 cm. SSD. -2CM---- 5CM 4CM #{149}DOSIMETRIC(90%). DOSIMETRIC (90%) NOMINAL (50%) FIG. u. Diagram of a cm. X cm. nominal telecobalt 6o field showing possible limits of the location of the 90% isodose line, depending upon source diameter, collimator, geometry, etc. devices and the skin to preserve proper skin-sparing. These characteristics contribute to variations between the nominal field size and the dosimetric field size (measured at a given isodose line). It is therefore recommended that, as a general rule, the radiation therapists records of telecobalt 6o field size give both the nominal field size (usually at the o% isodose line as recommended by the ICRU) and a dosimetric field size (preferably at the 90% isodose line). A nominal field size of cm. X cm., for example, is shown by a cm. X cm. light field projected onto the patient. This is usually at the o% isodose line and therefore corresponds to a dosimetric field which will irradiate a 5 cm. X cm. area of tissue to at least o% of the central ray dose. A dosimetric field size of #{231} cm. X cm. at the 90% isodose line, however, is one that will irradiate a 5 cm. X cm. area of tissue to at least 90% of the central ray dose. A cm. X cm. nominal field (at the 50% isodose line) could be 3 cm. wider than its dosimetric field at the 90% line. Dosimetric fields at the 90% line may differ from each other by as much as
276 B. H. Feder, J. W. Schaeflein and J. J. Stein FEBRUARY, 1970 Dosimetric (90%)* cm. Xcm. 3X3 5X5 8X8 IOXIO I2X12 15XI5 i8xis TABLE CORRELATION BETWEEN NOMINAL (o%) AND DOSIMETRIC (90%) FIELDS TELECOBALT 6o FIELD SIZES AT 8o cm. S5D Nominal (so%)t cm. Xcm. 4X4 5.5X5.5 7X7 ioxio I4X14 17XI7 2IX21 25X25 * \Vidth of 9o#{231}7 isodose curve, at 0.5 cm. depth. t Collimator settings for surface field, perpendicular to central ray. 2 cm. on different telecobalt 6o units (Fig. II). This has been documented by Hall7 and others. Hall stresses the need for the general employment of a convention to specify telecobalt 6o field sizes. Similarly, Karzmark et al. also emphasize this need. Karzmark et al. note the dependence of tumor dose homogeneity on penumbra width. They suggest one simple solution that can be employed if the telecobalt light localizer does not incorporate indicators for both a 50% and a 90% isodose field width. One can set the calibrated diaphragm controls to the measured width of the particular isodose curve desired. We prefer the 90% isodose lines, particularly when we are treating for cure and the volume of interest is relatively small. \Ve have prepared a table (Table II) which shows the consistent differences between width of fields on the surface, measured at the o%, and at the 90% isodose curves for our unit. If we assume that 6,ooo rads in 6 weeks is a desirable dose for carcinoma limited to the true cords, and that the entire volume of interest should be irradiated to a dose that is no less than 90% of the desired dose, that is to no less than 5,400 rads, then the location of the 90% isodose line becomes critical. This is particularly true for telecobait 6o equipment. Hence, it is our opinion that, particularly II where telecobalt 6o is used to treat vocal cord cancer, it may be well to employ fields that are measured at the 90% isodose line rather than at the o% isodose line. We prefer a 5 cm.x5 cm. dosimetric field, measured at the 90% line. SUMMARY 1. Kilovoltage therap\ of32 consecutive cases of carcinoma limited to one or both vocal cords, with no impairment of mobility, has resulted in the gross 4 year survival without disease of 88 per cent of the cases. 2. The field size description for telecobait 6o units, although clearl set forth by the ICRU, ma\ present the radiation therapist with unique difficulties. If one is to irradiate the volume of interest to within 90% of a desired (lose, the location of the 90% isodose line becomes critical. 3. The width of a nominal field (usually measured at the o% isodose line as recommended by the ICRU) and the width of a dosimetric field that is measured at the 90% isodose line may differ by more than 3 cm. for a small field. Therefore records of field size might give both the nominal and the dosimetric field size for each telecobalt 6o unit.. Clinically, it appears desirable to measure telecobalt 6o fields for vocal cord cancer at the 90% isodose line. We prefer a cm. X cm. dosimetric field, at the 90% line. B. H. Feder, M.D. Veterans Administration Hospital 5903 East Seventh Street Long Beach, California 9080! The assistance and advice of Dr. H. R. Haymond, Dr. C. A. Sondhaus, Dr. J. E. Morgan and Dr. Halvor Vermund are gratefully acknowledged. The cooperation of Dr. E. Stemmer, Dr. A. Swirsky, and Dr. J. Shramek of our Surgical Service made this report possible. Mr. Robert Dupree and Miss Billye Washington provided technical assistance. Mr. Timothy Dodge and staff prepared the illustrations. Mrs. Elinor Strain typed the manuscript.
VOL. io8, No. Early Carcinoma of Vocal Cords 277 RE FERENCES i. B.acisss, F. Comparative study of results obtained with conventional radiotherapy (2o0 kv) and cobalt teletherapy in treatment of cancer of larynx. C /in. Radio/., 1967, z8, 292-300. 2. BALLANTYNE, A. J., and FLETCHER, G. H. Preservation of larynx in surgical treatment of cancer, recurrent after radiation therapy. AM. J. ROENTGENOL., RAD. THERAPY & NUCLEAR MED., 3967, 99, 336-339. 3. CHAHBAZIAN, C. M., and DEL REGATO, J. A. Cobalt 6o teletherapy of early carcinoma of vocal cords. AM. J. ROENTGENOL., RAD. THERAPY & NUCLEAR MED., 3967, 99, 333-335. 4. DEL REGATO, J. A., and CHAHBAZIAN, C. M. Cobalt 6o teletherapy of early carcinoma of vocal cords. Tr. Pacific Coast Oto-Ophth. Soc., 3966, 47, 77-80. 5. FLETCHER, G. H., and KLEIN, R. Dose-timevolume relationship in squamous-cell carcinoma of larynx. Radio/ogy, 1964, 82, 1032-1042. 6. GRIMMETT, L. G., FLETCHER, G. H., and MOORE, E. B. Improved light localizer for x-ray therapy. Radio/ogy, 3954, 62, 589-593. 7. HALL, E. J. On the specification of field size for telecobalt units. AM. J. ROENTGENOL., RAn. THERAPY & NUCLEAR MED., 1964, 92, 207-212. 8. HIRBS, G. G., and HENDRICKSON, F. R. Telecobalt therapy of early malignant tumors of vocal cords. Radio/ogy, 1966, 86, 9. HOLT, J. A. G. Place of radiotherapy in management of laryngeal cancer. 7. Coil. Radiol. 4ust., 1965, 9, 399-203. Jo. JOLLE5, B. Long term results of treatment of cancer of larynx. C/in. Radio/., 3966, /7, 71-78. II. KARZMARK, C. J., SAMPIERE, V. A., and STOVALL, M. Standardisation of field size specification in megavoltage radiotherapy. Brit. 7. Radio/., 3968, 41, 712-713. 32. LATH ROP, F. D. Evaluation of supervoltage radiation therapy for carcinoma of larynx. 4nn. Otol., Rhin. & Laryng., 1968,77,493-507. 13. LE JEUNE, F. E., Cox, R. H., and HAINDEL, C. J. Review of available literature on larynx for 3963. Laryngoscope, 1964, 74, 1653-1675. 14. ORCHARD, P. G. Decrement lines: new presentation of data in cobalt 6o beam dosimetry. Brit. 7. Radio/., 3964, 37, 756-763. i. WANG, C. C., and SCHULTZ, M. D. Radiation therapy of malignant laryngeal neoplasms. Eye, Ear, Nose & Throat Month., 1965, 45, 33-38.
This article has been cited by: 1. P. C. Philip. 1976. The Study of Dose Distribution in Radiotherapy using Three Types of Thermoluminescent Dosimeters*. Australasian Radiology 20:2, 179-184. [CrossRef] 2. M Hjelm Hansen, K. Josrgensen, A. Sell. 1975. Carcinoma of the Larynx V. Relationship between biologic effect and failure of irradiation. Acta Radiologica: Therapy, Physics, Biology 14:4, 305-317. [CrossRef] 3. E. Müller. 1972. Die Frühformen des rezidivierenden Stimmlippencarcinoms, ihr morphologisches Bild und ihre Behandlung. Archiv für Klinische und Experimentelle Ohren- Nasen- und Kehlkopfheilkunde 202:2, 602-608. [CrossRef] 4. Richard D. Marks, G. Slaughter Fitz-Hugh, William C. Constable. 1971. Fourteen years' experience with Cobalt-60 radiation therapy in the treatment of early cancer of the true vocal cords. Cancer 28:3, 571-576. [CrossRef]