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381
Radiation Therapy for Glottic Cancer Using
6-MV Photons
Robert L. Foote, M.D.’
Gordon L. Grado, M.D!
Steven J. Buskirk, M.D?
Kerry D. Olsen, M.D?
James A. Bonner, M.D.’
John D. Earle, M.D.’
Jan L. Kasperbauer, M.D?
Thomas V. McCaffrey, M.D., Ph.D.2
Vera J. Suman, P ~ . D . ~
’
Division of Radiation Oncology, Mayo Clinic
and Mayo Foundation, Rochester, Minnesota.
Department of Otorhinolaryngology, Mayo
Clinic and Mayo Foundation, Rochester, Minnesota.
Section of Biostatistics, Mayo Clinic and Mayo
Foundation, Rochester, Minnesota.
Department of Radiation Oncology, Mayo
Clinic Scottsdale, Scottsdale, Arizona.
Department of Radiation Oncology, Mayo
Clinic Jacksonville, Jacksonville, Florida.
BACKGROUND. It has been recommended that cobalt-60 or 4-MV photons be used
when treating glottic cancer with radiation therapy. Underdosing may occur when
using higher energy photons, particularly when the anterior commissure is involved. The authors report their experience using higher energy photons (6 MV)
for the treatment of glottic cancer.
METHODS. Between January 1975 and July 1991, 73 patients with Tis, T1, ‘r2, or T3
glottic tumors underwent radiation therapy with curative intent. Cobalt-60 or 4MV photons were used to treat the cancers of 30 patients, and 6-MV photons
were used to treat 43 patients. Clinical records were reviewed retrospectively to
determine patterns of treatment failure, cancer deaths, and local tumor control in
the 43 patients receiving treatment with 6-MV photons. Patients were followed
until death or for a minimum of two years.
RESULTS. Treatment failures were: local recurrence, one patient; local recurrence
and distant metastasis, one patient; delayed neck metastasis, two patients; and
delayed neck metastasis with distant metastasis, one patient. Three patients who
had treatment failure are alive and free of cancer after salvage treatment. Two
patients died of neck and distant metastases. The 2-year initial local tumor control
rate was 94.8%.
CONCLUSIONS. Glottic cancer can be treated successfully with 6-MV photons. Local
tumor control is similar to that reported using cobalt-60 or 4-MV photons. Cancer
1996; 77:381-6. 0 1996 American Cancer Society.
KEYWORDS: glottic cancer, radiation therapy, higher energy photon beam, cobalt-60.
R
Presented at the 2nd World Congress on Laryngeal Cancer, Sydney, Australia, February 20 to
24, 1994.
Address for reprints: Robert L. Foote, M.D.,
Mayo Clinic, 200 First Street SW, Rochester,
MN 55905.
Received June 15, 1995; revision received September 8, 1995; accepted September 8, 1995.
8
0 1996 American Cancer Society
adiation therapy for early glottic cancer is a highly successful form of
treatment. Tumor control rates in large series of patients from major
cancer treatment centers have been reported to be 88% to 96% for Stage
T1 tumors and 52% to 80% for Stage T2 tumors after treatment with
c0balt-60.’-~It has been suggested that the optimal beam energy is cobalt60 or 4-MV photons.” Underdosing may occur when higher energy photons are used, particularly when the anterior commissure is involved,
because of inadequate buildup of electrons in the thin wedge-shaped
anterior neck and at the surface of the vocal cord-air interface. However,
the number of cobalt-60 machines in radiation therapy facilities in the
United States has decreased from 970 in 1975 to 504 in 1990. During this
same time, the number of linear accelerators has increased from 407
to 1893.’’ Therefore, fewer radiation facilities have cobalt-60 machines
available to treat early glottic cancer. If higher energy photons are found
to be less effective, perhaps patients with early stage glottic cancer should
be referred to treatment centers with cobalt-60 machines. The cobalt-60
machine at our institution was decommissioned in 1982. Since that time,
382
CANCER January 15, 1996 / Volume 77 I Number 2
the majority of patients have received treatment with 6MV photons. Our initial review revealed 5-year local tumor control rates of 77% for cobalt-60, 75% for 4-MV
photons, and 100% (16 patients) for 6-MV photons when
treating early (Tis, T1, T2) glottic cancer.” The purpose
of the present review is to determine the pattern of treatment failure, cause of death, and local tumor control rate
when using higher energy photons (6 Mv) for the treatment of glottic cancer.
MATERIALS AND METHODS
Between January 1975 and December 1991, 73 patients
with Stage Tis, T1, T2, or T3 squamous cell carcinoma of
the true vocal cord underwent treatment with curative
intent with radiation therapy at the Mayo Clinic in Rochester, Minnesota, Scottsdale, Arizona, and Jacksonville,
Florida. Of these 73 patients, 30 had treatment with cobalt-60 or 4-MV photons and 43 with 6-MV photons. Clinical records were reviewed retrospectively to determine
patterns of treatment failure and cancer-related deaths
in the 43 patients who underwent treatment with 6-MV
photons. Patients were followed until death or for a minimum of two years.
The tumors were staged according to the 1992 American Joint Committee on Cancer staging ~ y s t e m . Stage
’~
T1 tumors were stratified as follows: Tla, involvement of
one true vocal cord with or without involvement of the
anterior commissure; and Tlb, involvement of both true
vocal cords. Stage T2 tumors were stratified according to
the mobility of the vocal cord(s): T2a, normal mobility,
and T2b, reduced mobility.
The 43 patients consisted of 41 males and 2 females
(median age, 68 years; range, 36 to 88 years). Three patients had Stage Tis tumors, 27 had Stage T1 tumors (15
patients, Tla and 12 patients, Tlb), 10 had Stage T2 tumors (6 patients, T2a and 4 patients, TZb), and 3 patients
had stage T3 tumors. The anterior commissure was involved with tumor in 26 patients. All patients underwent
endoscopic biopsy before radiation therapy.
All radiotherapy was administered with a linear accelerator using a 6-MV photon beam. In four patients, a
three-field treatment technique was used that incorporated an anterior electron beam during the last few treatments. All patients underwent simulation and treatment
in the supine position except for one patient who was
treated in the lateral decubitus position. Since 1988, a
mask has been fabricated for immobilization. In addition
to weekly port films, the treatment fields were verified
each day on the treatment machine by a physician using
the anatomical landmarks of the thyroid and cricoid cartilages. For Tis, T1, and T2 tumors involving the entire true
vocal cord, the superior and inferior borders of the field
extended from the middle of the thyroid notch to the
bottom of the cricoid cartilage; the posterior border was
1 to 1.5 cm behind the posterior aspect of the thyroid
cartilage, and there was a 1- to 2-cm fall off anteriorly.
Planned reductions in the field size during treatment
were rarely performed. The field borders were modified
for each patient, depending on the anatomical extent of
the particular tumor. The median field length was 5 cm,
and the median field width was 6.5 cm. For Stage T3
tumors, the entire neck was initially treated before reducing the field size. Opposed lateral treatment fields were
used in the treatment of 33 patients. Six patients received
treatment with a three-field technique and four patients
with oblique fields. Multiple ( 2 2 ) fields were treated each
day in 40 patients; 1 field was treated each day in 3 patients. Wedge filters were used in 36 patients to improve
the dose homogeneity. Cerrobend blocks were fabricated
for 15 patients. One patient with a narrow anterior neck
received treatment with bolus. The lower cervical nodes
were treated with an anterior low neck field for T3 tumors.
The tumor dose was specified to the isodose volume
that encompassed the tumor, which was determined by
computer-generated dosimetry. The median total dose
delivered was 65.25 Gy (range, 27 to 74.4 Gy). The patient
receiving treatment with 27 Gy died of an unrelated intercurrent medical illness during radiation therapy. The
dose per fraction ranged from 1.2 to 2.3 Gy (median, 2
Gy). Beginning in 1988, the doses have been standardized
to 63 Gy, using 2.25-Gy fractions, for T1 tumors and to
74.4 to 76.8 Gy, using 1.2 Gy per fraction twice a day,
for T2 and T3 tumors. There were 41 patients who had
treatment with once-daily fractionation, and 2 patients
(1with T2 tumor and 1with T3 tumor) who had treatment
with twice-daily fractionation. Seventeen patients received continuous course treatment. In the 26 other patients, treatment was interrupted for a median of 3 days
(range, 1 to 86 days). The median number of days from
the start of radiation therapy to the end of radiation therapy was 45 (range, 16 to 141 days).
All patients were followed until death or for a minimum of two years. The median length of follow-up was
3.4 years. All three patients with Stage Tis tumors were
followed up for two or more years. Of the 15 patients with
Stage T l a tumors, 12 were followed for 2 or more years,
as were 10 of the 12 patients with Stage T l b tumors, 5 of
the 6 patients with Stage T2a tumors, 2 of the 4 patients
with Stage T2b tumors, and 1 of the 3 patients with Stage
T3 tumors. Ten patients died before two years of followup. Therefore, the majority of patients were followed for
two or more years. Most of the local recurrences (290%)
have been reported to occur during the first two years of
follow-up. The Kaplan-Meier method was used to estimate time to initial local failure, time to any initial failure
(local, nodal, or distant), and survival to death from any
cause.I4The log rank test was used to compare survival
and recurrence times. Initial local control was defined as
Radiation Therapy for Glottic Cancer/Foote et al.
383
TABLE 1
Initial Local Control According to T Stage for 43 Patients
Stage
No. of patients
Tis
3
Tla
15
12
6
4
Tlb
T?a*
T?b'
T3
All
3
43
2-year control (%)
SE (%)
100
100
100
83
75
100
94.8
I
SE: standard error.
0
1
2
3
4
5
Years from diagnosis
'lwo-year initial local control for id1 T2 IT?a and T2b) tiimors, 78.8%ISE. 13%).
FIGURE 1. Relapse-free and overall survival.
complete and continuous disappearance of disease at the
primary site. Variables evaluated for potential prognostic
impact on local control included tumor stage and anterior
commissure involvement.
RESULTS
Local Control
Initial local control according to T Stage is outlined in
Table 1. For all patients, the rate of initial local tumor
control at 2 years was 94.8% (standard error [SE] = 3.6%).
There were two local failures (one at 4 months and the
other at 16 months after diagnosis),which were surgically
managed with total laryngectomy. One of the two patients
initially presented with a Stage T2a tumor that would
have required partial vertical laryngectomy. The second
patient presented with a Stage T2b tumor that would have
required total laryngectomy. Pathologically, this tumor
was a Grade 4 spindle cell variant of squamous cell carcinoma. In both patients, the anterior commissure was initially involved and the anterior commissure and true vocal cords were involved with recurrent tumor. When the
anterior commissure was involved, the 2-year initial local
tumor control rate was 91.6% (SE = 5.7%). When the
anterior commissure was not involved, the 2-year initial
local tumor control rate was 100%. This difference was
not statistically significant ( P = 0.26).
Patterns of Treatment Failure and Relapse Free Survival
At latest follow-up evaluation, 38 patients had not experienced treatment failure (local recurrence, nodal metastasis, distant metastasis). Of the five other patients, cervical
node metastases had developed in two, both nodal and
distant metastases in one, local recurrence in one, and
both local recurrence and distant metastases in one. The
2-year relapse free survival was 92.3% (SE = 4.3%; Fig. 1).
Overall Survival
At latest follow-up evaluation, 21 patients were still alive
and free of cancer. Of the 18 patients who had died with
evidence of glottic cancer, 14 died of an intercurrent
medical illness and 4 of second malignancies. One patient
is alive with glottic cancer. Of the three patients who
died of glottic cancer, two died of uncontrolled nodal
or distant metastasis (or both) and the other died of an
intercurrent illness during the course of radiation therapy. The 2-year overall survival was 76.7% (SE = 6.4%;
Fig. 1).
Complications
Chondronecrosis did not develop in any of the patients.
Grade 3 laryngeal edema (according to the Radiation
Therapy Oncology Group (RTOG)/European Organization for Research and Treatment of Cancer (EORTC) late
radiation morbidity scoring schema) developed in one
patient (Stage T2b tumor). This required placement of a
permanent tracheostomy.
DISCUSSION
It has been recommended that the optimal beam energy
for treatment of early glottic cancer is cobalt-60 or 4-MV
photons." The concern is that with higher energy photon
beams underdosing may occur along the air-tumor tissue
interface because of the air cavity within the larynx, resulting in inadequate buildup of electrons at the surface
of the vocal cord cancer. Also, in the relatively thin wedgeshaped anterior neck, inadequate buildup of electrons in
the region of the anterior commissure may occur witn
higher energy photon beams. Dosimetry studies designed
to measure the degree of underdosing have yielded conflicting
(written communication, Dr. Michael
D. Sombeck, November, 1993). The relative merits of 6
MV compared with cobalt-60 for treatment of early vocal
cord cancer are still being investigated. Sombeck and associates (written communication, November, 1993) have
reported a comparison of dose values in an anthropomorphic phantom made of methyl methacrylate (Lucite) for
364
CANCER January 15,1996 I Volume 77 I Number 2
TABLE 2
Radiation Therapy for Glottic Cancer Using 6-MV Photons Literature Review-Local Control for T1 Glottic Cancer
Orthovoltage
Institution
Washington University
Columbia-Presbyterian
Shinshu University
National Cancer Center Hospital, Tokyo
Thomas Jefferson University Hospital
Northwestern University
Gunma University
Maya Clinic
Cobalt 60
Length of
follow-up (yr)
No. of
patients
5-yr control
rate (W)
No. of
patients
5-yr control
rate (96)
3
ND
60
13
78
100
54
16
17
85
87
88
19
46
15
84
85
68
ND
5
2
2
Nn
2
ND: no data.
a specific irradiation geometry (wedged parallel opposed
fields) and technique (prescription to an isodose surface).
Whether their observation of lower dose (16% to 35%) at
the skin surface and for 1 point 3 mm below the anterior
neck surface with 6 MV photons compared with cobalt60 is clinically relevant or universally applicable is still a
question. Doses measured along the surface of the vocal
cords and at the anterior commissure were essentially
identical. Initial reports of phantom dosimetric studies
suggested that the dose absorbed at the anterior commissure may be decreased by 12% with 6-MV photons and
by 18%with 10-MVphotons when compared with cobalt60.’* This is a practical concern because most cobalt-60
units in the United States are being replaced by higher
energy linear accelerators.” However, very limited clinical information is available to evaluate the efficacy of 6MV or higher photon beams in the treatment of early
glottic cancer. No prospective randomized clinical trials
exist. The following is a review of retrospective studies.
At the Mallinckrodt Institute of Radiologyin St. Louis,
281 patients with T1 glottic cancer received the following
treatment: 60 patients with 250-kilovolt peak (kVp) photons, 54 with cobalt-60 gamma rays, 144 with 4-MV photons, and 23 with 6-MV photons. Most patients received
6000 cGy in 200-cGy daily fractions. All patients were followed for a minimum of three years. The 5-year local
tumor control was reported to be 78% for patients receiving treatment with 250-kVp photons, 85% for those receiving cobalt-60,84% for those receiving 4-MV photons,
and 70% for those receiving 6-MV photon^.'^
Sung et al. reported 5-year local tumor control rates
for T1 and T2 glottic tumors of 88% for orthovoltage (26
patients), 84% for cobalt-60 (19 patients), 65% for 6-MV
photons (20 patients), and 76% for 22-MV photons (97
patients).’” The differences in tumor control rates were
significant ( P = 0.05) in favor of cobalt-60 versus 6-MV
photons for Stage T1 tumors (87% versus 60%, respectively). The differences in tumor control rates were also
significant (P= 0.03 and P = 0.01) in favor of orthovoltage
versus 6-MV and 22-MV photons for Stage T1 tumors
(100%versus 60% and 74%, respectively). No wedge filter
or bolus was used with the 22-MV photon beam. Doses
varied from 5000 to 6600 cGy in 200-cGy daily fractions.”
Izuno et al. reported the results of treatment of 53
patients who had TlNOMO glottic tumors.2’ Of these 53
patients, 17 underwent treatment with cobalt-60, 25 with
8- or 10-MV photons, and 11 with 4-MV photons. The
mean total dose was 6330 cGy in the cobalt-60 group,
6590 cGy in the 8- or 10-MV group, and 6380 cGy in the
4-MV group, with 180- or 200-cGy daily fractions. Wedge
filters were used for most treatments in the 8- to 10-MV
and 4-MV groups but not in the cobalt-60 group. The 5year local control rate was 88% (15 of 17) for cobalt-60
and 60% (15 of 25) for 8- or 10-MV photons (P = 0.05).
The follow-up period for the 4-MV photon group was
limited, but the 3-year local control rate was 91% (10
of 11).
These retrospective studies suggest that the estimated 5-year local tumor control rate may be lower with
6-Mv or higher energy photons when compared with cobalt-60 gamma rays or 4-MV photons. However, other
patient-, tumor-, or treatment-related factors maybe responsible for the differences noticed because these are
nonrandomized clinical data.
Akine et al. described the results for 154 patients with
Stage T1 tumors treated with 6-MV photons (8 of 151
patients receiving 6 MV had 6 MV and cobalt-60) or cobalt-60 (3 patients).” In 110 of the patients receiving
treatment with 6-MV photons, no bolus or wedge filter
was used. Doses varied from 5750 to 7250 cGy, with most
patients having 200-cGy daily fractions. The minimum
follow-up period was five years. The 5-year local control
Radiation Therapy for Glottic Cancer/Foote et al.
2Mv
No. of
patients
5-yr control
rate (%)
No. of
patients
5-yr control
rate (96)
144
84
No. of
patients
23
15
4
100
11
91
53
77
90
41
8/10 Mv
6MV
4Mv
5-yr control
rate (W)
5-yr control
rate (%)
25
60
33
89
70
60
151
15
14
86
92
27
100
rate was 89%, comparable with the 88% to 96% rate reported by others using ~ o b a l t - 6 0 . ’ - ~However,
~’~
for the
27 patients with anterior commissure involvement, the
5-year local control rate was 81%, in contrast to 91% for
the 127 patients without anterior commissure involvement. This was of borderline significance univariately ( P
= 0.061, and the analysis did not control for other important treatment factors, such as total dose, fraction size,
and overall treatment time.2z Nevertheless, a trend was
suggested and is of clinical concern, In contrast, a review
of approximately 2200 Stage TI vocal cord tumors treated
with cobalt-60 showed no increase in local failure with
anterior commissure involvement.I0
However, Rudoltz et al. reported the results of using
various photon energies to treat Stage TI glottic tumors
in 91 patient^.'^ The minimum follow-up period was two
years. Wedge filters were used in 50 patients. The median
dose was 6400 cGy. The 5-year local tumor control rate
for cobalt-60 was 84% (19 patients); for 2-MV photons,
100% (4 patients); for 4-MV photons, 77% (53 patients);
and for 6-MV photons, 86% (15 patients). None of these
differences were statistically significant with multivariate
analysis, including evaluation of anterior commissure
involvement ( P = 0.58).23
Small et al. reported the results of using various photon energies to treat Stage TI glottic tumors in 101 pat i e n t ~The
. ~ ~minimum follow-up period was two years.
Wedge filters were used in 25 patients. The mean total
dose and daily fraction dose were 6444 cGy and 228 cGy,
respectively. The 5-year recurrence rates reported for cobalt-60 were 15% (46 patients); for 4-MV photons, 10%
(41 patients); and for 6-MV photons, 8% (14 patients). No
significant differences in outcome were observed among
the groups.z4
Finally, Hayakawa et al. reported a 5-year local tumor
control rate of 68% for 15 patients with Stage TI glottic
No. of
patients
385
22 Mv
No. of
patients
5-yrcontrol
rate (%)
75
74
89
tumors treated with cobalt-60 and 45-degree wedge filters
compared with 85% in 27 patients having treatment with
10-MV photons without wedge filters and 89% in 33 patients receiving 10-MV photons without wedge filters either alone or in combination with cobalt-60.25Total doses
ranged from 6000 to 7000 cGy, with either 300 cGy per
fraction 3 days a week or 200 to 250 cGy per fraction 5
days a week.
These retrospective studies suggest that the estimated 5-year local tumor control rates with 6-, 8-, or 10MV photons may be similar to those achieved with cobalt60 or 4-MV photons. However, these nonrandomized series with a relatively small number of patients have the
same limitations as the studies that suggested inferior
results with the use of 6 - M v or higher energy photons
(Table 2).
We report a 2-year initial local tumor control rate of
100% for Stage T1 glottic tumors and 78.8% (SE = 13.4%)
for Stage T2 glottic tumors treated with 6-MV photons.
No significant difference was noted between the tumors
that involved the anterior commissure and those that did
not (2-year initial local tumor control 91.6% compared
with 100%; caution should be used when interpreting this
because of the small sample size). These results are comparable with the 88% to 96% local tumor control rates
reported for TI glottic tumors and 52% to 80% local tumor
control rates reported for T2 glottic tumors treated with
c0balt-60.*-~
We have previously reported 5-year local tumor control rates of 77% for cobalt-60, 75% for 4-MV
photons, and 100% €or 6-MV photons when treating Stage
Tis, TI, or T2 tumors with radiation therapy.” In that
review, we found that total dose (263 Gy) was a significant predictor of tumor control but photon energy was
not ( P = 0.007 and P = 0.082, respectively). Patients who
had treatment with cobalt-60 or 4-MV photons were more
likely to receive doses of less than 63 Gy. Therefore, on
386
CANCER January 15,1996 I Volume 77 I Number 2
the basis of our experience and that reported by others,
we believe that early glottic cancer can be treated safely
and effectively with 6-MV photons and that results very
similar to those reported using cobalt-60 can be achieved.
For patients with a very thin anterior neck and cancer
involving the anterior commissure, beam spoiling or bolus could be considered to aid in increasing the dose to
the anterior commissure.
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