Download the impact of positron emission tomography/computed tomography

Int. J. Radiation Oncology Biol. Phys., Vol. 68, No. 2, pp. 388 –395, 2007
Copyright © 2007 Elsevier Inc.
Printed in the USA. All rights reserved
0360-3016/07/$–see front matter
doi:10.1016/j.ijrobp.2006.12.029
CLINICAL INVESTIGATION
Head and Neck
THE IMPACT OF POSITRON EMISSION TOMOGRAPHY/COMPUTED
TOMOGRAPHY IN EDGE DELINEATION OF GROSS TUMOR VOLUME FOR
HEAD AND NECK CANCERS
HANI ASHAMALLA, M.D., F.C.C.P.,*† ADEL GUIRGIUS, M.D., M.S.,* EWA BIENIEK, M.D.,*
SAMEER RAFLA, M.D.,*† ALEX EVOLA, R.T.(R)(N),† GANESH GOSWAMI, PH.D.,†
RANDALL OLDROYD, M.D.,* BAHAA MOKHTAR, M.D.,*† AND KAPILA PARIKH, M.D.*†
*Department of Radiation Oncology, New York Methodist Hospital, Weill Medical College of Cornell University, New York, NY;
and †Leading Edge Radiation Oncology Services, Brooklyn, NY
Purpose: To study anatomic biologic contouring (ABC), using a previously described distinct halo, to unify
volume contouring methods in treatment planning for head and neck cancers.
Methods and Materials: Twenty-five patients with head and neck cancer at various sites were planned for
radiation therapy using positron emission tomography/computed tomography (PET/CT). The ABC halo was
used in all PET/CT scans to contour the gross tumor volume (GTV) edge. The CT-based GTV (GTV-CT) and
PET/CT-based GTV (GTV-ABC) were contoured by two independent radiation oncologists.
Results: The ABC halo was observed in all patients studied. The halo had a standard unit value of 2.19 ⴞ 0.28.
The mean halo thickness was 2.02 ⴞ 0.21 mm. Significant volume modification (>25%) was seen in 17 of 25
patients (68%) after implementation of GTV-ABC. Concordance among observers was increased with the use of
the halo as a guide for GTV determination: 6 patients (24%) had a <10% volume discrepancy with CT alone,
compared with 22 (88%) with PET/CT ( p < 0.001). Interobserver variability decreased from a mean GTV
difference of 20.3 cm3 in CT-based planning to 7.2 cm3 in PET/CT-based planning ( p < 0.001).
Conclusions: Using the “anatomic biologic halo” to contour GTV in PET/CT improves consistency among
observers. The distinctive appearance of the described halo and its presence in all of the studied tumors make
it attractive for GTV contouring in head and neck tumors. Additional studies are needed to confirm the
correlation of the halo with presence of malignant cells. © 2007 Elsevier Inc.
PET/CT planning, Head-and-neck cancers, GTV contouring, Anatomic biologic halo.
Positron emission tomography (PET) with the glucose analog 18F-fluoro-2-deoxy-D-glucose (FDG) is a functional
imaging method that detects areas of increased glucose
metabolism. Because neoplastic cells overexpress glucose
transporters and are, therefore, characterized by increased
glucose uptake, most neoplastic tissues can be detected by
PET. The fusion of PET with computed tomography (PET/
CT) adds anatomic information to the physiologic information of PET, thus allowing for improvement in spatial resolution. The role of PET/CT scan in detecting and staging
head and neck cancers is reported in several publications
(1–10). The sensitivity and specificity of PET/CT in this
disease varies from 67% to 100% (1–10). There is still
controversy regarding contouring methods for radiation
treatment planning. Riegel et al. (11) recently reported
significant variation in the delineation of gross tumor volume (GTV) using PET/CT. They concluded that the varia-
tions were due to a lack of consensus about contouring
protocol. One of the reasons for these inconsistencies is the
controversy among institutions regarding how to define the
threshold of malignant disease according to physiologic
imaging.
In a recent publication, we described a halo phenomenon
observed in 19 patients with lung cancer (12). The halo was
recognizable by a specific color, slim wall, low standard
unit value (SUV) uptake, and its location around the areas of
the maximum metabolic activity of the tumor. We also
described the anatomic biologic contour (ABC), representing the volume contoured based on PET/CT images, and the
counterpart of GTV contoured using CT alone. We showed
that use of the halo in delineating GTV resulted in a reduction of interobserver variability, as well as a modification of
GTV in 53% of cases when compared with CT-based planning. This halo may serve as a useful tool to unify the
contouring of volumes among observers.
Reprint requests to: Hani Ashamalla, M.D., F.C.C.P., 506 Sixth
Street, Brooklyn, NY 11215. Tel: (718) 780-3677; Fax: (718)
780-3688; E-mail: [email protected]
Conflict of interest: none.
Received Oct 30, 2006, and in revised form Dec 6, 2006.
Accepted for publication Dec 7, 2006.
INTRODUCTION
388
Head and neck cancers: PET/CT treatment planning
As reported in this article, we prospectively studied the
utility of the halo-based contouring method, using fully
integrated PET/CT planning in a group of head and neck
cancer patients. Our objectives were to: (1) study the presence of the anatomic biologic halo in head and neck cancer
and its utility in defining a standard treatment volume using
PET/CT images, (2) assess the degree of correlation between CT-based GTV (GTV-CT) and the corresponding
PET/CT-based ABC treatment volumes (GTV-ABC), and
(3) evaluate the magnitude of interobserver (radiation oncologist) variability in the delineation of GTV-ABC compared with that of GTV-CT treatment volumes.
METHODS AND MATERIALS
PET/CT simulation protocol
We used the GE Discovery ST (GE Healthcare, Waykesher, WI)
which combines a light-speed CT 16-slice, in-line with PET bismuth germinate oxide detectors. Ordered subset expectation maximization was used for the reconstruction algorithm. Slice smoothing was performed by posterior filter (5.14 mm) and loop filter
(4.69 mm), and the slice thickness was 3.75 mm. Full width at half
maximum for 1 cm was 4.8 mm and for 10 cm was 6.3 mm. Three
cross-laser pointers and a flat-topped table were integrated with the
machine for simulation purposes. Thermoplastic or vacuummolded immobilization devices needed for conformal radiotherapy
were custom fabricated. Patients were injected with a standard
dose of 10 mCi FDG and left in the designated “quiet room” in the
radiation oncology suite for an uptake period of 1 hour. Then,
patients were escorted to the PET/CT scanner in the adjacent room
(10 feet) and placed on the PET/CT machine in the treatment
position, using the previously constructed immobilization devices.
For the sake of reproducibility, an anterior and two lateral reference points were tattooed on the patient, using the laser crossmarks. A full-body PET/CT scan was then performed. The scan
was electronically transmitted to Xeleris (GE Healthcare) and
Eclipse (Varian Medical Systems, Palo Alto, CA) treatment-planning work stations. Coregistration was performed automatically.
Fused PET/CT images were adjusted to a window of 35,000
Bq/mL, whereas the level was adjusted at 30,000 Bq/mL; these
parameters resulted in a major concordance between the treatmentplanning images and the PET/CT scan without the need to vary the
PET threshold.
Treatment volume determination
Each FDG-PET study was reviewed with the interpreting nuclear radiologist before tumor volumes were contoured. To compare CT-based treatment planning with PET/CT-based planning,
treatment volumes were contoured independently by two radiation
oncologists. Blinded to the results of PET, the physicians first
contoured the GTV from the CT data sets. The GTV-CT volumes
included anatomically abnormal and contrast-enhancing regions,
as well as lymph nodes ⬎1 cm. The contrast-enhancing regions
were delineated using slice-by-slice comparison of the simulation
CT with the diagnostic contrast-CT performed before the simulation procedure. Clinical, endoscopic, and other radiologic studies
(e.g., magnetic resonance imaging [MRI]) were used in refining
the GTV. The GTV-ABC was then contoured separately using
fully fused PET/CT imaging. The GTV-ABC included PET-enhancing tumor and gross anatomic lesions, as well as any lymph
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389
node with an average SUV of ⱖ2.5, regardless of any deficiency
in adequate nodal size criteria for malignancy as visualized by CT
images alone. The anatomic biologic halo was recognized as the
edge to be used for delineation of the GTV-ABC.
An assessment of the degree of correlation between GTV-ABC
and GTV-CT was then performed, and interobserver variability
was estimated.
Additionally, to compare pre- and posttherapy values, we recorded the anatomic biologic value (ABV), defined as the product
of mean SUV multiplied by the diameter of GTV-ABC.
Patients
Twenty-five patients with head and neck cancers were studied.
The studied tumors included 6 oropharynx, 4 nasopharynx/paranasal sinus, 4 supraglottis, 4 lymph nodes with unknown primary,
3 oral cavity, 2 thyroid, 1 hypopharynx, and 1 parotid. Median age
was 68 years (range, 57– 81 years). Patient characteristics are listed
in Table 1. Intensity-modulated radiotherapy was used in 14 patients; the remaining patients were treated with standard radiation
techniques.
Statistical considerations
The values of GTV-CT and GTV-ABC and the mean of planned
treatment volumes were recorded. The absolute differences of
planned treatment volumes between the two observers were computed. Wilcoxon signed rank tests and sign tests for numeric
parameters were used, whereas chi-square or Fisher’s exact tests
were used for categoric data. For numeric parameters, the mean
values are given ⫾ 1 SD. SAS (SAS Institute, Cary, NC) and
STATA (Stata Corp., College Station, TX) software packages
were used for statistical analysis.
RESULTS
Significance of the anatomic biologic halo in defining a
standard edge for GTV delineation in radiation treatment
planning for head and neck tumors
The same halo that we previously described in lung
cancer cases (12) was observed around areas of maximal
Table 1. Patient characteristics
No. of patients
Age (y)
Median
Range
Gender
Male
Female
Tumor location
Oropharynx
Nasopharynx/paranasal sinus
Lymph nodes of unknown primary
Supraglottis
Oral cavity
Thyroid
Hypopharynx
Parotid
Stage
II
III
IV
25
68
57–81
16
9
6
4
4
4
3
2
1
1
6
9
10
Data are number of patients, unless otherwise specified.
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Fig. 1. Anatomic biologic halo. Intense 18F-fluoro-2-deoxy-D-glucose
uptake in nasopharyngeal carcinoma demonstrating the halo phenomenon in two color maps. (a) Computed tomography perfusion; (b) hot iron.
SUV uptake in all of the patients with head and neck
tumors included in this study. The identification of the
halo was made on the basis of its characteristic blush,
which was observed around the areas of the highest
Volume 68, Number 2, 2007
metabolic activity, regardless of the color map used (Fig.
1). The mean halo thickness was 2.02 ⫾ 0.21 mm (median, 1.2 mm; range, 1.7–2.4 mm).
In addition, we observed a gradual decrease in tumor
metabolic activity in all of the studied head and neck
lesions. The highest SUV registered in the center of the
tumor, with the values steadily decreasing toward the
periphery. The mean halo SUV was 2.19 ⫾ 0.28 (median,
2.3; range, 1.6 –2.9). The distinct appearance of the halo,
its relatively uniform thickness, characteristic color, and
low SUV uptake were used to define the edge of the
GTV-ABC.
Table 2 demonstrates the diameter of each patient’s
GTV-ABC volume, the maximum and mean SUV of
each contoured volume with the corresponding halo
thickness, and unit value reading. The halo SUV as a
percentage of the maximum (halo intensity level) and
mean SUV are also shown. The mean halo intensity level
was 24% ⫾ 13.06%. The median halo intensity level was
19.2%.
Table 2. Anatomic biologic halo parameters in head-and-neck cancer patients
Case no.
Diagnosis
ABC diameter
(cm)
Max SUV
Mean SUV
Halo thickness
(mm)
Halo SUV
Halo intensity
level*
ABC diameter
⫻ mean SUV
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Max sinus
Nasopharynx
Nasopharynx
Ethmoid
Parotid
Hypopharynx
Supraglottis
Supraglottis
Supraglottis
Supraglottis
Oropharynx
Oropharynx
Oropharynx
Oropharynx
Oropharynx
Oropharynx
LN†
LN†
LN†
LN†
Oral cavity
Oral cavity
Oral cavity
Thyroid
Thyroid
6.1
5.7
2.3
1.7
4.9
3.1
4.1
3.4
8.3
2.3
1.3
2.8
5.7
1.4
4.7
4.9
7.3
1.2
5.3
2.5
5.8
6.0
7.3
2.6
2.5
12.6
12
9.6
7.7
8.7
13
14.3
14
20.7
4.8
5.5
9.6
14
4.4
17.8
5.7
23.3
4.9
12.8
10.1
4.7
20
23.2
6
6.7
10
8.5
7
6.5
7.2
10.4
11.3
9.8
16
4.2
4.5
7.5
12.5
4.0
15.5
5.0
21.5
4.7
11
8.5
4.5
17
19.8
4.5
6.0
2.3
1.9
2
1.7
1.9
1.8
2.2
1.9
2.1
2.1
2.2
1.8
1.9
1.9
1.9
1.9
2.1
2.2
2.1
2.2
1.9
2.2
2.1
1.7
2.4
1.8
2.3
2.7
2.3
2.3
2.1
2.6
2.4
2.0
2.2
2.1
2.5
2.3
2.3
2.4
2.2
2.6
2.4
2.4
2.1
2.5
1.9
2.6
1.9
2.1
14.3
19.2
28.1
29.9
26.4
16.2
18.2
17.1
9.7
45.8
38.2
26
16.4
5.2
13.5
38.6
11.2
48.9
18.8
20.8
53.2
9.5
11.2
31.7
31.3
61
48.6
16.1
11
35.3
32.3
46.3
33.3
132.8
9.6
5.9
21
71.3
5.6
72.9
24.5
157
5.6
58.3
21.3
26.1
102
144.5
11.7
15
Abbreviations: ABC ⫽ anatomic biologic contouring; SUV ⫽ standard unit value.
* Halo intensity level ⫽ halo SUV as a percentage of maximum target SUV.
†
Lymph nodes with unknown primary.
Fig. 2. (a) Oropharyngeal cancer demonstrating modified computed tomography (CT)-based gross tumor volume (GTV)
(yellow) and positron emission tomography (PET)/CT-based anatomic biologic contouring (ABC) (green). (b) Baseof-tongue cancer; CT-GTV (red), PET/CT-ABC (blue). (c) Floor-of-mouth cancer; CT-GTV (red), PET/CT-ABC
(blue). (d) Floor-of-mouth cancer; CT-GTV (yellow), PET/CT-ABC (green). (e) Nasopharyngeal cancer; CT-GTV
(yellow), PET/CT-ABC (green).
Head and neck cancers: PET/CT treatment planning
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Impact of PET/CT-based edge delineation on
volume modification
Overall (ⱖ25%) modification of GTV using PET/CT
planning vs. CT-based planning was seen in 17 of 25
patients (68%). Of these 17, 11 demonstrated reduction in
the GTV-CT. The reason for volume reduction was overstated lymph nodes in 9 cases (6 on the opposite side, and
3 on the same side), and the other 2 patients had volume
reduction due to inclusion of postoperative scarring. Six
patients demonstrated increase in volume with PET/CT; in
4 this was owing to unrecognized contralateral lymph nodes
and in 2 to unsuspected base of skull involvement. Figure 2
shows modification of GTV using PET/CT planning in
various neck cancer sites.
Influence of GTV-ABC contouring on
interobserver variability
The use of the proposed halo-based contouring approach
resulted in significant decrease in variability of GTV outlines
done by the two physicians. The amplitude of difference in
GTV-CT and GTV-ABC is illustrated in Fig. 3. The differences between the two observers using CT and PET/CT in
planning are further illustrated in a box plot in Fig. 4. Interobserver GTV variability decreased from a mean volume difference (mean of sum of differences between the two volumes) of
20.3 cm3 (in CT-based planning) to 7.2 cm3 (in PET/CT-based
planning), with a respective decrease in SD from 17.1 to 7.9 ( p
⬍ 0.001). Regression analysis (Fig. 5) demonstrates that if the
absolute difference between two observers using CT planning
is 1 cm3, then the absolute difference between the same two
observers using PET/CT is only 0.34 cm3 (95% confidence
interval [CI], 0.21– 0.47). The variations in treatment volumes
between observers using CT alone is illustrated in Fig. 6a, and
the concordance in delineating ABC by using PET/CT is
demonstrated in Fig. 6b.
DISCUSSION
The integration of PET and CT scans allows the simultaneous utilization of biologic and anatomic imaging data.
Although anatomic information obtained from CT has been
routinely used as a part of radiation treatment planning, the
use of functional images for planning and contouring of the
tumor volume remains to be standardized.
Part of the current controversy stems from interinstitutional variability in defining the threshold for delineating
malignant disease according to physiologic images. Application of different guidelines for tumor volume delineation
can result in profound differences in the contouring of final
target volumes. Some have arbitrarily advocated the FDGavid volume as the region encompassed by the 50% intensity level relative to the tumor maximum intensity (13–17),
whereas Bradley et al. (18) used the 40% intensity level. In
an editorial, Paulino and Johnstone (19) suggested autocontouring of all areas with an SUV of 2.5. Scarfone et al.
(20) observed that the threshold of PET images needed to be
adjusted on a case-by-case basis to adequately depict FDG-
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Fig. 3. Magnitude of changes in computed tomography– based
gross tumor volume (GTV-CT) and anatomic biologic contouring–
based GTV (GTV-ABC) among observers of head and neck cancer
patients.
avid disease relative to the background. In a similar study
(11) of 16 head-and-neck cancer patients, four observers
contoured GTV in CT and PET/CT images. Significant
variation across physicians’ PET/CT volumes was observed
( p ⫽ 0.0002). The investigators concluded the need for a
unified contouring protocol for PET/CT treatment planning.
We recently reported (12) on the use of PET/CT as a
treatment planning tool for lung cancer. We advocated the
use of the anatomic biologic volume to represent the contoured GTV when PET/CT is used. A halo was found
without the need to vary the PET threshold in all 19 lung
cancer patients studied. Having adjusted the window and
level to 35,000 and 30,000 Bq/mL, respectively, the halo
was used to contour the PET/CT-based GTV with a decrease in interobserver GTV variability from a mean volume difference of 28.3 cm3 (in CT-based planning) to 9.12
cm3 (in PET/CT-based planning) ( p ⬍ 0.0001), with a
respective decrease in SD from 20.99 to 6.47.
The same halo phenomenon was also true in 25 headand-neck cancer patients studied. The mean SUV of the halo
was 2.19 ⫾ 0.28 (range, 1.6 –2.9) at the peripheral edge of
the ABC. This relatively low SUV at the edge of PET-based
GTV is ⬍2.5, the currently accepted value for detecting
malignancy, which has a sensitivity, specificity, and accuracy of 97%, 82%, and 92%, respectively (21). The anatomic biologic halo was detected with a mean thickness of
2.02 ⫾ 0.21 mm (range, 1.7–2.4 mm). The same halo phenomenon was also observed in abdominopelvic sites. The latter
will be discussed in a separate work. The halo thickness and
SUV reading in lung, head-and-neck, and abdominopelvic
sites were found to be consistent (Table 3). Such consistency
and reproducibility may, therefore, indicate use of the halo as
a potential standardized method for contouring.
The main weakness in this work, as well as in similar
studies, is the lack of histopathologic confirmation. While
these confirmatory studies are desperately awaited, indirect
proofs of the specificity of PET/CT as a potential planning
tool were offered by Schwartz et al. (7). Twenty patients
had preoperative CT scans and PET/CT images before neck
dissection. The agreement between the imaging results and
pathology findings of neck dissection was stronger for
Head and neck cancers: PET/CT treatment planning
Fig. 4. Box plot of the differences among observers using computed tomography (CT) and positron emission tomography
(PET)/CT scans.
FDG-PET/CT (␬, 0.95; 95% CI, 0.82– 0.99) than for CT
alone (␬, 0.81; 95% CI, 0.63– 0.91; p ⫽ 0.06), indicating
that FDG-PET/CT is superior to CT alone for geographic
localization of diseased neck node levels.
Other studies continued to show the superiority of
PET/CT as a planning tool. Ciernik et al. (22) evaluated the
feasibility of integrated PET/CT in radiation planning.
Among 39 patients with solid tumors studied, 12 were of
head and neck origin. The addition of overlay PET data
significantly modified the target volume in 6 of them.
Daisne et al. (23) studied 30 patients with oropharyngeal
cancers who underwent MRI and PET coregisteration with
CT planning images. There was significant modification of
the target volume in 37%. Nishioka et al. (24) observed
higher detection of involved nodes; 39 nodes were detected
by PET/CT, compared with only 28 by clinical examination
and CT/MRI in 21 patients with nasopharyngeal or oropharyngeal cancers. Nodal target volume was increased in 4 of
them (19%). Furthermore, parotid sparing was more feasible in 71% of patients after reducing target volume accord-
Fig. 5. A scatter plot of the correlation between computed tomography (CT) and positron emission tomography (PET)/CT differences among observers, with the regression line and the 95%
confidence intervals. ABC ⫽ anatomic biologic contouring;
GTV ⫽ gross tumor volume.
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Fig. 6. The variation in treatment volumes between observers
using computed tomography (CT) alone (a) vs. positron emission
tomography/CT (b) for planning.
ing to PET/CT planning. At 18-month follow-up, no recurrences were seen. Koshy et al. (21) studied 36 patients with
intact primary head and neck cancers, using PET/CT as part
of treatment planning. Radiotherapy volume and dose were
altered in 5 patients (14%) and 4 patients (11%), respectively. Wong and Saunders (25) evaluated 17 patients with
neck cancers using PET/CT images, which resulted in modification of the radiation therapy plan in 53% of cases.
Interobserver variability was addressed by several investigators. Ciernik et al. (22) reported that the mean difference in
GTV with CT alone was 26.6 cm3, compared with a mean
difference of only 9.1 cm3 with PET/CT. Syed et al. (26)
demonstrated that FDG-PET planning has resulted in an improvement of the interobserver degree of confidence in anatomic localization of 51%. They noted that interobserver
agreement in assigning primary and nonprimary lesions to
anatomic territories was moderate with FDG-PET alone (␬
coefficients of 0.45 and 0.54, respectively) but almost perfect
with FDG-PET/CT (␬ coefficients of 0.90 and 0.93, respectively).
There is considerable variation among investigators in
defining the edge of PET/CT-based GTV. Other investigators
have used 40 –50% of the maximum SUV intensity levels as
the edge of GTV (15–18). We defined halo intensity level as
the percentage ratio of halo SUV to the maximum SUV (Table
2). The mean of this value in 25 patients was 24%. Therefore,
using the halo to define the GTV edge encompasses the previously proposed PET/CT-based contouring techniques. In addition, it includes all areas with an SUV of 2.5, which reportedly corresponds to threshold of malignancy (21).
In this work, a clinically significant (ⱖ25%) modification of
GTV was observed in 17 of 25 patients (68%). The concordance among two observers, defined as the percentage of cases
that had ⱕ10% volume discrepancy relative to the mean treatment volumes, was increased from 24% with CT-based planning to 88% with PET/CT planning ( p ⬍ 0.001). The mean
absolute volume difference was reduced from 20.3 cm3 (in
CT-based planning) to 7.2 cm3 (in PET/CT-based planning),
with a respective decrease in SD from 17.1 to 7.9 ( p ⬍ 0.001).
There are two reasons for the reduction in observer
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Table 3. Consistency of anatomic biologic halo in different sites
Site
No. of patients
Halo thickness (mm)*
Halo SUV (mm)*
Lung
Head and neck
Abdominopelvic
20
25
16
2.03 ⫾ 0.22 (1.5–2.5)
2.02 ⫾ 0.21 (1.7–2.4)
1.9 ⫾ 0.14 (1.7–2.1)
1.98 ⫾ 0.19 (1.5–2.3)
2.19 ⫾ 0.28 (1.6-2.9)
2 ⫾ 0.17 (1.6–2.3)
Abbreviation: SUV ⫽ standard unit value.
* Mean ⫾ SD (range).
discrepancies. First, the use of PET/CT in treatment
planning, as illustrated by others, reduces ambiguities in
target delineation. The second plausible reason is the
application of the halo in delineation of the GTV edge.
The halo’s characteristic hue, relatively uniform SUV, and
small thickness make it a clearly identifiable edge around the
regions of maximum uptake. This in turn allows for improved
reproducibility in gross tumor edge delineation between independent observers.
CONCLUSIONS
Positron emission tomography/computed tomography
planning is a promising tool for refining traditional treat-
ment volumes for radiotherapy for head and neck cancer.
The ABC is advocated to replace CT-based GTV because it
combines data about both anatomy and function of lesions.
The characteristic appearance of the halo, with its thin wall
and low SUV, provides excellent parameters for future
standard use for contouring. Its use in treatment planning
resulted in overall alteration of GTV in 68% of patients. In
addition, interobserver variability is significantly reduced
when an ABC-based definition of GTV is used. Thus, the
anatomic biologic halo is proposed as a border of contouring owing to its reproducibility in various types of head and
neck cancers. These data need to be further validated by
other groups to plan future studies with minimal variations
among participating institutions.
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