12 PET/CT in Lung, Head and Neck Cancer
Hans C. Steinert, Gerhard Goerres, and Gustav K. von Schulthess
H. C. Steinert, MD; G. Goerres, MD;
G. K. von Schulthess, MD, PhD
Nuclear Medicine, Department of Medical Radiology, University Hospital of Zurich, 8091 Zurich, Switzerland
scanner is a state of the art four-slice CT (GE Medical Systems, Milwaukee, Wisconsin) with a gantry rota- tion of 0.5” minimum. Extensive evaluation of the CT breath hold breathing pattern best matching the free breathing pattern of PET data acquisition, has led us to conclude that an unforced end-expiratory state is best during the period where the CT images the regions adjacent to the diaphragm (Goerres et al. 2002a). Thus patients are instructed to expire and hold their breath when the CT scanner scans this body region. CT scanning is accomplished in less than 30 s. Subsequently, PET scanning is started from the pelvis up. PET scanning is performed using six to seven table positions. As the PET scanner covers an axial field of view of around 15 cm with 32 slices per table position, roughly 90–105 cm of the patient is covered, which includes the anatomic regions from the brain to the upper thighs in almost all patients.
With table position imaging times of 3–4 min, typical scan length of a PET/CT partial body scan covering the patient from the head to the mid upper thighs is 20–30 min.
After this “baseline” PET/CT data acquisi- tion, additional standard CT protocols can be run depending upon the clinical requirements. It is desirable in some settings to also perform a CT scan enhanced with intravenous contrast, which can better delineate the lesions in relation to vascular structures. The overall protocol design for such all- encompassing PET/CT examinations is not defined and the next years will have to tell where and when additional contrast enhanced CT scanning is useful.
Other groups have advocated the use of i.v. contrast enhanced CT scans from the beginning and as only CT scan. It is our opinion, that this is less than opti- mal for two reasons. First, in many settings, CT con- trast is not needed, as FDG is mostly a much better
“contrast agent” than the contrast agents used in CT.
Second, vascular contrast, which for vessel delinea- tion is dense and transient, causes the CT data not to be ideal attenuation maps for the subsequent PET scan. It has to be emphasized that iodinated contrast behaves in such a way that it can be difficult to use
CONTENTS
12.1 General Aspects of PET/CT 205 12.1.1 Clinical Protocols of PET/CT 205 12.1.2 Critical Appraisal of Clinical PET/CT 206 12.2 PET/CT in Lung Cancer 206
12.2.1 Solitary Lung Nodule 206 12.2.2 T Staging 207
12.2.3 N Staging 207 12.2.4 M Staging 209
12.2.5 Recurrent Lung Cancer 209 12.2.6 Pitfalls 209
12.2.7 Small Cell Lung Cancer 209 12.2.8 Malignant Pleural Mesothelioma 210 12.3 PET/CT in Head and Neck Cancer 210 12.3.1 Staging 210
12.3.2 Treatment Planning 211 12.3.3 Follow-Up 211
12.3.4 The Pros and Cons of PET/CT in Head and Neck Oncology 212
12.4 Conclusion 213 References 213
12.1
General Aspects of PET/CT
12.1.1
Clinical Protocols of PET/CT
As PET/CT is a novel technology, the clinical protocols
in PET/CT are subject to fast evolution. At our institu-
tion we routinely give diluted bowel contrast agent 1 h
prior to scanning (Dizendorf et al. 2002) followed
by supine FDG injection and patient rest of at least
45 min. After bladder voiding just prior to scanning,
we first perform a low-dose CT scan at 40 mAs (Hany
et al. 2002a). This covers 100 cm of axial field of view
in less than 30 s with a slice thickness of 4.5 mm,
which is matched to the PET slice thickness. The CT
a scan enhanced with intravenous contrast agent for attenuation correction (Dizendorf et al. 2003).
Iodine looks like bone at 80–140 keV, but like soft tissue at 511 keV. Thus, the Hounsfield Unit trans- formation used for attenuation correction of the PET scan may not work consistently without segmenting the vessels out of the CT and replacing the vessels with soft tissue density on the CT data.
12.1.2
Critical Appraisal of Clinical PET/CT
Clinical experience with PET/CT is currently lim- ited. However, after performing approximately 5000 clinical scans, we can confidently appraise at least some aspects of clinical PET/CT.
PET/CT is easy to use and in the overwhelming number of patients, image co-registration is excel- lent. This is probably due to the fact that oncology patients, who represent the major patient group imaged, are generally cooperative. To our surprise, routine availability of an anatomic reference frame has led to the identification of various pitfalls not fully appreciated in PET prior to the introduction of PET/CT. The superimposition of an FDG avid focus onto an anatomically identifiable structure makes the image interpreter more confident of what he sees and reduces interobserver variability. This also applies to the use of PET/CT data in radiation planning. The software infrastructure has now been developed which permits transfer of PET and CT data into the planning environment of radiation oncologists by DICOM standard formats.
Viewing PET and CT images next to each other is not adequate for lesion localization once lesions are below 1.5 cm, and it is particularly in such lesions, where PET excels and where standard morphologi- cal criteria of malignancy in CT no longer are very useful. Thus co-registration and fusion or linked cursor viewing are mandatory. All currently avail- able data suggest that the software approach is at least logistically difficult and obviously does not provide easily accessible data for attenuation cor- rection.
Any new modality has also new artifacts and pit- falls. The most relevant artifact identified so far is misregistration around the diaphragm (Goerres et al. 2002a). This can lead to abdominal lesions apparently located in the supradiaphragmatic lung zones and a photon deficit of variable importance overlying the diaphragmatic domes and looking like “bananas” on the coronal CT attenuation cor-
rected PET scans. A second artifact, which may be more prominent in CT corrected PET scans than PET scans corrected with the standard transmis- sion scan methods, is that arising around metallic implants (Goerres et al. 2002b; Kamel et al. 2003a).
Slight misregistrations can lead to overcorrections, which then result in focal or linear regions of appar- ently increased FDG uptake. The last artifact comes from major misregistration due to patient motion between the acquisition of the two scans.
12.2
PET/CT in Lung Cancer
The majority of PET imaging work has been done in non-small cell lung cancer. It has been shown that FDG PET is highly accurate in classifying lung nodules as benign or malignant. Whole-body PET improves the rate of detection of mediastinal lymph node metastases as well as extrathoracic metastases when compared to conventional imaging methods, such as CT, MR, ultrasound or bone scan. Since com- mercial PET scanners provide nominal spatial reso- lution of 4.5–6 mm in the center of the axial field of view, even lesions less than 1 cm with an increased FDG uptake can be detected. This represents a criti- cal advantage of PET over CT and MR. Integrated PET/CT enables the exact matching of focal abnor- malities on PET to anatomic structures on CT. First clinical results show, that integrated PET/CT enables the exact matching of focal abnormalities on PET to anatomic structures on CT resulting in an increased diagnostic accuracy (Lardinois et al. 2003; Bar- Shalom et al. 2003; Antoch et al. 2003a).
12.2.1
Solitary Lung Nodule
The ability of PET to separate between benign and malignant lesions is high, but not perfect. For benign lesions, a high specificity for FDG PET has been dem- onstrated. It has been shown that FDG PET is highly accurate in differentiating malignant from benign solitary pulmonary nodules (0.6–3 cm) when radio- graphic findings were indeterminate (Gupta et al.
1996). In a series of 61 patients, PET had a sensitivity
of 93% and a specificity of 88% for detecting malig-
nancy. However, FDG PET may show negative results
for pulmonary carcinoid tumors and bronchiolo-
alveolar lung carcinoma. Lesions with increased FDG
uptake should be considered malignant, although false-positive results have been reported in cases of inflammatory and infectious processes, such as histoplasmosis, aspergillosis, or active tuberculosis.
PET is clinically useful in patients with a solitary pulmonary nodule less than 3 cm in diameter, espe- cially where biopsy may be risky or where the nodule carries a low risk for malignancy based on patients’
history or radiographic findings. With integrated PET/CT an additional certainty to the presence or absence of FDG uptake in the pulmonary nodule can be achieved.
12.2.2 T Staging
Without image fusion, the use of PET in T staging lung cancer is limited. Recently, it has been shown that integrated PET/CT is superior to CT alone, PET alone, and visual correlation of PET and CT in T staging of patients with non-small cell lung cancer (Lardinois et al. 2003). Due to the exact anatomic correlation of the extent of FDG uptake, the delinea- tion of the primary tumor can be defined precisely.
Therefore the diagnosis of chest wall infiltration and the mediastinal invasion by the tumor is improved.
Lesions with chest wall infiltration are classified as stage T3 and are potentially resectable. Integrated PET/CT provides important information on medi- astinal infiltration too. However, PET/CT imaging is unable to distinguish contiguity of tumor with the mediastinum from the direct invasion of the walls of mediastinal structures. It has been shown that FDG PET is a useful tool for the differentiation between tumor and peritumoral atelectasis. This is particu- larly important for the planning of radiotherapy in patients with lung cancer associated with an atelec- tasis. The information provided by FDG PET results in a change in the radiation field in approximately 30%–40% of patients (Nestle et al. 1999).
12.2.3 N Staging
PET has proven to be a very effective staging modal- ity for mediastinal nodal staging (Steinert et al.
1997; Vansteenkiste et al. 1998; Dwamena et al.
1999; Pieterman et al. 2000; Hellwig et al. 2001).
CT and MR imaging are limited in depicting small mediastinal lymph node metastases. Several studies have demonstrated that FDG PET is significantly
more accurate than CT in determination of nodal status. In our own study, PET assigned the correct N stage in 96% of cases, CT was correct in 79%
of cases (Lardinois et al. 2003). A meta-analytic comparison of PET and CT in mediastinal staging of NSCLC was performed (Dwamena et al. 1999).
The mean sensitivity and specificity (±95% CI) were 0.79±0.03 and 0.91±0.02, respectively, for PET and 0.60±0.02 and 0.77±0.02, respectively, for CT. These results were confirmed in another meta-analysis with a total of more than 1000 patients (Hellwig et al. 2001).
Even if mediastinoscopy remains the gold stan- dard for mediastinal staging, not all mediastinal lymph nodes can be routinely accessed by medi- astinoscopy, particularly in the para-aortic region and in aorto-pulmonary window. The limited view through the scope and the single direction in which biopsies can be carried out prevents 100% accuracy.
The accuracy of mediastinoscopy is approximately 90% and is surgeon dependent (Patterson et al.
1987). It has been demonstrated that PET is useful to assist mediastinoscopy (Fig. 12.1). Due to the knowl- edge from the PET scan, mediastinoscopy revealed additional mediastinal disease in 6% of patients (Kerstine et al. 2002).
Exact allocation of focal abnormalities on PET to specific lymph nodes is difficult or even impos- sible due to the poor anatomic information pro- vided by PET alone. The presence and site of lymph node metastases should be recorded according to the revised American Thoracic Society lymph node station-mapping system (Mountain and Dresler 1997). In patients with bulky mediastinal disease or multilevel nodal involvement the assessment of N stage is easy. However, the exact localization of lymph node metastases in the hilum is difficult.
Lymph nodes distal to the mediastinal pleural reflection and within the visceral pleura are clas- sified as N1 nodes. Lymph nodes within the medi- astinal pleural envelope are classified as N2 nodes.
Because the pleura is visible neither in CT nor in PET
the exact classification of a hilar lesion as a N1 node
or N2 node remains difficult. The difficulty of PET
is the localization of small single nodes, particularly
in patients with a mediastinal shift due to atelecta-
sis or anatomical variants. In our experience, inte-
grated PET/CT imaging will become the new stan-
dard of mediastinal staging. The high reliability of
integrated PET/CT in the exact localization of extra-
thoracic vs. intrathoracic and mediastinal vs. hilar
lymph nodes might have very important therapeutic
implications.
Fig. 12.1a–c. Whole-body PET/CT examination of a 50-year-old male patient who had been suffering hoarseness and a swal- lowing disorder for 6 weeks. Full screen with maximum intensity projection, CT, PET, and PET/CT scans (a). PET/CT reveals a supraglottic laryngeal cancer with ipsilateral lymph node metastases. There is central necrosis in an enlarged lymph node (b; white and black arrows). This was a G1 cT2 N2b squamous cell carcinoma. The patient underwent supraglottic partial lar- yngectomy with radical neck dissection. In the same PET/CT examination a bronchogenic cancer was found with ipsilateral lymph node metastasis in the aortic-pulmonary window (c; white and black arrowheads). This was a pT2 pN2 squamous cell carcinoma of the left upper lung lobe. The patient underwent resection of the upper lung lobe and mediastinal lymph nodes 3 weeks later. After surgery the patient was scheduled for subsequent combined radiation and chemotherapy
a
b c
Until now, our group used non-enhanced CT scans for integrated PET/CT imaging. We could not ethically justify the use of vascular contrast material because all patients had a conventional contrast enhanced CT for staging before the PET/CT. In non-enhanced CT scans delineation of vessels was considerably poorer than with contrast enhancement or impossible. How- ever, in our patient series non-enhanced PET/CT scans are sufficient for planning surgery in approximately 80% of patients. Further evaluation is necessary to define conditions in which the application of intra- vascular contrast material might have an additional diagnostic impact in integrated PET/CT imaging.
However, regarding infiltration of hilar and medias- tinal vessels, a relatively low sensitivity, specificity, and accuracy (68%, 72%, and 70%, respectively) of conventional CT scan with contrast enhancement has been observed (Rendina et al. 1987).
Microscopic foci of metastases within very small lymph nodes cannot be detected with any imag- ing modality. If there is no increased FDG uptake in PET, integrated PET/CT will not provide further information based on FDG accumulation. It has been reported that FDG PET after induction therapy is less accurate in mediastinal staging than in stag- ing of untreated NSCLC (Akhurst et al. 2002). PET over-staged nodal status in 33% of patients, under- staged nodal status in 15%, and was correct in 52%.
Future studies are required to correlate FDG PET before and after treatment.
12.2.4 M Staging
Whole-body FDG PET is an excellent method to screen for extrathoracic metastases (Weder et al.
1998). In a meta-analysis of 581 patients, sensitivity, specificity, and accuracy of FDG PET were 94%, 97%
and 96%, respectively (Hellwig et al. 2001). Current imaging methods are inadequate for accurate M stag- ing of patients. PET detects unexpected extrathoracic metastases in 10%–20% of patients and changes ther- apeutic management in about 20% of patients. FDG PET is more accurate than CT in the evaluation of adrenal metastases (Erasmus et al. 1997). Marom et al. (1999) compared the accuracy of FDG PET to con- ventional imaging in 100 patients with newly diag- nosed NSCLC. Comparing bone scintigraphy and FDG PET in detecting bone metastases, the accuracy was 87% and 98%, respectively. All hepatic metasta- ses were correctly identified with PET and CT. With CT, however, benign liver lesions were over-staged as
metastases, thus accuracy of PET was superior to CT in the diagnosis of liver metastases.
The clinical significance of a single focal abnor- mality on PET remains unclear, especially when no morphological alterations occur on CT images. The advantage of integrated PET/CT imaging is the exact localization of a focal abnormality on PET. This was the case in 20% of all patients with extrathoracic metastases in our study on the value of integrated PET/CT (Lardinois et al. 2003).
12.2.5
Recurrent Lung Cancer
A very high accuracy of FDG PET in distinguishing recurrent disease from benign treatment effects has been shown. If PET images demonstrate areas of tumor viability, they can direct biopsy for pathologic confirmation. Patients should be evaluated a mini- mum of 2 months after completion of therapy. Oth- erwise post-therapeutic healing processes or radia- tion pneumonitis may result in false positive PET findings. These abnormal findings return to normal at variable times without further intervention. In the experience of Inoue et al. (1995) a curvilinear contour of increased FDG accumulation was seen mostly in inflammatory lesions, while focal nodular uptake was seen mostly in recurrent tumors. Their data suggest that FDG PET can be clinically used for selecting biopsy sites because of its high sensitivity in detecting recurrent lung cancer.
12.2.6 Pitfalls
False negative FDG PET results have been reported in pulmonary carcinoid tumors and in bronchiolo-alve- olar carcinomas. Some active infectious or inflam- matory lesions may have an increased FDG uptake.
Tuberculosis, eosinophilic lung disease, histoplasmo- sis, aspergillosis and other infections may have a sig- nificant uptake of FDG (Strauss 1996). Furthermore, sarcoid shows a typical bilateral relatively symmet- ric hilar uptake pattern. Therefore, lesions with an increased FDG accumulation should be histologically confirmed. However, most chronic inflammatory processes do not significantly take up FDG.
It is well known that active muscles accumulate
FDG. In some patients with lung cancer an intense
focal FDG accumulation is seen in the lower anterior
neck just lateral to the midline. Co-registered PET/
CT images revealed that the focal FDG uptake was localized in the internal laryngeal muscles (Kamel et al. 2002). This finding is a result of compensatory laryngeal muscle activation caused by contralateral recurrent laryngeal nerve palsy due to direct nerve invasion by lung cancer of the left mediastinum or lung apices (Hany et al. 2002).
FDG accumulation in brown adipose tissue can easily be identified and discriminated from muscle uptake or a soft tissue mass using co-registered PET- CT.
12.2.7
Small Cell Lung Cancer
The staging procedures for SCLC do not differ from those for NSCLC. The primary role of imaging is to separate accurately limited disease (LD) from extended disease (ED). Based upon the widespread dissemination of SCLC, a battery of imaging tests is performed such as CT of the chest and abdomen, CT or MRI of the brain and a bone scan. Recently, it has been shown that whole-body FDG PET is a useful tool for staging SCLC (Kamel et al. 2003b).
FDG PET is superior to conventional staging in the detection of all involved sites, and particularly in the assessment of mediastinal lymph node metastases.
Our first experience demonstrated that integrated PET/CT imaging in SCLC is a highly valuable tool for planning radiation treatment. It is useful for accu- rate target definition by reducing the probability of overlooking involved areas.
12.2.8
Malignant Pleural Mesothelioma
Similarly to lung cancer, excellent FDG uptake in malignant pleural mesothelioma (MPM) has been pre- viously described (Marom et al. 2002). Schneider et al. (2000) demonstrated that PET is particularly valu- able for distinguishing between benign and malignant pleural processes. FDG is not taken up in pleural fibro- sis, thus differential diagnosis of the pleural lesions is possible. PET imaging is useful in localizing the areas involved with MPM. However, PET and CT are unable to differentiate MPM from pleural adenocarcinoma, so that histology is needed for confirmation.
The role of PET is to document the extent of pleural disease, to establish mediastinal lymph node involve- ment, to evaluate tumor invasion, and to diagnose recurrence. Our experience demonstrates that inte-
grated PET/CT imaging is an excellent method for staging patients with MPM. With the co-registration of anatomic and metabolic information, the extent of the tumor can be precisely defined. Small mediasti- nal lymph node metastases can be detected and pre- cisely localized. Integrated PET/CT imaging is helpful to identify the optimal biopsy site thereby increasing diagnostic accuracy of the histological examination.
12.3
PET/CT in Head and Neck Cancer
In head and neck oncology, precise identification and localization of a lesion is frequently decisive, as many patients undergo surgical treatment after the PET and CT examinations. Therefore, availability of co-registered images is useful and sometimes criti- cal. The combination of PET and CT is helpful for staging and treatment planning as shown in recent reports (Giraud et al. 2001; Kluetz et al. 2000).
12.3.1 Staging
In most patients with squamous cell carcinoma of the head and neck (HNSCC) a staging PET examination will be done after having obtained cytological/ his- tological proof of the cancer. In many patients the extent of a lesion can be reliably assessed with clinical evaluation. In most regions of the head and neck the T stages T1, T2, and T3 describe lesions with increas- ing size. In all regions a T4 stage describes a carcinoma invading into adjacent structures such as bone, carti- lage, deep muscles, or vessels and nerve sheaths.
It has been questioned whether PET alone is suit-
able for routine evaluation of head and neck cancer
patients, because of the lack of anatomic informa-
tion (Keyes et al. 1997). T staging needs anatomic
information and the possibility to exactly measure
tumor size. Furthermore, the precise identification,
localization, and delineation of size and anatomic
extent of a primary lesion is very important to cor-
rectly plan surgical interventions and radiation treat-
ment. Based on our experience in many patients the
information obtained by a PET scan read side-by-side
together with a separate spiral contrast-enhanced CT
scan is equivalent to the information obtained from a
co-registered PET/CT scan without intravenous con-
trast. Some primaries or lymph node metastases will
be missed on PET images due to the low resolution of
PET cameras and partial volume effects. Therefore, small lesions and lesions with a superficial growth pattern may be missed (Goerres et al. 2002c, 2004a).
Some of these lesions will be identified with structural imaging methods. On the other hand, a primary can be missed with CT or MRI when located in an area with metal induced artifacts. Furthermore, it might be overlooked due to a small size and location in an anatomical area difficult to assess. However, early publications have shown that with image co-registra- tion of PET with CT or MRI, one can better evaluate HNSCC (Chisin et al. 1993; Wong et al. 1996).
Because HNSCC mainly spreads regionally, the correct lymph node staging is the key to an optimal treatment strategy. In patients treated for oral cavity cancer and an N0 neck the 5-year survival will be 73%, in patients with pathologically positive lymph nodes 50%, and in patients with extracapsular spread of lymph node involvement 30% (Myers et al. 2001). For the assessment of loco-regional lymph node status, numerous publications have shown that PET has a higher sensitivity and specificity than contrast enhanced CT or MRI. However, contrast- enhanced CT, sonography, or MRI are routinely used for lymph node assessment, because precise anatomical localization of lesions is important for the planning of surgical treatment and radiation therapy. Therefore, the CT information facilitates the identification and correct localization of lymph nodes with an increased FDG uptake.
Patients with HNSCC have a high risk of devel- oping secondary cancers in the head and neck, esophagus or lung. Léon et al. (1999) reported an incidence to develop a second neoplasm of 4% per year. The incidence of distant metastases depends on the primary site of the lesion and the stage of disease at the time of first diagnosis. An advan- tage of the PET/CT technique is the ability to pro- vide whole-body scanning in one imaging session.
Several studies have suggested that imaging of the whole-body can have an impact on further treat- ment decisions by detecting distant metastases or secondary cancers (Goerres et al. 2003; Kitagawa et al. 2002; Schwartz et al. 2003; Stokkel et al.
1999; de Bree et al. 2000). Most secondary cancers observed in HNSCC patients, such as bronchogenic cancer and carcinomas of the esophagus, show a high FDG uptake (Fig. 12.1). Therefore, whole-body PET is a good imaging tool to also identify these secondary malignancies. Whole-body PET or PET/
CT for screening of distant metastases seems to be most useful in patients with advanced stage HNSCC.
Because secondary carcinomas are not only a prob-
lem at initial staging but may arise during the later disease course, whole-body imaging can be recom- mended for follow-up examinations.
12.3.2
Treatment Planning
PET/CT data have been shown to improve staging and treatment planning (Hany et al. 2002a; Kluetz et al. 2000). The co-registration of PET and CT data sets as acquired with routine clinical PET/CT stud- ies is precise enough for the pre-surgical and pre- radiotherapeutic evaluation of patients. Intrave- nous contrast is important to assess the extension of a tumor and invasion into adjacent structures such as vessels. CT and MRI are currently acquired with intravenous contrast enhancement. However, small tumors and superficial mucosal lesions can be overlooked with morphological and functional imaging and some lesions may remain undetected on CT without intravenous contrast enhancement.
Therefore, it is prudent to perform PET/CT with intravenous contrast enhancement if no additional CT scan is available. However, if the PET/CT data shall be used for radiation treatment planning, the CT scan has to be acquired without intravenous con- trast enhancement and with a high enough mAs to reliably define attenuation values of the different tissues for dose calculations. The patient has to be positioned in exactly the same way during PET/CT as for the radiation treatment. With the co-regis- tration PET information the radiation oncologist can better define the metabolically active tissue and adapt the target volume of the radiation field.
It seems that this PET information influences the accuracy of target volume definition. However, to date no studies are available showing that adapta- tions of radiation therapy planning based on the area of increased FDG uptake improves local cancer control and outcome of the patient.
12.3.3 Follow-Up
At re-staging the most important question is to find
out whether a patient can still be treated for example
with salvage surgery for a locoregional problem or
if the disease has spread systemically. Early after
radiation treatment, edema and other post-thera-
peutic changes of the soft tissues can render the
clinical evaluation of a patient difficult. Therefore,
structural imaging is most important in the follow- up of patients with HNSCC. Post-therapeutic soft tissue alterations may influence FDG uptake in the neck muscles and uptake in brown adipose tissue generating an asymmetric appearance. A common finding in patients undergoing follow-up PET/CT for HNSCC is the tracheotomy. It is important to insert a plastic tube before scanning to avoid metal induced artifacts in such patients. An increased FDG uptake at the borders of the tracheotomy due to inflam- matory (post surgical) reaction is not uncommon and therefore additional clinical and other imag- ing information is needed for image interpretation in patients with suspected malignancy at this site (Goerres et al. 2002d). Common post-radiother- apy changes include inflammatory reactions of the mucosal membrane in the upper aerodigestive tract leading to increased FDG uptake. Such a reaction has usually normalized within a few weeks after the end of treatment. However, esophagitis and phar- yngitis can be caused by fungal infection leading to increased FDG uptake (Goerres et al. 2002d).
Some authors recommend waiting 4 months after completion of radiation treatment before scanning patients to identify residual cancer/early recurrence (Greven et al. 1994). However, it has been shown that already 6–8 weeks after the end of a combined chemotherapy and radiation treatment a reliable assessment of residual viable cancer tissue is possi- ble with >90% sensitivity and specificity (Goerres et al. 2004b).
In difficult areas such as the base of the skull and in the paranasal sinuses with a wide range of mor- phological variants it is necessary to reliably co-reg- ister functional findings onto the structural findings in an individual patient. For imaging evaluation of the base of the skull MRI is often the first choice.
However, future studies should clarify whether PET/
CT has superior accuracy compared to MRI for the assessment of patients with carcinomas at the base of the skull and in the paranasal sinuses.
12.3.4
The Pros and Cons of PET/CT in Head and Neck Oncology
The ability to co-register PET data onto structural information facilitates the reading of images and speeds up image interpretation, because a lesion is reliably allocated to the correct anatomic region.
For example, in a patient with muscle uptake, it is
still possible to reliably identify FDG uptake in adja- cent lymph nodes, because the lymph node is also anatomically delineated. FDG uptake facilitates the detection of cancer in non-enlarged lymph nodes or in lymph nodes which would have been missed on the CT image. Additionally, enlarged lymph nodes are often found on the CT but show no increased FDG uptake. Therefore, the correct localization of a lesion is easier with co-registered PET/CT than with PET read together with a separate CT. Physiological FDG accumulations such as in brown adipose tissue can easily be identified and discriminated from muscle uptake or a soft tissue mass, because on the co-registered CT image, this uptake will be local- ized within normal appearing fatty tissue (Hany et al. 2002b). Additionally, physiologic FDG uptake in normal lymphatic tissue of Waldeyer’s ring and in the salivary glands may render interpretation of PET images difficult (Goerres et al. 2002d). PET/
CT allows for the exact delineation of viable cancer tissue even in large lesions with necrotic areas. PET information may be used to guide interventions and improve planning of further treatment. The prob- lems generated with the co-registration of data sets obtained by different devices at different time points such as patient repositioning can be overcome by using an integrated PET/CT scanner (Beyer et al.
2000). This is especially important when for exam- ple a lesion adjacent to the midline has to be assessed and it is not clear if this lesion crosses the midline or not. In this situation a reader may feel more con- fident with PET/CT images than with PET and con- trast enhanced CT images read together side by side, because the area of edema around a lesion or con- trast medium enhancement, respectively, may not correspond to the area of increased FDG uptake.
It has been shown that co-registered PET/CT improves whole-body staging of oncologic patients (Antoch et al. 2003b). Furthermore, a readers’ con- fidence in image interpretation is improved and the number of equivocal findings decreases with PET/CT. More lesions will be judged either as posi- tive or negative and the discrimination of benign from malignant lesions and thus the specificity is improved. However, based on our experience PET/
CT and PET read together with (contrast enhanced) CT will identify an equal number of malignant lesions as long as FDG avid regions are examined.
Another important issue is the work-up time and
patient scheduling. Because HNSCC are cancers that
may grow very fast, it is very important to perform
the different diagnostic steps efficiently. In busy
centers patients often have to wait between the CT-
or MRI-scan and the PET examination. This sup- ports the use of PET/CT for shortening the work-up of HNSCC patients.
The use of CT data for attenuation correction of PET emission scans has been established (Kinahan et al. 1998; Burger et al. 2002). CT based attenua- tion corrected PET scans can show some PET/CT specific artifacts in the head and neck area. Move- ment of the head between the PET and CT acquisition will lead to erroneous measurement of attenuation data with mistaken correction of PET emission data.
Additionally, such movement will lead to misreg- istration ranging from slight to very severe image deterioration (Goerres et al. 2002d). However, the
“intrinsic hardware” co-registration of both image data sets and the relatively fast acquisition provide high fusion accuracy. Co-registration of PET and CT data sets in the head and neck is possible to within only a few millimeters even in patients undergo- ing scanning without a face mask (Goerres et al.
2004c). However, in routine head and neck PET/CT imaging the major image quality degradation is due to metallic dental implants (Goerres et al. 2002b;
Kamel et al. 2003a). Therefore, all artificial dentures and metal parts should be removed before scanning if possible.
12.4 Conclusion
In patients with HNSCC the combination of PET and anatomic information is indispensable. PET/
CT offers many advantages in this patient group: it facilitates the interpretation of PET information and can offer adequate anatomic information to plan surgical interventions and radiation treatment. At staging and restaging, PET/CT improves identifica- tion of a lesion, allocation to the correct anatomic site and definition of the extension of a primary lesion. This is crucial in the complex anatomical situation of HNSCC patients especially in suspected recurrence.
The confidence of image interpretation is increased and images are read faster. Whole-body imaging can detect distant metastases and secondary tumors and, thus, influence patient management. However, future studies have to elucidate: (a) whether the CT portion of the PET/CT examination should always be done with a contrast agent, (b) the role of PET/CT for radiation treatment planning, and (c) if PET/CT can be cost-effective in defined patient groups.
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