Positron emission tomography in lung cancer

PET-CT in Clinical Adult Oncology: II. Primary Thoracic and Breast Malignancies

Simple Summary

Positron emission tomography (PET), typically combined with computed tomography (CT), has become a critical advanced imaging technique in oncology. With PET-CT, a radioactive molecule (radiotracer) is injected in the bloodstream and localizes to sites of tumor because of specific cellular features of the tumor that accumulate the targeting radiotracer. The CT scan, performed at the same time, provides information to facilitate assessment of the amount of radioactivity from deep or dense structures, and to provide detailed anatomic information. PET-CT has a variety of applications in oncology, including staging, therapeutic response assessment, restaging, and surveillance. This series of six review articles provides an overview of the value, applications, and imaging and interpretive strategies of PET-CT in the more common adult malignancies. The second article in this series addresses the use of PET-CT in breast cancer and other primary thoracic malignancies.

Abstract

Positron emission tomography combined with x-ray computed tomography (PET-CT) is an advanced imaging modality with oncologic applications that include staging, therapy assessment, restaging, and surveillance. This six-part series of review articles provides practical information to providers and imaging professionals regarding the best use of PET-CT for the more common adult malignancies. The second article of this series addresses primary thoracic malignancy and breast cancer. For primary thoracic malignancy, the focus will be on lung cancer, malignant pleural mesothelioma, thymoma, and thymic carcinoma, with an emphasis on the use of FDG PET-CT. For breast cancer, the various histologic subtypes will be addressed, and will include ¹⁸F fluorodeoxyglucose (FDG), recently Food and Drug Administration (FDA)-approved ¹⁸F-fluoroestradiol (FES), and ¹⁸F sodium fluoride (NaF). The pitfalls and nuances of PET-CT in breast and primary thoracic malignancies and the imaging features that distinguish between subcategories of these tumors are addressed. This review will serve as a resource for the appropriate roles and limitations of PET-CT in the clinical management of patients with breast and primary thoracic malignancies for healthcare professionals caring for adult patients with these cancers. It also serves as a practical guide for imaging providers, including radiologists, nuclear medicine physicians, and their trainees.

Positron Binding and Annihilation Properties of Amino Acid Systems

Despite the fact that the positron annihilation has been used in biomedical applications, the detailed mechanism of the positron annihilation on biological molecules remains poorly understood so far. In this work, we investigated the positron binding and positron annihilation properties for both global minimum and hydrogen-bonded structures of 20 amino acid molecules using the multicomponent molecular orbital method. By regression analysis, we confirmed that positron affinity can increase with an increase of the permanent dipole moment of the parent amino acids as reported in previous studies, while the annihilation rate linearly increases with respect to the square root of positron affinity. By the one-particle property analyses for probabilities of electron-positron contacts, we found that delocalization characteristics of both electrons and positrons play key roles to enhance the positron annihilation rate arising from both the valence electrons in σ- and π-type molecular orbitals from 2p atomic orbitals but not from the highest occupied molecular orbital electrons, particularly for comparatively weakly bound positronic amino acid systems.

Theoretical investigation of the enhancement of positron affinity by the vibration and dimerization of non-polar carbon disulfide

The positronic bound state for the non-polar carbon disulfide (CS₂) has been experimentally identified, although previous theoretical investigations, which were dedicated to studying the positronic CS₂ monomer, could not reasonably reproduce the experimentally measured positron affinity. In the present study, we performed analysis of the vibrational averaged positron affinity for the positronic CS₂ dimer, [C₂S₄; e⁺], using the Hartree-Fock and configuration interaction levels of the multi-component molecular orbital method combined with the self-consistent field level of the vibrational variational Monte Carlo method. We demonstrated that the equilibrium structure of the non-polar C₂S₄ can have the positronic bound state with a positron affinity of about 46.18 meV in the configuration interaction level, while this is 0 meV in the Hartree-Fock level. Furthermore, by taking into account the vibrational effect, we succeeded in reproducing the resonant positron kinetic energies lying close to the experimental value, where the vibrational averaged positron affinity becomes greater with an increased dipole moment and dipole polarizability. We also showed possible mechanisms to effectively enhance the resonant positron capture for [C₂S₄; e⁺], associated with both the infrared active and infrared inactive vibrational modes.

18F-Fluoro-2-Deoxy-d-Glucose PET/Computed Tomography Evaluation of Lung Cancer in Populations with High Prevalence of Tuberculosis and Other Granulomatous Disease

Pulmonary tuberculosis infects one-third of world's population and is responsible for the high mortality and morbidity in developing countries. The presence of a high number of macrophages and lymphocytes in active tuberculosis granulomas is associated with high uptake of ¹⁸F-fluoro-2-deoxy-d-glucose on PET imaging mimicking lung cancer. In many cases, radiological features of pulmonary tuberculosis are undistinguishable from lung cancer, which makes the diagnosis difficult. Clinical history and computed tomographic (CT) findings on a hybrid PET/CT are as important as findings on a PET in the diagnosis of lung cancer.

PET/CT imaging in lung cancer: indications and findings

The use of PET/CT imaging in the work-up and management of patients with lung cancer has greatly increased in recent decades. The ability to combine functional and anatomical information has equipped PET/CT to look into various aspects of lung cancer, allowing more precise disease staging and providing useful data during the characterization of indeterminate pulmonary nodules. In addition, the accuracy of PET/CT has been shown to be greater than is that of conventional modalities in some scenarios, making PET/CT a valuable noninvasive method for the investigation of lung cancer. However, the interpretation of PET/CT findings presents numerous pitfalls and potential confounders. Therefore, it is imperative for pulmonologists and radiologists to familiarize themselves with the most relevant indications for and limitations of PET/CT, seeking to protect their patients from unnecessary radiation exposure and inappropriate treatment. This review article aimed to summarize the basic principles, indications, cancer staging considerations, and future applications related to the use of PET/CT in lung cancer.

Unabsorbed polylactide adhesion barrier mimicking recurrence of gynecologic malignant diseases with increased ¹⁸F-FDG uptake on PET/CT

PURPOSE

To evaluate the incidence and characteristics of the unabsorbed polylactide adhesion barrier with increased (18)F-fluorodeoxyglucose ((18)F-FDG) uptake after surgeries for gynecologic malignancies.

METHODS

Between September 2006 and November 2009, we reviewed the charts of 75 patients who were provided a polylactide adhesion barrier after surgery for gynecologic malignant diseases. We surveyed the cases of increased (18)F-FDG uptake on positron emission tomography/computed tomography (PET/CT), and evaluated the effectiveness of polylactide adhesion barrier using an adhesion scoring system.

RESULTS

Ten patients (13.3 %) had a solitary pelvic mass with increased (18)F-FDG uptake in the follow up PET/CT. The characteristics of patients and tumors are described below. The median age was 48 years (range 19-66 years). The median tumor size was 1.9 cm (range 1.0-2.3 cm), and the median SUVmax of the pelvic mass was 5.1 (range 3.7-7.9). The median time between initial operations and second operation was 13.5 months (range 8-23 months). We performed laparoscopic excision of the pelvic mass, and the biopsy revealed foreign body reactions with the exception of 1 case, which contained tumor cells under the unabsorbed polylactide adhesion barrier. The median adhesion grade was 1 (range 0-2).

CONCLUSIONS

A solitary pelvic mass found in the PET/CT with increased (18)F-FDG uptake after usage of a polylactide adhesion barrier may be an unabsorbed remnant. The adhesion barrier should be used with caution in patients with gynecologic malignant diseases.

Physiology and pathophysiology of incidental findings detected on FDG-PET scintigraphy

A routine feature of positron emission tomography/computed tomography (PET/CT) imaging is whole-body acquisition that results in many unexpected findings identified outside of the primary region of abnormality. Furthermore, (18)F-fluorodeoxyglucose (FDG) is a marker of glycolysis and does not specifically accumulate in malignancy. Understanding the physiology and pathophysiology of normal FDG distribution and common incidental findings is therefore essential to the physician interpreting whole-body FDG-PET/CT studies. Whereas many incidental findings are benign and of limited clinical significance, others represent uncommon manifestations of the primary malignancy, second malignancies, or various clinically significant pathologic processes. Patients with a single malignancy are at greater risk of developing synchronous or metachronous second malignancies, possibly related to exposure to shared carcinogenic agents or presence of prooncogenic mutations. The decision of how to pursue an intervention on the basis of an incidental finding is generally left to clinical judgment.

Usefulness of fluoro-2-deoxyglucose positron emission tomography for investigating unexplained rising carcinoembryonic antigen levels that occur during the postoperative surveillance of lung cancer patients

BACKGROUND

Positron emission tomography with 18-fluorodeoxyglucose (FDG-PET) has been useful for the detection of recurrent lung cancer. However, such technology is expensive and is not always widely available. In this study, we evaluated the selected use of FDG-PET for lung cancer patients with re-elevated levels of serum carcinoembryonic antigen (CEA) after curative surgery.

METHODS

Among 327 patients who underwent curative resection for primary lung cancer from May 2002 through March 2007, 199 had adenocarcinoma, of whom 78 (39%) had elevated levels of serum CEA before treatment. After surgery, the level was monitored every 1 to 3 months. Re-elevation of CEA was recognized in 39 (50%) patients, 22 of whom underwent FDG-PET and were included in this study. FDG-PET images were visually inspected, and abnormally increased FDG uptake was interpreted as recurrence. The sensitivity, specificity, positive predictive value, and negative predictive value of PET in detecting recurrent lung cancer were determined.

RESULTS

FDG-PET correctly identified 14 out of 15 relapses and gave true negative results in six out of seven remissions, i.e., one false-negative and one false-positive were observed. The sensitivity, specificity, positive predictive value, and negative predictive value were 93%, 86%, 93%, and 86%, respectively.

CONCLUSION

In 64% of the patients with unexplained increased CEA levels, FDG-PET provided decisive diagnostic clues guiding further diagnostic and therapeutic interventions. The selected use of FDG-PET for patients with re-elevated serum CEA levels after surgery can be a practical and effective mode of surveillance for detecting recurrent lung cancer.

Positron emission tomography/computed tomography potential pitfalls and artifacts

With the recent use of 18-fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) for tumor staging and treatment response, it is important to recognize many pitfalls, artifacts, and benign uptakes that are commonly encountered. Normal physiology can explain many regions of increased FDG activity, as well as incidental benign tumors and benign metabolic conditions. Recognition of characterization of benign causes and physiologic variants for FDG uptake are discussed to avoid improper characterization as a malignancy. A basic understanding of PET/computed tomographic physics is also discussed, in relation to attenuation correction artifacts caused by metallic implants and contrast agents in the gastrointestinal tract, as well as artifacts caused in fused images due to patient motion. Also presented is the rationale for expected, benign uptake in various metabolic diseases, as well as pharmacologic methods for decreasing the artifacts caused by metabolic diseases. PET/computed tomographic evaluation of the thyroid, thymus, adrenal adenomas, uterus and ovaries, infection/inflammatory changes, and postradiation/chemotherapy changes are also discussed, with expected normal changes, as well as pitfalls and artifacts.

Dual-time point 18F-FDG PET/CT scan for differentiation between 18F-FDG-avid non-small cell lung cancer and benign lesions

OBJECTIVE

The aim of this study is to clarify the difference of F-18 FDG uptake kinetics between FDG-avid non-small-cell lung cancer (NSCLC) and benign lesions associated with various etiologies on dual-time point PET/CT scan, and to determine the optimal parameter for differentiation.

MATERIALS AND METHODS

The materials were 76 FDG-avid solitary NSCLC in 76 patients and 57 FDG-avid solitary benign lesions associated with various etiologies in 61 patients. FDG PET/CT scan was performed at 60 and 120 min after intravenous injection of 4.4 MBq/kg F-18 FDG. The maximum standardized uptake value (SUVmax) on early and delayed scans and the percent change of SUVmax (%DeltaSUVmax) between the two time points were measured. The optimal differential parameter was determined by receiver-operating characteristic curve analysis and evaluation of diagnostic accuracy.

RESULTS

The mean +/- SD of early SUV max, delayed SUVmax and %DeltaSUVmax were 8.3 +/- 5.2, and 10.2 +/- 6.5, and 21.9% +/- 18.9 in FDG-avid NSCLC, and 3.8 +/- 3.2, 4.0 +/- 3.7, and 11.3% +/- 26.0 in FDG-avid benign lesions, respectively. Delayed SUVmax in NSCLC was significantly higher than early SUVmax (P < 0.0001); while not different in benign lesions. Percent change of SUVmax in NSCLC was also significantly higher than that in benign lesions (P < 0.01). The optimal parameter for the differentiation was delayed SUVmax > 5.5 and yielded sensitivity of 77.6%, specificity of 80.7% and accuracy of 78.9%, which provided better differentiation than the use of %DeltaSUVmax or the traditional parameter of early SUVmax > 2.5. However, 11 (19.2%) benign lesions were indistinguishable from NSCLC.

CONCLUSION

Although delayed PET/CT scan enhances the difference of FDG uptake between FDG-avid NSCLC and benign lesions, and the use of delayed SUVmax > 5.5 appears to improve the differentiation of these hypermetabolic lesions compared with an early scan, careful interpretation and management for correct differentiation are still required.

Follow-up and surveillance of the lung cancer patient following curative intent therapy: ACCP evidence-based clinical practice guideline (2nd edition)

BACKGROUND

To develop an evidence-based approach to follow-up of patients after curative intent therapy for lung cancer.

METHODS

Guidelines on lung cancer diagnosis and management published between 2002 and December 2005 were identified by a systematic review of the literature, and supplemental material appropriate to this topic was obtained by literature search of a computerized database (Medline) and review of the reference lists of relevant articles.

RESULTS

Adequate follow-up by the specialist responsible for the curative intent therapy should be ensured to manage complications related to the curative intent therapy and should last at least 3 to 6 months. In addition, a surveillance program should be considered to detect recurrences of the primary lung cancer and/or development of a new primary lung cancer early enough to allow potentially curative retreatment. A standard surveillance program for these patients, coordinated by a multidisciplinary tumor board and overseen by the physician who diagnosed and initiated therapy for the original lung cancer, is recommended based on periodic visits with chest imaging studies and counseling patients on symptom recognition. Smoking cessation and, if indicated, facilitation in participation in special programs is recommended for all patients following curative intent therapy for lung cancer.

CONCLUSIONS

The current evidence favors follow-up of complications related to curative intent therapy, and a surveillance program at regular intervals with imaging and review of symptoms. Smoking cessation after curative intent therapy to prevent recurrence of lung cancer is strongly supported by the available evidence.

Improvement in sensitivity with delayed imaging of pulmonary lesions with FDG-PET

PURPOSE

This study was undertaken to determine the value of using dual-time point 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) imaging to distinguish malignant from benign pulmonary lesions after lesion detection by conventional computed tomography chest imaging.

METHODS

Patients referred for characterization of lung lesions were included in this prospective study. Eighty-three patients had histopathologic confirmation of disease. Patients underwent FDG-PET coincidence imaging, performed with a dual-headed gamma camera at 1 h ("early" scan) and 3 h ("late" scan) after injection of 185 MBq of FDG. Studies were read independently by 2 physicians who had knowledge of the lesion location but not the final diagnosis. For both early and late images, readers graded FDG lesion uptake intensity on a scale of 1 (definitively benign) to 5 (definitively malignant) and classified studies dichotomously for malignancy. Tumor-to-background (T:B) ratios were computed using contralateral lung sites as controls.

RESULTS

Sixty one lesions (74 %) were non-small cell lung cancer, and 10 (12 %) were other primary tumors or metastases. Twelve lesions (14 %) were benign. T:B ratios were significantly higher for early versus late scans (+ 5.1 +/- 4.9 versus + 8.2 +/- 8.7, p = 0.01, n = 71) for malignancies but not for benign lesions (+ 3.1 +/- 3.4 versus + 2.6 +/- 2.2, n = 12). The percent change of T:B ratios was higher for malignant than benign lesions (+ 48.3 +/- 40.2 % versus + 7.2 +/- 22.8 %, p = 0.0009). No malignant lesion of any type demonstrated a time-decrease in FDG T:B ratios. The accuracy and sensitivity of lesion characterization were significantly higher for late scans than early scans for dichotomous visual readings. Quantitative analysis was found to provide significantly higher sensitivity and accuracy than visual analysis for lesion characterization, with no significant difference in test specificity.

CONCLUSIONS

In malignant pulmonary nodules, there is a progressive, although variable, increase in FDG uptake over time. Increasing FDG uptake is a nonspecific finding, as some benign lesions also demonstrate increasing uptake, particularly those associated with granulomas. The use of late PET images increases the accuracy and sensitivity of visual detection of malignancy.

Clinical implications of defining the gross tumor volume with combination of CT and 18FDG-positron emission tomography in non-small-cell lung cancer

PURPOSE

To compare the planning target volume (PTV) definitions for computed tomography (CT) vs. positron emission tomography (PET) in non-small-cell lung cancer (NSCLC).

METHODS AND MATERIALS

A total of 21 patients with NSCLC underwent three-dimensional conformal radiotherapy planning. All underwent a staging F-18 fluorodeoxyglucose-position emission tomography (18FDG-PET) scan and underwent treatment simulation using CT plus a separate planning 18FDG-PET scan. Three sets of target volumes were defined: Set 1, CT volumes (CT tumor + staging PET nodal disease); Set 2, PET volumes (planning PET tumor {gross tumor volume (GTV) = [(0.3069 x mean standardized uptake value) + 0.5853])}; Set 3, composite CT-PET volumes (fused CT-PET tumor). Sets 1 and 2 were compared using a matching index. Three-dimensional conformal radiotherapy plans were created using the Set 1 (CT) volumes; and coverage of the Set 3 (composite) volumes was evaluated. Separate three-dimensional conformal radiotherapy plans were designed for the Set 3 volumes.

RESULTS

For the primary tumor GTV, the Set 1 (CT) volume was larger than the Set 2 (PET) volume in 48%, smaller in 33%, and equal in 19%. The mean matching index was 0.65 (35% CT-PET mismatch). Although quantitatively similar, the volumes differed qualitatively. The Set 3 (composite) volume was larger than either CT or PET alone in 62%, smaller in 24%, and equal in 14%. The dose-volume histogram parameters did not differ among the plans for Set 1 (CT) vs. Set 3 (composite) volumes. Small portions of the Set 3 PTV were significantly underdosed in 40% of cases using the CT-only plan.

CONCLUSION

Computed tomography and PET are complementary and should be obtained in the treatment position and fused to define the GTV for NSCLC. Although the quantitative absolute target volume is sometimes similar, the qualitative target locations can be substantially different, leading to underdosage of the target when planning is done using CT alone without PET fusion.

Sensitivity and positive predictive value of CT, MRI and 123I-MIBG scintigraphy in localizing pheochromocytomas: A prospective study

AIM

To establish a standardized non-invasive imaging protocol for patients with pheochromocytoma undergoing surgery.

METHODS

A series of 32 consecutive patients (16 men, 16 women; median age 43 years, range 15-71 years) with biochemically confirmed pheochromocytoma underwent computed tomography (CT) scanning, magnetic resonance imaging (MRI) and meta-[I]iodobenzylguanidine (MIBG) whole-body scintigraphy prior to adrenalectomy or excision of extra-adrenal tumour (paraganglioma).

RESULTS

At final pathology no malignant pheochromocytomas were found. The tumour was right-sided in 16 (50%) patients, left-sided in 13 (41%), extra-adrenal (sympathetic ganglia, upper abdomen) in two (6%) and bilateral in one (3%) patient. Overall, the median greatest diameter (size) of the tumour was 35 mm (range, 15-90 mm). The sensitivity of CT, MRI and MIBG scintigraphy was 90%, 93% and 91%, and the specificity was 93%, 93% and 100%, respectively. The three patients with false negative scintigraphy had an intra-adrenal tumour, ranging from 20 to 50 mm in size. The presence of necrosis within the mass might justify the lack of significant uptake of radiopharmaceutical in two patients, and the small size (15 mm) of the mass in the other. There were two false positive results with both CT and MRI, and no false positive MIBG scintigraphy, which had the highest (100%) positive predictive value. The combination of MRI+MIBG scintigraphy reached 100% sensitivity and positive predictive value.

CONCLUSION

Our data suggest that this imaging protocol should be used in all patients with biochemically confirmed pheochromocytoma.

How few cancer cells can be detected by positron emission tomography? A frequent question addressed by an in vitro study

PURPOSE

Positron emission tomography (PET) has gained widespread use in cancer diagnosis and treatment, but how many malignant cells are required for a tumour to be detected by PET?

METHODS

Three human cancer cell lines [glioblastoma and two subtypes of small cell lung cancer (SCLC)] in concentrations from 10(4) to 10(7) were seeded on six-well plates or plastic tubes and treated with [(18)F]fluorodeoxy-glucose (FDG) in vitro. FDG retention was measured in a PET/CT scanner and in a calibrated well counter. The clinical situation was simulated using a cylinder phantom with a background concentration of FDG.

RESULTS

The theoretical detection limit was found to be around 10(5) malignant cells. In a cylinder phantom the detection limit was increased by a factor of 10. The FDG retention by the glioblastoma cell line was significantly higher than the activity of the SCLC cell line. FDG retention measured by PET and a gamma counter was closely correlated to the number of cells and a linear relationship was found.

DISCUSSION

The detection limit of PET is in the magnitude of 10(5) to 10(6) malignant cells. The experimental set-up was robust and well suited as a platform for further investigations of factors influencing the detection limit of PET.

Radiolabeled peptides in oncology: role in diagnosis and treatment

There has been an exponential growth in the development of radiolabeled peptides for diagnostic and therapeutic applications in the last decade. The automated means of synthesizing these compounds in large quantities and the simplified methods of purifying, characterizing, and optimizing them have kindled attention to peptides as carrier molecules. These new techniques have accelerated the commercial development of radiolabelled peptides, which has provided additional radiopharmaceuticals for the nuclear medicine community. Peptides have many key properties including fast clearance, rapid tissue penetration, and low antigenicity, and can be produced easily and inexpensively. However, there may be problems with in vivo catabolism, unwanted physiologic effects, and chelate attachment. Radiolabeled peptides have made their greatest impact in the management of relatively rare neuroendocrine malignancies. Indeed, Indium-111 ((111)In)-pentetreotide ((111)In-DTPA-octreotide, Octreoscan), which binds to somatostatin receptors (SSTRs), has become the diagnostic 'gold standard' in these diseases. However, (111)In-pentetreotide has been less successful in the diagnosis of other more prevalent diseases in which SSTRs are upregulated. Technetium-99m (99mTc)-depreotide (NeoTect), a 99mTc-labeled SSTR-analog, could have wider impact since it has high sensitivity and specificity for lung cancer lesion detection. However, this impact may be minimized by the increased availability of positron emission tomography imaging with Fluorine-18 (18F)-flourodeoxyglucose, which has similar sensitivity and specificity for lesion identification in this disease, and is currently more widely used. The receptors for bombesin, alpha-melanocyte-stimulating hormone, neurotensin, and the integrin alpha(v)beta3, are under active investigation as targets for radiolabelled peptides, but are still in the pre-clinical stage. Compounds directed at the cholecystokinin-B/gastrin receptor have shown promising results in clinical trials in humans. Radiolabelled peptide therapy is usually indicated for patients with widespread disease that is not amenable to focused radiation therapy or is refractory to chemotherapy. Phase I/II studies using various radiolabelled peptides (including (111)In-pentetreotide, Yttrium-90 [90Y]-DOTA-Phe1-Tyr3-octreotide, 90Y-DOTA-lanreotide, and Lutetium-177 [177Lu]-DOTA-octreotate) for the treatment of patients with neuroendocrine malignancy are in progress. Over 400 patients have been treated, and the response rate has ranged from 60% to 75%, although few patients have had a complete response. Patients have been given individual doses ranging from 2 to 11 GBq with a slow infusion every 4-8 weeks (up to 12 times). The kidney is the dose-limiting organ and most patients experience a transient decline in blood cell counts. A concomitant infusion of an amino acid mixture can reduce kidney toxicity and increase the effective tumor dose. Other peptides currently under investigation, some of which have shown promising results, include Rhenium-188 (188Re)-P2045 and 90Y-alpha(v)beta3 antagonist.

Planning evaluation of radiotherapy for complex lung cancer cases using helical tomotherapy

Lung cancer treatment is one of the most challenging fields in radiotherapy. The aim of the present study was to investigate what role helical tomotherapy (HT), a novel approach to the delivery of highly conformal dose distributions using intensity-modulated radiation fan beams, can play in difficult cases with large target volumes typical for many of these patients. Tomotherapy plans were developed for 15 patients with stage III inoperable non-small-cell lung cancer. While not necessarily clinically indicated, elective nodal irradiation was included for all cases to create the most challenging scenarios with large target volumes. A 2 cm margin was used around the gross tumour volume (GTV) to generate primary planning target volume (PTV2) and 1 cm margin around elective nodes for secondary planning target volume (PTV1) resulting in PTV1 volumes larger than 1000 cm3 in 13 of the 15 patients. Tomotherapy plans were created using an inverse treatment planning system (TomoTherapy Inc.) based on superposition/convolution dose calculation for a fan beam thickness of 25 mm and a pitch factor between 0.3 and 0.8. For comparison, plans were created using an intensity-modulated radiation therapy (IMRT) approach planned on a commercial treatment planning system (TheraplanPlus, Nucletron). Tomotherapy delivery times for the large target volumes were estimated to be between 4 and 19 min. Using a prescribed dose of 60 Gy to PTV2 and 46 Gy to PTV1, the mean lung dose was 23.8+/-4.6 Gy. A 'dose quality factor' was introduced to correlate the plan outcome with patient specific parameters. A good correlation was found between the quality of the HT plans and the IMRT plans with HT being slightly better in most cases. The overlap between lung and PTV was found to be a good indicator of plan quality for HT. The mean lung dose was found to increase by approximately 0.9 Gy per percent overlap volume. Helical tomotherapy planning resulted in highly conformal dose distributions. It allowed easy achievement of two different dose levels in the target simultaneously. As the overlap between PTV and lung volume is a major predictor of mean lung dose, future work will be directed to control of margins. Work is underway to investigate the possibility of breath-hold techniques for tomotherapy delivery to facilitate this aim.

Defining a radiotherapy target with positron emission tomography

PURPOSE

F-18 fluorodeoxyglucose positron emission tomography (FDG-PET) imaging is now considered the most accurate clinical staging study for non-small-cell lung cancer (NSCLC) and is also important in the staging of multiple other malignancies. Gross tumor volume (GTV) definition for radiotherapy, however, is typically based entirely on computed tomographic data. We performed a series of phantom studies to determine an accurate and uniformly applicable method for defining a GTV with FDG-PET.

METHODS AND MATERIALS

A model-based method was tested by a phantom study to determine a threshold, or unique cutoff of standardized uptake value based on body weight (standardized uptake value [SUV]) for FDG-PET based GTV definition. The degree to which mean target SUV, background FDG concentration, and target volume influenced that GTV definition were evaluated. A phantom was constructed consisting of a 9.0-L cylindrical tank. Glass spheres with volumes ranging from 12.2 to 291.0 cc were suspended within the tank, with a minimum separation of 4 cm between the edges of the spheres. The sphere volumes were selected based on the range of NSCLC patient tumor volumes seen in our clinic. The tank and spheres were filled with a variety of known concentrations of FDG in several experiments and then scanned using a General Electric Advance PET scanner. In the initial experiment, six spheres with identical volumes were filled with varying concentrations of FDG (mean SUV = 1.85 approximately 9.68) and suspended within a background bath of FDG at a similar concentration to that used in clinical practice (0.144 muCi/mL). The second experiment was identical to the first, but was performed at 0.144 and 0.036 muCi/mL background concentrations to determine the effect of background FDG concentration on sphere definition. In the third experiment, six spheres with volumes of 12.2 to 291.0 cc were filled with equal concentrations of FDG and suspended in a standard background FDG concentration of 0.144 muCi/mL. Sphere images in each experiment were auto-contoured (simulating a GTV) using the threshold SUV that yielded a volume matching that of the known sphere volume. A regressive function was constructed to represent the relationship between the threshold SUV and the mean target SUV. This function was then applied to define the GTV of 15 NSCLC patients. The GTV volumes were compared to those determined by a fixed image intensity threshold proposed by other investigators.

RESULTS

There was a strong linear relationship between the threshold SUV and the mean target SUV. The linear regressive function derived was: threshold SUV = 0.307 x (mean target SUV) + 0.588. The background concentration and target volume indirectly affect the threshold SUV by way of their influence on the mean target SUV. We applied the linear regressive function, as well as a fixed image intensity threshold (42% of maximum intensity) to the sphere phantoms and 15 patients with NSCLC. The results indicated that a much smaller deviation occurred when the threshold SUV regressive function was utilized to estimate the phantom volume as compared to the fixed image intensity threshold. The average absolute difference between the two methods was 21% with respect to the true phantom volume. The deviation became even more pronounced when applied to true patient GTV volumes, with a mean difference between the two methods of 67%. This was largely due to a greater degree of heterogeneity in the SUV of tumors over phantoms.

CONCLUSIONS

An FDG-PET-based GTV can be systematically defined using a threshold SUV according to the regressive function described above. The threshold SUV for defining the target is strongly dependent on the mean target SUV of the target, and can be uniquely determined through the proposed iteration process.

Feasibility of optimizing the dose distribution in lung tumors using fluorine-18-fluorodeoxyglucose positron emission tomography and single photon emission computed tomography guided dose prescriptions

The information provided by functional images may be used to guide radiotherapy planning by identifying regions that require higher radiation dose. In this work we investigate the dosimetric feasibility of delivering dose to lung tumors in proportion to the fluorine-18-fluorodeoxyglucose activity distribution from positron emission tomography (FDG-PET). The rationale for delivering dose in proportion to the tumor FDG-PET activity distribution is based on studies showing that FDG uptake is correlated to tumor cell proliferation rate, which is shown to imply that this dose delivery strategy is theoretically capable of providing the same duration of local control at all voxels in tumor. Target dose delivery was constrained by single photon emission computed tomography (SPECT) maps of normal lung perfusion, which restricted irradiation of highly perfused lung and imposed dose-function constraints. Dose-volume constraints were imposed on all other critical structures. All dose-volume/function constraints were considered to be soft, i.e., critical structure doses corresponding to volume/function constraint levels were minimized while satisfying the target prescription, thus permitting critical structure doses to minimally exceed dose constraint levels. An intensity modulation optimization methodology was developed to deliver this radiation, and applied to two lung cancer patients. Dosimetric feasibility was assessed by comparing spatially normalized dose-volume histograms from the nonuniform dose prescription (FDG-PET proportional) to those from a uniform dose prescription with equivalent tumor integral dose. In both patients, the optimization was capable of delivering the nonuniform target prescription with the same ease as the uniform target prescription, despite SPECT restrictions that effectively diverted dose from high to low perfused normal lung. In one patient, both prescriptions incurred similar critical structure dosages, below dose-volume/function limits. However, in the other patient, critical structure dosage from the nonuniform dose prescription exceeded dose-volume/function limits, and greatly exceeded that from the uniform dose prescription. Strict compliance to dose-volume/ function limits would entail reducing dose proportionality to the FDG-PET activity distribution, thereby theoretically reducing the duration of local control. Thus, even though it appears feasible to tailor lung tumor dose to the FDG-PET activity distribution, despite SPECT restrictions, strict adherence to dose-volume/function limits could compromise the effectiveness of functional image guided radiotherapy.

Incorporation of functional imaging data in the evaluation of dose distributions using the generalized concept of equivalent uniform dose

Advances in the fields of IMRT and functional imaging have greatly increased the prospect of escalating the dose to highly active or hypoxic tumour sub-volumes and steering the dose away from highly functional critical structure regions. However, current clinical treatment planning and evaluation tools assume homogeneous activity/function status in the tumour/critical structures. A method was developed to incorporate tumour/critical structure heterogeneous functionality in the generalized concept of equivalent uniform dose (EUD). The tumour and critical structures functional EUD (FEUD) values were calculated from the dose-function histogram (DFH), which relates dose to the fraction of total function value at that dose. The DFH incorporates flouro-deoxyglucose positron emission tomography (FDG-PET) functional data for tumour, which describes the distribution of metabolically active tumour clonogens, and single photon emission computed tomography (SPECT) perfusion data for critical structures. To demonstrate the utility of the method, the lung dose distributions of two non-small cell lung cancer patients, who received 3D conformal external beam radiotherapy treatment with curative intent, were evaluated. Differences between the calculated lungs EUD and FEUD values of up to 50% were observed in the 3D conformal plans. In addition, a non-small cell lung cancer patient was inversely planned with a target dose prescription of 76 Gy. Two IMRT plans (plan-A and plan-B) were generated for the patient based on the CT, FDG-PET and SPECT treatment planning images using dose-volume objective functions. The IMRT plans were generated with the goal of achieving more critical structures sparing in plan-B than plan-A. Results show the target volume EUD in plan-B is lower than plan-A by 5% with a value of 73.31 Gy, and the FEUD in plan-B is lower than plan-A by 2.6% with a value of 75.77 Gy. The FEUD plan-B values for heart and lungs were lower than plan-A by 22% and 18%, respectively. While EUD values show plan-A is marginally better than plan-B in terms of target volumetric coverage, the FEUD plan-B values show adequate target function coverage with significant critical structure function sparing. In conclusion, incorporating functional data in the calculation of EUD is important in evaluating the biological merit of treatment plans.

The Utility of 99m Tc Depreotide Compared With F-18 Fluorodeoxyglucose Positron Emission Tomography and Surgical Staging in Patients With Suspected Non-small Cell Lung Cancer

STUDY OBJECTIVES

The findings from conventional imaging modalities, such as chest CT, are frequently unreliable in patients with lung cancer. This study was designed to compare the relative diagnostic accuracies and utility of the two most widely used functional imaging examinations, F-18-2-fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) and (99m)Tc depreotide scintigraphy, for the diagnosis and staging of lung cancer.

DESIGN

Prospective, experimental investigation.

SETTING

Academic medical center.

PATIENTS

One hundred sixty-six subjects with suspected lung cancer were enrolled in the study.

INTERVENTIONS

Whole-body and single-photon emission CT imaging of the chest was performed after IV administration of (99m)Tc depreotide. Attenuation-corrected FDG PET imaging was performed after IV administration of FDG. Image findings were compared with the biopsy results or clinical follow-up.

MEASUREMENTS AND RESULTS

In 157 subjects with evaluable lung lesions, the sensitivities and specificities for detecting malignant disease (95% confidence intervals) of FDG PET are 96% (90 to 98%) and 71% (54 to 85%), and of (99m)Tc depreotide are 94% (88 to 98%) and 51% (34 to 68%). In the 139 subjects with available complete staging data, FDG PET correctly staged 76 of 139 patients (55%), and (99m)Tc depreotide correctly staged 63 of 139 patients (45%).

CONCLUSIONS

The sensitivity for detection of lung cancer in the primary lesion is equally high for FDG PET and (99m)Tc depreotide. The specificity is superior for FDG PET. The staging accuracy of FDG PET and (99m)Tc depreotide is similar, but when read with the chest CT neither scintigraphic examination is sufficiently accurate to stage patients with non-small cell lung cancer.

Limitations of PET for imaging lymphoma

The uptake of fluorine-18 fluorodeoxyglucose (FDG) is increased in processes with enhanced glycolysis, including malignancy. It is this property of FDG which is exploited in positron emission tomography (PET) imaging for lymphoma. FDG, whilst a good oncology tracer, is not perfect and there are limitations to its use. FDG may have low uptake in some types of lymphoma, predominantly low-grade lymphomas. High physiological uptake may occur within the bowel, urinary tract, muscle, salivary glands and lymphoid tissue. FDG is not specific for malignancy and increased uptake occurs in benign conditions with increased glycolysis such as infection, inflammation and granulomatous disease. Benign conditions usually have lower uptake than malignancy but there is overlap. These limitations of FDG mean that tumour may be 'missed', 'masked' or 'mimicked' by other pathology. These limitations are described in this article and methods to circumvent them where possible are discussed. These include performing baseline scans at presentation with lymphoma for comparison with post-treatment scans, simple manoeuvres to reduce physiological uptake such as administration of frusemide and diazepam and remaining alert to the possibility of alternative pathology in immunosuppressed patients. Patients with disease secondary to human immunodeficiency virus are a particular challenge in this regard as they often have dual or multiple pathology. One of the most important skills in PET reporting may be to recognise its limitations and be clear when a definitive answer cannot be given to the referring clinician's question. This may require using PET to direct the clinician to biopsy the site most likely to yield the correct diagnosis.