Automated functional image-guided radiation treatment planning for rectal cancer

Integrated FDG PET/CT: Utility and Applications in Clinical Oncology

Accurate diagnosis and staging are essential for an optimal management of cancer patients. Positron emision tomography with 2-deoxy-2-fluorine-18-fluoro-D-glucose (¹⁸FDG-PET) and, more recently, ¹⁸FDG-PET/computed tomography (¹⁸FDG-PET/CT) have emerged as powerful imaging tools in oncology, because of the valuable functional information they provide. The combined acquisition of PET and CT has synergistic advantages over its isolated constituents and minimizes their limitations. It decreases examination times by 25%–40%, leads to a higher patient throughput and unificates two imaging procedures in a single session. There is evidence that ¹⁸FDG-PET/CT is a more accurate test than either of its components for the evaluation of various tumors. It is a particularly valuable tool for detection of recurrence, especially in asymptomatic patients with rising tumor markers and those with negative or equivocal findings on conventional imaging tests. Yet, there are some limitations and areas of uncertainty, mainly regarding the lack of specificity of the ¹⁸FDG uptake and the variable ¹⁸FDG avidity of some cancers. This article reviews the advantages, limitations and main applications of ¹⁸FDG-PET/CT in oncology, with especial emphasis on lung cancer, colorectal cancer, lymphomas, melanoma and head and neck cancers.

Single photon emission computed tomography-based three-dimensional conformal radiotherapy for hepatocellular carcinoma with portal vein tumor thrombus

PURPOSE

To evaluate the safety and efficacy of three-dimensional conformal radiotherapy (3D-CRT) using single photon emission computed tomography (SPECT) in unresectable hepatocellular carcinoma (HCC) with portal vein tumor thrombus (PVTT).

METHODS AND MATERIALS

Patients with HCC with PVTT in the first branch and/or main trunk were selected for this study. The optimal beam directions for 3D-CRT were explored using a Tc-99m-galactosyl human serum albumin SPECT image for guidance. The SPECT image was classified as either wedge type or localized type. The clinical target volume to a total dose of 45 or 50 Gy per 18-20 fractions included the main tumor and PVTT in the wedge type and PVTT alone in the localized type.

RESULTS

Twenty-six patients were enrolled: 18 with wedge type and 8 with localized type. Mean tumor size was 7.1 cm (range, 4.4-12.3 cm). Clinical target volumes of wedge type vs. localized type were 111.2 cm(3) vs. 48.4 cm(3) (p = 0.010), respectively. Mean dose to normal liver and mean dose to functional liver were 1185 cGy and 988 cGy (p = 0.001) in wedge type and 1046 cGy and 1043 cGy (p = 0.658) in localized type, respectively. Despite an incidence of Child-Pugh B and C of 57.7%, no patients experienced radiation-induced liver disease. The progression of PVTT was inhibited, with an incidence of 92.2%; survival rates at 1 and 2 years were 44% and 30%, respectively.

CONCLUSION

Single photon emission computed tomography-based 3D-CRT enables irradiation of both the main tumor and PVTT with low toxicity and promising survival.

A contrast-oriented algorithm for FDG-PET-based delineation of tumour volumes for the radiotherapy of lung cancer: derivation from phantom measurements and validation in patient data

PURPOSE

An easily applicable algorithm for the FDG-PET-based delineation of tumour volumes for the radiotherapy of lung cancer was developed by phantom measurements and validated in patient data.

METHODS

PET scans were performed (ECAT-ART tomograph) on two cylindrical phantoms (phan1, phan2) containing glass spheres of different volumes (7.4-258 ml) which were filled with identical FDG concentrations. Gradually increasing the activity of the fillable background, signal-to-background ratios from 33:1 to 2.5:1 were realised. The mean standardised uptake value (SUV) of the region-of-interest (ROI) surrounded by a 70% isocontour (mSUV(70)) was used to represent the FDG accumulation of each sphere (or tumour). Image contrast was defined as C=(mSUV(70)-BG)/BG where BG is the mean background - SUV. For the spheres of phan1, the threshold SUVs (TS) best matching the known sphere volumes were determined. A regression function representing the relationship between TS/(mSUV(70) - BG) and C was calculated and used for delineation of the spheres in phan2 and the gross tumour volumes (GTVs) of eight primary lung tumours. These GTVs were compared to those defined using CT.

RESULTS

The relationship between TS/(mSUV(70) - BG) and C is best described by an inverse regression function which can be converted to the linear relationship TS=a x mSUV(70)+b x BG. Using this algorithm, the volumes delineated in phan2 differed by only -0.4 to +0.7 mm in radius from the true ones, whilst the PET-GTVs differed by only -0.7 to +1.2 mm compared with the values determined by CT.

CONCLUSION

By the contrast-oriented algorithm presented in this study, a PET-based delineation of GTVs for primary tumours of lung cancer patients is feasible.

Threshold segmentation for PET target volume delineation in radiation treatment planning: the role of target-to-background ratio and target size

A multivariable approach was adopted to study the dependence of the percentage threshold [TH(%)] used to define the boundaries of 18F-FDG positive tissue on emission scan duration (ESD) and activity at the start of acquisition (Aacq) for different target sizes and target-to-background (T/B) ratios. An anthropomorphic model, at least for counting rate characteristics, was used to study this dependence in conditions resembling the ones that can be encountered in the clinical studies. An annular ring of water bags of 3 cm thickness was fitted over an International Electro-technical Commission (IEC) phantom in order to obtain counting rates similar to those found in average patients. The scatter fraction of the modified IEC phantom was similar to the mean scatter fraction measured on patients, with a similar scanner. A supplemental set of microhollow spheres was positioned inside the phantom. The NEMA NU 2-2001 scatter phantom was positioned at the end of the IEC phantom to approximate the clinical situation of having activity that extends beyond the scanner field of view. The phantoms were filled with a solution of water and 18F (12 kBq/mL) and the spheres with various T/B ratios of 22.5, 10.3, and 3.6. Sequential imaging was performed to acquire PET images with varying background activity concentrations of about 12, 9, 6.4, 5.3, and 3.1 kBq/mL. The ESD was set to 60, 120, 180, and 240 s/bed. Data were fitted using two distinct multiple linear regression models for sphere ID < or = 10 mm and sphere ID > 10 mm. The fittings of both models were good with an R2 of 0.86 in both cases. Neither ESD nor Aacq resulted as significant predictors of the TH(%). For sphere ID < or =10 mm the target size was the most significant predictor of the TH(%), followed by the T/B ratio, while for sphere ID > 10 mm the explanatory power of the target size and T/B ratio were reversed, the T/B ratio being now the most important predictor of the TH(%). Both the target size and T/B ratio play a major role in explaining the variance of the TH(%), throughout the whole range of target sizes and T/B ratios examined. Thus, algorithms aimed at automatic threshold segmentation should incorporate both variables with a relative weight which critically depends on target size.

Comparison of FDG-PET/CT and CT for delineation of lumpectomy cavity for partial breast irradiation

PURPOSE

The success of partial breast irradiation critically depends on proper target localization. We examined the use of fluorodeoxyglucose-positron emission tomography (FDG-PET)/computed tomography (CT) for improved lumpectomy cavity (LC) delineation and treatment planning.

METHODS AND MATERIALS

Twelve breast cancer patients underwent FDG-PET/CT on a GE Discovery scanner with a median time from surgery to PET/CT of 49 days. The LC was contoured on the CT scan by a radiation oncologist and, together with a nuclear medicine physician, on the PET/CT scan. The volumes were calculated and compared in each patient. Treatment planning target volumes (PTVs) were calculated by expanding the margin 2 cm beyond the LC, maintaining a 5-mm margin from the skin and chest wall, and the treatment plans were evaluated. In addition, a study with a patient-like phantom was conducted to evaluate the effect that the window/level settings might have on contouring.

RESULTS

The margin of the LC was well visualized on all FDG-PET images. The phantom results indicated that the difference between the known volume and the FDG-PET-delineated volume was <10%, regardless of the window/level settings. The PET/CT volumes were larger than the CT volumes in all cases (median volume ratio, 1.68; range, 1.24-2.45; p = 0.004). The PET/CT-based PTVs were also larger than the CT-based PTV (median volume ratio, 1.16; range, 1.08-1.64; p = 0.006). In 9 of 12 patients, a CT-based treatment plan did not provide adequate coverage of the PET/CT-based PTV (99% of the PTV received <95% of the prescribed dose), resulting in substantial cold spots in some plans. In these cases, treatment plans were generated which were specifically designed to cover the larger PET/CT-based PTV. Although these plans showed an increased dose to the normal tissues, the increases were modest: the non-target breast volume receiving > or =50 Gy, lung volume receiving > or =30 Gy, and heart volume receiving > or =5 Gy increased by 5.7%, 0.8%, and 0.2%, respectively. The normal tissue dose-volume objectives were still met with these plans.

CONCLUSION

The results of our study have shown that FDG-PET/CT can be used to define the LC volume. The increased FDG uptake was likely a result of postoperative inflammation in the LC. The targets defined using PET/CT were significantly larger than those defined with CT alone. Our results have shown that treatment plans can be generated to cover these larger PET/CT target volumes with only a modest increase in irradiated tissue volume compared with CT-determined PTVs.

Impact of PET on radiation therapy planning in lung cancer

The superiority of PET imaging to structural imaging in many cancers is rapidly transforming the practice of radiotherapy planning, especially in lung cancer. Although most lung cancers are potentially treatable with radiation therapy, only patients who have truly locoregionally confined disease can be cured by this modality. PET improves selection for high-dose radiation therapy by excluding many patients who have incurable distant metastasis or extensive locoregional spread. In those patients suitable for definitive treatment, PET can help shape the treatment fields to avoid geographic miss and minimize unnecessary irradiation of normal tissues. PET will allow for more accurately targeted dose escalation studies in the future and could potentially lead to better long-term survival.

[18FDG] PET-CT-based intensity-modulated radiotherapy treatment planning of head and neck cancer

PURPOSE

To define the best threshold for tumor volume delineation of the (18) fluoro-2-deoxy-glucose positron emission tomography ((18)FDG-PET) signal for radiotherapy treatment planning of intensity-modulated radiotherapy (IMRT) in head and neck cancer.

METHODS AND MATERIALS

In 25 patients with head-and-neck cancer, CT-based gross tumor volume (GTV(CT)) was delineated. After PET-CT image fusion, window level (L) was adapted to best fit the GTV(CT), and GTV(PET) was delineated. Tumor maximum (S) and background uptake (B) were measured, and the threshold of the background-subtracted tumor maximum uptake (THR) was used for PET signal segmentation. Gross tumor volumes were expanded to planning target volumes (PTVs) and analyzed.

RESULTS

The mean value of S was 40 kBq/mL, S/B ratio was 16, and THR was 26%. The THR correlated with S (r = -0.752), but no correlation between THR and the S/B ratio was seen (r = -0.382). In 77% of cases, S was >30 kBq/mL, and in 23% it was </=30 kBq/mL, with a mean THR of 21.4% and 41.6%, respectively (p < 0.001). Using PTV(PET) in radiotherapy treatment planning resulted in a reduced PTV in 72% of cases, while covering 88.2% of GTV(CT), comparable to the percentage of GTV(PET) covered by PTV(CT) (p = 0.15).

CONCLUSIONS

A case-specific PET signal threshold is optimal in PET-based radiotherapy treatment planning. Signal gating using a THR of 20% in tumors with S >30% +/- 1.6% kBq/mL and 40% in tumors with S </=30% +/- 1.6% kBq/mL is suitable.

FDG-PET/CT imaging for staging and target volume delineation in preoperative conformal radiotherapy of rectal cancer

PURPOSE

To investigate the potential impact of using (18)F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) on staging and target volume delineation for patients affected by rectal cancer and candidates for preoperative conformal radiotherapy.

METHODS AND MATERIALS

Twenty-five patients diagnosed with rectal cancer T3-4 N0-1 M0-1 and candidates for preoperative radiotherapy underwent PET/CT simulation after injection of 5.18 MBq/kg of FDG. Clinical stage was reassessed on the basis of FDG-PET/CT findings. The gross tumor volume (GTV) and the clinical target volume (CTV) were delineated first on CT and then on PET/CT images. The PET/CT-GTV and PET/CT-CTV were analyzed and compared with CT-GTV and CT-CTV, respectively.

RESULTS

In 4 of 25 cases (24%), PET/CT affected tumor staging or the treatment purpose. In 3 of 25 cases (12%) staged N0 M0, PET/CT showed FDG uptake in regional lymph nodes and in a case also in the liver. In a patient with a single liver metastasis PET/CT detected multiple lesions, changing the treatment intent from curative to palliative. The PET/CT-GTV and PET/CT-CTV were significantly greater than the CT-GTV (p = 0.00013) and CT-CTV (p = 0.00002), respectively. The mean difference between PET/CT-GTV and CT-GTV was 25.4% and between PET/CT-CTV and CT-CTV was 4.1%.

CONCLUSIONS

Imaging with PET/CT for preoperative radiotherapy of rectal cancer may lead to a change in staging and target volume delineation. Stage variation was observed in 12% of cases and a change of treatment intent in 4%. The GTV and CTV changed significantly, with a mean increase in size of 25% and 4%, respectively.

A novel iterative method for lesion delineation and volumetric quantification with FDG PET

OBJECTIVES

The determination of lesion boundaries on FDG PET is difficult due to the point-spread blurring and unknown uptake of activity within a lesion. Standard threshold-based methods for volumetric quantification on PET usually neglect any size dependence and are biased by dependence on the signal-to-background ratio (SBR). A novel, model-based method is hypothesized to provide threshold levels independent f the SBR and to allow accurate measurement of volumes down to the resolution of the PET scanner.

METHODS

A background-subtracted relative-threshold level (RTL) method was derived, based on a convolution of the point-spread function and a sphere with diameter D. Validation of the RTL method was performed using PET imaging of a Jaszczak phantom with seven hollow spheres (D=10-60 mm). Activity concentrations for the background and spheres (signal) were varied to obtain SBRs of 1.5-10. An iterative procedure was introduced for volumetric quantification, as the optimal RTL depends on a priori knowledge of the volume. The feasibility of the RTL method was tested in two patients with liver metastases and compared to a standard method using a fixed percentage of the signal.

RESULTS

Phantom data validated that the theoretically optimal RTL depends on the sphere size, but not on the SBR. Typically, RTL=40% (D=15-60 mm), and RTL>50% for small spheres (D<12 mm). The RTL method is better applicable to patient data than the standard method.

CONCLUSIONS

Based on an iterative procedure, the RTL method has been shown to provide optimal threshold levels independent of the SBR and to be applicable in phantom and in patient studies. It is a promising tool for lesion delineation and volumetric quantification of PET lesions.

PET-CT-based auto-contouring in non-small-cell lung cancer correlates with pathology and reduces interobserver variability in the delineation of the primary tumor and involved nodal volumes

PURPOSE

To compare source-to-background ratio (SBR)-based PET-CT auto-delineation with pathology in non-small-cell lung cancer (NSCLC) and to investigate whether auto-delineation reduces the interobserver variability compared with manual PET-CT-based gross tumor volume (GTV) delineation.

METHODS AND MATERIALS

Source-to-background ratio-based auto-delineation was compared with macroscopic tumor dimensions to assess its validity in 23 tumors. Thereafter, GTVs were delineated manually on 33 PET-CT scans by five observers for the primary tumor (GTV-1) and the involved lymph nodes (GTV-2). The delineation was repeated after 6 months with the auto-contour provided. This contour was edited by the observers. For comparison, the concordance index (CI) was calculated, defined as the ratio of intersection and the union of two volumes (A intersection B)/(A union or logical sum B).

RESULTS

The maximal tumor diameter of the SBR-based auto-contour correlated strongly with the macroscopic diameter of primary tumors (correlation coefficient = 0.90) and was shown to be accurate for involved lymph nodes (sensitivity 67%, specificity 95%). The median auto-contour-based target volumes were smaller than those defined by manual delineation for GTV-1 (31.8 and 34.6 cm(3), respectively; p = 0.001) and GTV-2 (16.3 and 21.8 cm(3), respectively; p = 0.02). The auto-contour-based method showed higher CIs than the manual method for GTV-1 (0.74 and 0.70 cm(3), respectively; p < 0.001) and GTV-2 (0.60 and 0.51 cm(3), respectively; p = 0.11).

CONCLUSION

Source-to-background ratio-based auto-delineation showed a good correlation with pathology, decreased the delineated volumes of the GTVs, and reduced the interobserver variability. Auto-contouring may further improve the quality of target delineation in NSCLC patients.

PET/CT: form and function

Functional imaging with positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of malignant disease, image-guided therapy planning, and treatment monitoring. PET with the labeled glucose analogue fluorine 18 fluorodeoxyglucose (FDG) is a relatively recent addition to the medical technology for imaging of cancer, and FDG PET complements the more conventional anatomic imaging modalities of computed tomography (CT) and magnetic resonance imaging. CT is complementary in the sense that it provides accurate localization of organs and lesions, while PET maps both normal and abnormal tissue function. When combined, the two modalities can help both identify and localize functional abnormalities. Attempts to align CT and PET data sets with fusion software are generally successful in the brain; other areas of the body is more challenging, owing to the increased number of degrees of freedom between the two data sets. These challenges have recently been addressed by the introduction of the combined PET/CT scanner, a hardware-oriented approach to image fusion. With such a device, accurately registered anatomic and functional images can be acquired for each patient in a single scanning session. Currently, over 800 combined PET/CT scanners are installed in medical institutions worldwide, many of them for the diagnosis and staging of malignant disease and increasingly for monitoring of the response to therapy. This review will describe some of the most recent technologic developments in PET/CT instrumentation and the clinical indications for which combined PET/CT has been shown to be more useful than PET and CT performed separately.

Fully automated segmentation of oncological PET volumes using a combined multiscale and statistical model

The widespread application of positron emission tomography (PET) in clinical oncology has driven this imaging technology into a number of new research and clinical arenas. Increasing numbers of patient scans have led to an urgent need for efficient data handling and the development of new image analysis techniques to aid clinicians in the diagnosis of disease and planning of treatment. Automatic quantitative assessment of metabolic PET data is attractive and will certainly revolutionize the practice of functional imaging since it can lower variability across institutions and may enhance the consistency of image interpretation independent of reader experience. In this paper, a novel automated system for the segmentation of oncological PET data aiming at providing an accurate quantitative analysis tool is proposed. The initial step involves expectation maximization (EM)-based mixture modeling using a k-means clustering procedure, which varies voxel order for initialization. A multiscale Markov model is then used to refine this segmentation by modeling spatial correlations between neighboring image voxels. An experimental study using an anthropomorphic thorax phantom was conducted for quantitative evaluation of the performance of the proposed segmentation algorithm. The comparison of actual tumor volumes to the volumes calculated using different segmentation methodologies including standard k-means, spatial domain Markov Random Field Model (MRFM), and the new multiscale MRFM proposed in this paper showed that the latter dramatically reduces the relative error to less than 8% for small lesions (7 mm radii) and less than 3.5% for larger lesions (9 mm radii). The analysis of the resulting segmentations of clinical oncologic PET data seems to confirm that this methodology shows promise and can successfully segment patient lesions. For problematic images, this technique enables the identification of tumors situated very close to nearby high normal physiologic uptake. The use of this technique to estimate tumor volumes for assessment of response to therapy and to delineate treatment volumes for the purpose of combined PET/CT-based radiation therapy treatment planning is also discussed.

Tumor delineation using PET in head and neck cancers: threshold contouring and lesion volumes

Tumor boundary delineation using positron emission tomography (PET) is a promising tool for radiation therapy applications. In this study we quantify the uncertainties in tumor boundary delineation as a function of the reconstruction method, smoothing, and lesion size in head and neck cancer patients using FDG-PET images and evaluate the dosimetric impact on radiotherapy plans. FDG-PET images were acquired for eight patients with a GE Advance PET scanner. In addition, a 20 cm diameter cylindrical phantom with six FDG-filled spheres with volumes of 1.2 to 26.5 cm3 was imaged. PET emission scans were reconstructed with the OSEM and FBP algorithms with different smoothing parameters. PET-based tumor regions were delineated using an automatic contouring function set at progressively higher threshold contour levels and the resulting volumes were calculated. CT-based tumor volumes were also contoured by a physician on coregistered PET/CT patient images. The intensity value of the threshold contour level that returns 100% of the actual volume, I(V100), was measured. We generated intensity-modulated radiotherapy (IMRT) plans for an example head and neck patient, treating 66 Gy to CT-based gross disease and 54 Gy to nodal regions at risk, followed by a boost to the FDG-PET-based tumor. The volumes of PET-based tumors are a sensitive function of threshold contour level for all patients and phantom datasets. A 5% change in threshold contour level can translate into a 200% increase in volume. Phantom data indicate that I(V100) can be set as a fraction, f, of the maximum measured uptake. Fractional threshold values in the cylindrical water phantom range from 0.23 to 0.51. Both the fractional threshold and the threshold-volume curve are dependent on lesion size, with lesions smaller than approximately 5 cm3 displaying a more pronounced sensitivity and larger fractional threshold values. The threshold-volume curves and fractional threshold values also depend on the reconstruction algorithm and smoothing filter with more smoothing requiring a higher fractional threshold contour level. The threshold contour level affects the tumor size, and therefore the ultimate boost dose that is achievable with IMRT. In an example head and neck IMRT plan, the D95 of the planning target volume decreased from 7770 to 7230 cGy for 42% vs. 55% contour threshold levels. PET-based tumor volumes are strongly affected by the choice of threshold level. This can have a significant dosimetric impact. The appropriate threshold level depends on lesion size and image reconstruction parameters. These effects should be carefully considered when using PET contour and/or volume information for radiotherapy applications.

Incorporating PET information in radiation therapy planning

PET scanning, because of its impressive sensitivity and accuracy, is being incorporated into the standard staging workup for many cancers. These include lung cancer, lymphomas, head and neck cancers, and oesophageal cancers. PET often provides incremental information about the patient’s disease status, adding to the data obtained from structural imaging methods, such as, CT scan or MRI. PET commonly upstages patients into more advanced disease categories. Incorporation of PET information into the radiotherapy planning process has the potential to reduce the risks of geographic miss and can help minimise unnecessary irradiation of normal tissues. The best means of incorporating PET information into radiotherapy planning is uncertain, and considerable effort is being expended in this area of research.

The role of 18F-fluoro-deoxy-glucose positron emission tomography (FDG-PET) in the management of patients with colorectal cancer

AIM

In patients with colorectal cancer an accurate diagnostic work-up is mandatory in order to perform the most specific treatment. At this moment 18F-fluoro-deoxy-glucose positron emission tomography (FDG-PET) is considered an accurate imaging technique in staging/restaging several malignancies. The aim of this paper is to review the scientific literature available about the role of FDG-PET in the management of patients with colorectal cancer.

METHODS

An overview on Medline of scientific literature concerning FDG-PET and colorectal cancer was performed. The most relevant studies are reported. Advantages, limitations and new chances in using FDG-PET in these subsets of patients are summarized.

RESULTS

FDG-PET is a useful tool in the evaluation of colorectal cancer. In comparison to conventional imaging technique, FDG-PET has an additional diagnostic value because it allows to metabolically characterize undetermined lesions suspected for recurrence of disease, to perform a complete pre-surgical staging and to identify occult metastatic disease. In clinical practice its use leads to a change in therapeutic choices in a high percentage of cases.

CONCLUSIONS

FDG-PET should be considered an essential diagnostic tool in the management of patients with colorectal cancer, especially in recurrent disease evaluation.

Physics and imaging for targeting of oligometastases

Oligometastases refer to metastases that are limited in number and location and are amenable to regional treatment. The majority of these metastases appear in the brain, lung, liver, and bone. Although the focus of interest in the past within radiation oncology has been on the treatment of intracranial metastases, there has been growing interest in extracranial sites such as the liver and lung. This is largely because of the rapid development of targeting techniques for oligometastases such as intensity-modulated and image-guided radiation therapy, which has made it possible to deliver single or a few fractions of high-dose radiation treatments, highly conformal to the target. The clinical decision to use radiation to treat oligometastases is based on both radiobiological and physics considerations. The radiobiological considerations involve improvement of treatment schema for time, dose, and volume. Areas of interests are hypofractionation, tumor and normal tissue tolerance, and hypoxia. The physics considerations for oligometastases treatment are focused mainly on ensuring treatment accuracy and precision. This article discusses the physics and imaging aspects involved in each step of the radiation treatment process for oligometastases, including target definition, treatment simulation, treatment planning, pretreatment target localization, radiation delivery, treatment verification, and treatment evaluation.

Assessment of 18F PET signals for automatic target volume definition in radiotherapy treatment planning

INTRODUCTION

Positron emission tomography (PET) alone or in combination with computer tomography (PET/CT) is increasingly used in target volume assessment. A standardized way of converting PET signals into target volumes is not available at present.

MATERIALS AND METHODS

Assuming a uniform signal emission from a tumour and surrounding normal tissues, a model-based method was developed to determine a relative threshold level (Th(rel)) for gross tumour volume delineation. Two phantoms consisting of cylindrical and spherical sources of diameter ranging from 4.5 to 43 mm in a tank and (18)F activities ranging from 0.001 to 0.15 MBq/ml for tank and sources, respectively, were used for PET/CT imaging. A Th(rel) was calculated that best corresponded to the physical diameter of the cylindrical sources. Software (SW) was generated to automatically delineate volumes based on this threshold. The SW was validated for in vitro and in vivo PET signals.

RESULTS

The Th(rel) best representing the source diameter was 41+/-2.5% (95% confidence level) of the background-subtracted signal. The mean deviation for sources of diameter > or =12.5 mm was < or =1.5 mm. The Th(rel) was constant for diameters > or =12.5 mm. For source diameters <12.5 mm, the 41% level over-estimated the real source diameter by a factor depending on the diameter. In an in vitro set-up the SW was capable of segmenting solitary PET volumes to within 1.4 mm (1SD). For non-homogeneous signals in a clinical set-up minimal manual intervention is presently required to separate target from non-target signals. The SW may slightly underestimate target volumes when compared with CT-based volumes, but works well as a first approximation. The volume can be manually adapted to give the ultimate target volume.

CONCLUSIONS

SW-based automatic delineation of the volume of (18)F activity is feasible and highly reproducible. Volumes can be subsequently modified by the clinician if necessary. This approach will increase the efficiency of the planning process.

PET/CT today: System and its impact on cancer diagnosis

Over the past six years, PET/CT has spread rapidly and replaced conventional PET. Although PET/CT is a combination of PET for functional information and CT for morphological information, their combination is synergistic. PET/CT fusion images result in higher diagnostic accuracy with fewer equivocal findings. This results in a greater impact on cancer diagnosis. With attenuation correction performed by the CT component, PET/CT can provide higher quality images over shorter examination times than conventional PET. As with all modalities, PET/CT has several characteristic artifacts such as misregistration due to respiration, overattenuation correction due to metals, etc. Awareness of these pitfalls will help the imaging physician use PET/CT effectively in daily practice.

Neoadjuvant radiochemotherapy in the treatment of fixed and semi-fixed rectal tumors. Analysis of results and prognostic factors

PURPOSE

To report the retrospective analysis of patients with locally advanced rectal cancer treated with neodjuvant radiochemotherapy.

METHODS AND MATERIALS

From January 1994 to December 2003, 101 patients with fixed (25%) or semi-fixed (75%) rectal adenocarcinoma were treated by preoperative radiotherapy with a dose of 45 Gy at the whole pelvis and 50.4 Gy at primary tumor, concomitant to four weekly chemotherapies with 5-Fluorouracil (425 mg/m2) and Leucovorin (20 mg/m2). In 71 patients (70.3%) the primary tumor was located up to 6 cm from the anal verge and in 30 (29.7%) from 6.5 cm to 10 cm. Age, gender, tumor fixation, tumor distance from the anal verge, clinical response, surgical technique, and postoperative TNM stage were the prognostic factors analyzed for overall survival (OS), disease-free survival (DFS), and local control (LC) at five years.

RESULTS

Median follow-up time was 38 months (range, 2-141). Complete response was observed in eight patients (7.9%), partial in 54 (53.4%) and absence in 39 (38.7%). OS, DFS and LC were 52.6%, 53.8%, and 75.9%, respectively. Distant metastasis occurred in 40 (39.6%) patients, local recurrence in 20 (19.8%) and both in 16 (15.8%). Patients with fixed tumors had lower OS (17% Vs 65.6%; p < 0.001), DFS (31.2% Vs 60.9%; p = 0.005), and LC (58% Vs 82%; p = 0.004). Patients with tumors more than 6 cm above the anal verge had better LC (93% Vs 69%; p = 0.04). The postoperative TNM stage was a significant factor for DFS (I:64.1%, II:69.6%, III:35.2%, IV:11.1%; p < 0.001) and for LC (I:75.7%, II: 92.9%, III:54.1%, IV:100%; p = 0.005). Patients with positive lymph nodes had worse OS (37.9% Vs 70.4%, p = 0.006), DFS (32% Vs 72.7%, p < 0.001) and LC (56.2% Vs 93.4%; p < 0.001).

CONCLUSION

This study suggests that the neoadjuvant treatment employed was effective for local control. Fixation of the lesion and lymph nodes metastasis were the main adverse prognostic factors. Distant failures were frequent, supporting the need of new drugs for adjuvant chemotherapy.

The current status of FDG-PET in tumour volume definition in radiotherapy treatment planning

Positron emission tomography (PET) scan, mainly using 18 F-fluoro-deoxyglucose (FDG) as a tracer, is currently widely accepted as a diagnostic tool in oncology. It may lead to a change in staging and therefore in treatment management. PET can also be used to define the target volume in radiation treatment planning and to evaluate treatment response. In this review, we focused on issues concerning the role of PET in target volume delineation, both for the primary tumour and regional lymph nodes. A literature search was performed using MEDLINE. Furthermore, the following questions were addressed: does PET allow accurate tumour delineation and does it improve the outcome of radiotherapy, in terms of reduced toxicity or a higher tumour control probability? Combined computer tomography (CT) and PET information seems to influence target volume delineation. Using (CT-) PET scan, interobserver variability is being reduced. Only few studies compared delineation based on PET with pathologic examination, showing a complex relation. Preliminary results concerning incorporation of PET information in to target volume delineation varies in different tumour sites. In the field of lung cancer, incorporation of PET seems to improve tumour coverage and spare normal tissues, which may lead to less toxicity or the possibility to escalate dose. In oesophageal cancer and in lymphoma, PET scan can be used to include PET positive lymph nodes in the target volume. In most other tumour sites not enough data are currently available to draw definitive conclusions about the role of PET in radiation treatment planning.