[A study of collimator scatter factors of rectangular irradiation fields using the virtual source plane].

Kim introduced the geometric weighting factor concept into the field mapping method, and estimated collimator scatter factors of rectangular fields by correcting collimator exchange effects. The source plane is present at a specific position in the field mapping method, and, accordingly, the geometric weighting factor is constant. In this study, we changed the position of the source plane based on the measurement results, and we estimated the collimator factors of rectangular fields using the field mapping method. A geometric weighting factor at which the measured collimator scatter factors optimally fitted a square collimator scatter factor was calculated in each field. Collimator scatter factors can be accurately calculated recursively by changing the geometric weighting factor, resulting in altering the position of the source plane, as in this method.

[Application of PET in radiation oncology].

Positron emission tomography (PET), especially F-18 fluorodeoxyglucose (FDG)-PET, has been recently used to verify the target volume in radiation treatment planning (RTP) for malignancies. The utility of FDG-PET/CT in defining gross tumor volume (GTV) has been shown in many studies, and the target delineation by a fixed threshold of the maximum standardized uptake value (40-50%) is suggested to be useful in RTP for lung cancer, head and neck cancer, etc. But, the spatial resolution, sensitivity, and specificity of PET are not always enough to define the difference between the GTV and the clinical target volume(CTV). Furthermore, FDG-PET is frequently used in the clinical staging before the treatment, and is also applied to the response evaluation after the treatment. This review focuses on the developing applications of PET in radiation oncology.

Physical and histopathological assessment of the effects of metallic stents on radiation therapy.

To evaluate whether simultaneous metallic stent (MS) placement and radiotherapy are feasible, phantom and animal experiments were performed. The interface dose by external irradiation (EI) or intracavity irradiation (II) to 5 kinds of MS was measured using the charge-coupled device (CCD) camera with a thermoluminescent (TL) sheet, and backscatter and absorption were evaluated using composite method. Lineac 10 MV X-ray irradiated the MS in close contact with the TL sheet. II was performed using (192)Ir, and the irradiation dose transmitted through the MS was measured using the TL sheet. The ratio of the CCD value of the MS wire region to that of the MS non-wire region was defined as the dose perturbation factor (DPF). Furthermore, the effects of a combination of (60)Co gamma-ray EI and MS placement in the normal common bile duct were histopathologically evaluated in dogs. In the phantom experiments of EI, in backscatter by the MS, the DPF was 1.09 for CZ, and 1.03 for Pal, but no backscatter was detected in the remaining 3 MS. In absorption by the MS, the DPF was 0.92, 0.97, 0.97, and 0.98 for CZ, Wall, Pal, and Vel, respectively, but no absorption was detected in U. Flex. In those of II, the DPF of absorption was 0.91, 0.98, and 0.98 for CZ, U. Flex, and Wall, respectively, but no absorption was detected in Pal and Vel. The animal experiments showed infiltration of inflammatory cells and fibrosis in the case of both MS placement and EI. These changes were marked in EI treating after MS placement, but neither severe ulcer nor perforation was found. In conclusion, these results suggested that the effect of MS should be considered carefully when simultaneous MS placement and EI is performed clinically.

Efficacy of FDG-PET for defining gross tumor volume of head and neck cancer.

We analyzed the data for 53 patients with histologically proven primary squamous cell carcinoma of the head and neck treated with radiotherapy between February 2006 and August 2009. All patients underwent contrast-enhanced (CE)-CT and (18)F-fluorodeoxyglucose (FDG)-PET before radiation therapy planning (RTP) to define the gross tumor volume (GTV). The PET-based GTV (PET-GTV) for RTP was defined using both CE-CT images and FDG-PET images. The CE-CT tumor volume corresponding to a FDG-PET image was regarded as the PET-GTV. The CE-CT-based GTV (CT-GTV) for RTP was defined using CE-CT images alone. Additionally, CT-GTV delineation and PET-GTV delineation were performed by four radiation oncologists independently in 19 cases. All four oncologists did both methods. Of these, PET-GTV delineation was successfully performed in all 19 cases, but CT-GTV delineation was not performed in 4 cases. In the other 15 cases, the mean CT-GTV was larger than the PET-GTV in 10 cases, and the standard deviation of the CT-GTV was larger than that of the PET-GTV in 10 cases. Sensitivity of PET-GTV for identifying the primary tumor was 96%, but that of CT-GTV was 81% (P < 0.01). In patients with oropharyngeal cancer and tongue cancer, the sensitivity of CT-GTV was 63% and 71%, respectively. When both the primary lesions and the lymph nodes were evaluated for RTP, PET-GTV differed from CT-GTV in 19 cases (36%). These results suggested that FDG-PET is effective for defining GTV in RTP for squamous cell carcinoma of the head and neck, and PET-GTV evaluated by both CE-CT and FDG-PET images is preferable to CT-GTV by CE-CT alone.

[Investigation of vessel visibility in the iterative reconstruction method in coronary computed tomography angiography using simulated vessel phantom].

Iterative reconstruction methods can reduce the noise of computed tomography (CT) images, which are expected to contribute to the reduction of patient dose CT examinations. The purpose of this study was to investigate impact of an iterative reconstruction method (iDose(4), Philips Healthcare) on vessel visibility in coronary CT angiography (CTA) by using phantom studies. A simulated phantom was scanned by a CT system (iCT, Philips Healthcare), and the axial images were reconstructed by filtered back projection (FBP) and given a level of 1 to 7 (L1-L7) of the iterative reconstruction (IR). The vessel visibility was evaluated by a quantitative analysis using profiles across a 1.5-mm diameter simulated vessel as well as visual evaluation for multi planar reformation (MPR) images and volume rendering (VR) images in terms of the normalized-rank method with analysis of variance. The peak CT value of the profiles decreased with IR level and full width at half maximum of the profile also decreased with the IR level. For normalized-rank method, there was no statistical difference between FBP and L1 (20% dose reduction) for both MPR and VR images. The IR levels higher than L1 sacrificed the spatial resolution for the 1.5-mm simulated vessel, and their visual vessel visibilities were significantly inferior to that of the FBP.

Axillary lymph node accumulation on FDG-PET/CT after influenza vaccination.

OBJECTIVE: 2-[(18)F]fluoro-2-deoxy-D: -glucose (FDG) is known to accumulate in benign conditions such as infection and inflammation as well as in malignancy. Vaccination may cause transient inflammation of lymph nodes, which may induce false-positive findings on FDG-positron emission tomography (PET) imaging. This study investigated the influence of influenza vaccination on FDG-PET/CT imaging in normal subjects. METHODS: Between November 2008 and March 2009, a total of 172 examinees underwent FDG-PET/CT during an annual cancer-screening program at our hospital, 83 of whom had a history of recent non-adjuvanted seasonal influenza vaccination. They were asked the date and injection site of the vaccination. Examinees were divided into 2 groups based on the interval after vaccination using a cutoff value of 7 days (1 week). Two double board-certified nuclear medicine physicians and radiologists visually interpreted the FDG-PET/CT images with reference to PET/CT fusion and CT images and checked the location and the number of abnormal accumulations by consensus reading. RESULTS: Intervals between vaccination and FDG-PET were less than 7 days in 5 examinees, and 7 days or more in 78 examinees. Unexpected accumulations were visualized in 4 examinees in the axilla and medial upper arm, and all of them belonged to the group who underwent vaccination less than 7 days previously. In the second group there was no abnormal FDG accumulation. CONCLUSIONS: Recent influenza vaccination before FDG-PET/CT examination may cause ipsilateral axillary lymph node accumulations, especially within several days after vaccination. Questionnaires about vaccination can help to avoid false interpretation of FDG avid axillary lymph nodes.

Phantom study on radiotherapy planning using PET/CT--delineation of GTV by evaluating SUV.

We assessed the usefulness of PET/CT images to determine the target volume in radiotherapy planning by evaluating the standardized uptake value (SUV). We evaluated the imaging conditions and image-reconstruction conditions of PET/CT useful for treatment planning by collecting (18)F-FDG images of acrylic spheres (10-48 mm in diameter) in a phantom. The (18)F-FDG concentration in the spheres was 10-fold higher than that of the phantom. The contours were delineated according to the SUV by the threshold and regions of interest (ROI) methods. Comparisons of two- and three-dimensional (2D and 3D) acquisition images indicated that the sharpness and quantitative qualities of the sphere boundaries were better in the former than in the latter. In the extraction of outlines using the SUV, outlines obtained at an SUV of 40-50% of the maximum agreed well with the actual acrylic sphere size. 2D acquisition images are more suitable for delineating target volume contours, although 3D acquisition images are more popular in diagnostic imaging. An SUV of 40-50% of the maximum is suggested to be appropriate for GTV contouring of sphere tumors with homogenously distributed (18)F-FDG.