Basic evaluation of a novel 4D target and human body phantom.

Stereotactic body radiation therapy (SBRT) is usually verified with a dynamic phantom or solid phantom, but there is a demand for phantoms that can accurately simulate tumor dynamics within an individual that would allow customized validation in every patient. We developed a new 4D dynamic target phantom (multi-cell 4D phantom) that allows simulation of tumor movement in patients. The basic quality and dynamic reproducibility of this new phantom was verified in this investigation. The newly developed multi-cell 4D phantom comprises four main components: soft tissue, bones, lungs, and tumor (target). The phantom structure was based on computed tomography (CT) data of a male. In this study, we investigated the basic performance of a multi-cell 4D phantom. All the CT numbers of the phantom were very close to those of human data. The geometric maximum amplitudes were 4.57 mm in the lateral direction, 4.59 mm in the ventrodorsal direction, and 3.68 mm in the cranio-caudal direction. Geometric errors were 0.84, 0.58, and 0.40 mm, respectively. Movements of the abdominal surface were stable for 60 s. Repeated measurements show no actual differences in target movements between multiple measurements and indicated high reproducibility (r > 0.97). End-to-end tests using Gafchromic film revealed a gamma pass rate of 98% or above (2 mm/3%). Although our phantom performed limited reproducibility in the movement of the patient tumor at present, a satisfactory level of precision was confirmed in general. This is a very promising device for use in the verification of radiation therapy for moving targets.

An analysis of the survival rate after radiotherapy in lung cancer patients with bone metastasis: is there an optimal subgroup to be treated with high-dose radiation therapy?

We investigated the prognostic factors after radiotherapy for bone metastasis from lung cancer while taking the recent findings in the treatment of such cases into consideration. A total of 132 patients with bone metastases from pathologically confirmed lung cancer were evaluated regarding the following potential prognostic factors: treatment for primary site (surgery vs. other), treatment site (spine vs. other), number of bone metastases (solitary vs. multiple), number of metastatic organs (0 vs. 1 vs. ≥2), neurological symptoms (no symptoms vs. numbness vs. paresis), degree of pain (no pain vs. mild pain vs. severe pain), performance status [PS] (0-1 vs. ≥2), biological effective dose [BED] (≥40 Gy vs. <40Gy), time to distant metastasis (≥1 year vs. <1 year), histology (adenocarcinoma vs. others), and use of epidermal growth factor receptor [EGFR]-targeted agents (Yes vs. No). The univariate analysis demonstrated that all factors except for the treatment site were significant. Surgery as treatment for primary site, solitary bone metastasis, no visceral organ metastasis, no symptoms or numbness, no pain, PS<2, BED≥40 Gy, time to distant metastasis ≥ 1year, adenocarcinoma histology, and use of EGFR-targeted agents were correlated with a favorable prognosis. In a multivariate analysis, solitary bone metastasis, PS<2, BED≥40 Gy, adenocarcinoma histology, and the use of EGFR-targeted agents were significantly correlated with a better survival (p = 0.038, 0.006, 0.003, 0.014, and <0.001, respectively). A contingency table to assess the relationship between each variable and the median survival time of the patients according to the administered BED showed that in patients with the time to distant metastasis ≥ 1year and the use of EGFR-targeted agents, the subgroups treated with BED≥40 Gy had a favorable prognosis. Our study suggests that high-dose radiotherapy is associated with a better prognosis in combination with other favorable prognostic factors.

Comparison of clinical, tumour-related and dosimetric factors in grade 0-1, grade 2 and grade 3 radiation pneumonitis after stereotactic body radiotherapy for lung tumours.

OBJECTIVE: The aim of this study was to investigate significant clinical, tumour-related and dosimetric factors among patients with grade 0-1, grade 2 and grade 3 radiation pneumonitis (RP) after stereotactic body radiotherapy (SBRT) for lung tumours. METHODS: Patients (n=128) with a total of 133 lung tumours treated with SBRT of 50 Gy in 5 fractions were analysed. RP was graded according to the Common Terminology Criteria for Adverse Events v.3.0. Significant factors were identified by univariate and multivariate analyses. Threshold dose-volume histograms (DVHs) were constructed to identify the incidence of RP. RESULTS: The median follow-up period was 12 months (range, 6-45 months). In univariate analyses, gender, operability, forced expiratory volume in 1 s (FEV1), internal target volume, lung volumes treated with doses >5-30 Gy (V5-30) and mean lung dose were significant factors differentiating between grade 0-1 and grade 2 RP, and V15-30 were significant factors differentiating between grade 2 and grade 3. However, no factors were significant between grade 0-1 and grade 3 RP. Multivariate analysis showed that female gender, high FEV1 and high V15 were significant factors differentiating between grade 0-1 and grade 2 RP. Threshold DVH curves were created based on ≤5% and ≤15% risk of grade 2 RP among patients with grade 0-2 RP. CONCLUSIONS: Grade 0-2 RP was dose-volume dependent, and female gender and high FEV1 were significant predictive clinical factors for grade 2 RP among patients with grade 0-2 RP. However, incidences of V15-30 in grade 3 RP were significantly lower than those in grade 2 RP, and no significant clinical or tumour-related factors were found. Further studies are needed to identify the mechanism underlying the development of grade 3 RP after SBRT for lung tumours.

SU-E-J-100: Feasibility of Dose Calculation Using Combined Information of Cone-Beam and Multi-Slice CT Images.

PURPOSE: The purpose of this study is to evaluate the feasibility of kilovoltage cone-beam CT (CBCT) images that are obtained with the Varian On-Board Imager in dose calculation at each radiation therapy. METHODS: CBCT images are commonly degraded by scattered radiations originating in the patient's body, and so the CT numbers of the CBCT images depend on data acquisition conditions and the patient size. However, the anatomical shape of each organ is not likely affected by scattered radiations, and so we used only the shape of major organs such as lungs and bones in the CBCT images, and replaced these CT numbers with those of the multi-slice CT (MSCT) images that were used for dose calculation in a treatment planning. As regards this alternative CT number we adopted the median of MSCT numbers in a segmented region of a major organ each corresponding to that in the CBCT images. We evaluated the validity of our segmented region (SR) method with images of eight patients with lung diseases. The number of irradiation beams was four. In this evaluation we used the distance-to-agreement (DTA) and y analysis, and the dose-volume-histogram (DVH) analysis. RESULTS: The pass rates of the DTA analysis (2mm) and γ analysis (2mm, 2%) between the dose distributions calculated with our method were 90.4±6.0% and 99.1±1.1%, respectively. The results of the DVH analysis showed that the differences in doses (average, maximum and minimum) for a target volume were 1.3±0.5%, 0.9±0.8% and 3.4±3.0%, respectively. These results showed that our method was acceptable in the calculation of a dose distribution. CONCLUSION: We evaluated the dose calculation method with a combination of CBCT and MSCT images. This method could yield an accurate dose distribution and achieved an easier verification of radiation therapy on each treatment day.

SU-E-I-63: Performance Study of An Electron-Tracking Compton Camera for Medical Imaging.

PURPOSE: Conventional gamma-ray detector, PET and SPECT, have the limitation of energy and field of view. These limitations are major problems of studying for a new medical imaging. Therefore, we have developed the new imaging detector which is an electron-tracking Compton camera (ETCC). METHODS: A reconstruction method of Compton camera (CC) is using the physics principle. Because of using physics principle, CC can have a wide energy dynamic range and wide field of view. Conventional CC, however, cannot catch Compton recoil electron tracks, and this is one of the reasons of low imaging power. We have developed a time projection chamber (TPC) using micro pixel chamber (µPIC) as the new detector for ETCC. The µPIC is 2-dimensional gaseous detector and this position resolution is less than 400 µm. Using this detector, ETCC can get electron tracks which are generated from Compton scattering. In this paper, we show the prototype ETCC performance and imaging results. RESULTS: ETCC achieved a wide energy dynamic range (200-1300keV) and wide field of view (3 steradian). Also we succeeded in imaging new imaging reagents using mice as follows; (1) F-18-FDG (511 keV) and I-131-MIBG (364 keV) simultaneous imaging for double clinical tracer imaging, (2) Zn-65- porphyrin (1116 keV) imaging for high energy gamma-ray imaging and, (3) imaging of some minerals (Mn-54, Zn-65) in mice and so on. And we succeeded in 3-D imaging which has imaged only one direction using one head camera. CONCLUSIONS: We have developed the ETCC for new medical imaging device and succeeded in imaging the some imaging reagents. We started to develop the new ETCC which can image the mouse within 30 min. Thus, this detector has the possibility of new medical imaging.

CT evaluations of focal liver reactions following stereotactic body radiotherapy for small hepatocellular carcinoma with cirrhosis: relationship between imaging appearance and baseline liver function.

This study aimed to assess the imaging appearances of focal liver reactions following stereotactic body radiotherapy (SBRT) for small hepatocellular carcinoma (HCC) and to examine relationships between imaging appearance and baseline liver function. We retrospectively studied 50 lesions in 47 patients treated with SBRT (30-40 Gy in 5 fractions) for HCC, who were followed up for more than 6 months. After SBRT, all patients underwent regular follow-ups with blood tests and dynamic CT scans. At a median follow-up of 18.1 months (range 6.2-43.7 months), all lesions but one were controlled. 3 density patterns describing focal normal liver reactions around HCC tumours were identified in pre-contrast, arterial and portal-venous phase scans: iso/iso/iso in 4 patients (Type A), low/iso/iso in 8 patients (Type B) and low/iso (or high)/high in 38 patients (Type C). Imaging changes in the normal liver surrounding the treated HCC began at a median of 3 months after SBRT, peaked at a median of 6 months and disappeared 9 months later. Liver function, as assessed by the Child-Pugh classification, was the only factor that differed significantly between reactions to treatment showing "non-enhanced" (Type A and B) and "enhanced" (Type C) appearances in CT. Hence, liver tissue with preserved function is more likely to be well enhanced in the delayed phase of a dynamic contrast-enhanced CT scan. The CT appearances of normal liver seen in reaction to the treatment of an HCC by SBRT were therefore related to background liver function and should not be misread as recurrence of HCC.

Spinal deformity after intra-operative radiotherapy for paediatric patients.

The purpose of this study was to clarify the incidence and characteristics of late-onset complications of the spine in children who underwent intra-operative radiation therapy (IORT) for common paediatric malignant tumours. 12 children with more than 4 years of follow-up after IORT were included and, in 11 of these, thoracic and/or lumbar spines were irradiated. To compare doses of irradiation to the spine with the resulting deformities, dose simulations of IORT were carried out on two selected cases using a radiation treatment planning system with a pencil-beam algorithm. The mean follow-up period was 135 months (range, 53-234 months). Radiographic reviews found spinal deformity in six patients. Only one patient was symptomatic and the spinal deformity was severe (Grade 3), whereas spinal deformity was mild in the remaining five patients without clinical symptoms (Grade 1). In all of the six patients, anterior wedge-shaped deformity was dominant, and scoliosis was found in only two patients. In one particular case with nephrectomy, irradiation had penetrated much deeper than usual at the site of nephrectomy, and dose distribution was asymmetric, causing clinically significant spinal deformity with scoliosis. In conclusion, specific deformities of the spine observed after IORT can be explained on the basis of dose distribution of the electron beam to the spine.

Gadolinium-enhanced three-dimensional magnetization-prepared rapid gradient-echo (3D MP-RAGE) imaging is superior to spin-echo imaging in delineating brain metastases.

BACKGROUND: Precisely defining the number and location of brain metastases is very important for establishing a treatment strategy for malignancies. Although magnetic resonance imaging (MRI) is now considered the best modality, various improvements in sequences are still being made. PURPOSE: To prospectively compare the diagnostic ability of three-dimensional, magnetization-prepared rapid gradient-echo (3D MP-RAGE) imaging in detecting metastatic brain tumors, with that of two-dimensional spin-echo (2D SE) T1-weighted imaging. MATERIAL AND METHODS: A total of 123 examinations were included in this study, and 119 examinations from 88 patients with known malignancies were analyzed. All patients underwent T1- and T2-weighted 2D SE transverse imaging, followed by gadolinium-enhanced T1-weighted transverse and coronal 2D SE imaging and 3D MP-RAGE transverse imaging. Four radiologists interpreted the images to compare the accuracy and the time required for interpretation for each imaging. RESULTS: 3D MP-RAGE imaging was significantly better than 2D SE imaging for detecting metastatic brain lesions, regardless of the readers' experience. The sensitivities of the 3D MP-RAGE and 2D SE imaging for all observers were 0.81 vs. 0.80 (P>0.05), specificities were 0.93 vs. 0.87 (P<0.05), positive predictive values were 0.92 vs. 0.86 (P<0.05), negative predictive values were 0.78 vs. 0.75 (P>0.05), and accuracies were 0.84 vs. 0.78 (P<0.05), respectively. There was no significant difference in the time required for image interpretation between the two modalities (15.6+/-4.0 vs. 15.4+/-4.1 min). CONCLUSION: 3D MP-RAGE imaging proved superior to 2D SE imaging in the detection of brain metastases.

Variation of dose distribution of stereotactic radiotherapy for small-volume lung tumors under different respiratory conditions.

PURPOSE: To clarify the effects of respiratory condition on dose calculation for stereotactic radiotherapy of small lung tumors. METHODS AND MATERIALS: Computed tomography (CT) data were obtained for nine tumors (diameter, 2.1-3.6cm; mean, 2.7cm) during the stable state, deep expiration, and deep inspiration breath-hold states. Rotational Irradiation with 3 non-coplanar arcs (Rotational Irradiation) and static irradiation with 18 non-coplanar ports (Static Irradiation) using 6-MV photons were evaluated using Fast Fourier Transform (FFT) convolution and Multigrid (MG) superposition algorithms. Dose-volume histograms (DVHs), mean path-length (PL) and mean effective path-length (EPL) were calculated. RESULTS: Although the PL was larger for the inspiration state than for the stable state and the expiration state, the EPL was 0.4-0.5cm smaller in the inspiration state than in the expiration state (p=0.01 for Rotational Irradiation; p=0.03 for Static Irradiation). The isocenter dose obtained by the FFT convolution algorithm was 7-12% higher than that obtained with the MG superposition algorithm. A leftward shift of the DVH obtained by MG superposition was noted for the inspiration state compared with the expiration state. CONCLUSIONS: The choice of the proper algorithm is important to accounting for changes in respiration state. Differences in isocenter dose were not large among the respiratory states analyzed. EPL was a little shorter for inspiration than for expiration, although there were larger and reverse trends in path length. A leftward shift of the DVH obtained for the inspiration state when MG superposition was used.

Dose calculation system for remotely supporting radiotherapy.

The dose calculation system IMAGINE is being developed keeping in mind remotely supporting external radiation therapy using photon beams. The system is expected to provide an accurate picture of the dose distribution in a patient body, using a Monte Carlo calculation that employs precise models of the patient body and irradiation head. The dose calculation will be performed utilising super-parallel computing at the dose calculation centre, which is equipped with the ITBL computer, and the calculated results will be transferred through a network. The system is intended to support the quality assurance of current, widely carried out radiotherapy and, further, to promote the prevalence of advanced radiotherapy. Prototypes of the modules constituting the system have already been constructed and used to obtain basic data that are necessary in order to decide on the concrete design of the system. The final system will be completed in 2007.

Indications of stereotactic irradiation for brain lesions.

Stereotactic radiosurgery (SRS: single-fraction stereotactic irradiation) was originally developed to treat benign lesions in the brain, and has been adopted for the treatment of small primary or metastatic brain tumours. It has recently been recommended that stereotactic radiotherapy (SRT: fractionated stereotactic irradiation) be applied to the treatment of brain tumours; however, it requires much more time and work than SRS, and sufficient radiobiological evaluations of these techniques have never been carried out. Biologically effective doses were calculated to determine the indications for SRS and SRT, and to decide on an effective SRT schedule, incorporating the factors of 'repair' and 'cell proliferation'. The results suggest that SRS would be recommended for arteriovenous malformations and benign tumours that have distinct margins separating them from surrounding normal tissue and SRT would be recommended for benign or malignant brain tumours without clearly defined boundaries. The recommended SRT schedules would be 7 Gy x 7 fractions every other day for malignant tumours and 3.5 Gy x 12 fractions every other day for benign tumours. However, clinically, these schedules should be modified according to many other factors in individual cases.

[Hard-copy (film) versus soft-copy (CRT) reading performance between compressed and uncompressed images: SOLs in abdominal CT images].

The objective of this study was to examine the feasibility of soft-copy (CRT) reading and suitable compression. Forty abdominal CT images with a space-occupying lesion (SOL) in liver and 40 normal images were selected for receiver-operating-curve (ROC) analysis. Each image was compressed by JPEG extended mode into 1/10 its original capacity, and then an expanded image was printed on film. Ten radiologists evaluated the presence of liver SOLs (primary and secondary tumors) on soft-copy (CRT) and hard-copy (film) images. Each radiologist reviewed four types of images (original and compressed hard-copy and original and compressed soft-copy images). Values of the area under the curve in the various ROC analyses were 0.858 (FILM) and 0.842 (CRT) for original images and 0.879 (FILM) and 0.846 (CRT) for compressed images. The results of ROC analysis showed better reading performance with hard-copy than soft-copy images, but the difference was not statistically significant. Compressed images showed a higher value (0.879) than original images (0.858), a difference that was statistically significant (p < 0.029) by the paired t-test but not by the jackknife method. The results indicate that soft-copy reading is a clinically acceptable alternative to hard-copy reading. We have had no difficulty in reading abdominal CT images compressed to 1/10 of the original size by the JPEG method. This study was supported in part by a grant from the Japanese Ministry of Health and Welfare.

[Evaluation of parathyroid imaging methods with 99mTc-MIBI--the comparison of planar images obtained using a pinhole collimator and a parallel-hole collimator].

Parathyroid scintigraphy with 99mTc-MIBI was performed using two kinds of collimators, namely, a pinhole one and a parallel-hole one, to evaluate which one was more suitable for the detection of hyperfunctioning parathyroid lesions. In the studies using 99mTc source, the pinhole collimator showed better efficiency and spatial resolution in the distance where the parathyroid scan are actually performed. In the phantom study, the nodular activities modeling parathyroid lesions were visualized better on the images obtained using the pinhole collimator. In clinical studies for 30 patients suspicious of hyperparathyroidism, hyperfunctioning parathyroid nodules were better detected when the pinhole collimator was used. In conclusion, the pinhole collimator was thought to be more suitable for parathyroid scintigraphy with 99mTc-MIBI than the parallel-hole collimator.

Scintigraphic evaluation of myocardial ischaemia using a new fatty acid analogue: iodine-123-labelled 15-(p-iodophenyl)-9-(R, S)-methylpentadecanoic acid (9MPA).

Iodine-123-labelled 15-(p-iodophenyl)-9-(R,S)-methylpentadecanoic acid (9MPA) is a branched fatty acid analogue for myocardial imaging, which has been recently designed for medium washout rates from the myocardium. The purpose of this study was to assess the clinical feasibility of use of 9MPA for the evaluation of myocardial ischaemia. Twenty-one patients were injected with 9MPA at rest, and sequential single-photon emission tomography (SPET) acquisitions were performed 5, 45 and 240 min after administration to calculate washout rates from the myocardium. The findings of 9MPA images were analysed in comparison with those of perfusion images with thallium-201 or sestamibi, coronary angiography and left venticulography. In general, reduction of 9MPA uptake was more remarkable than that of perfusion tracers. The findings of 9MPA early images correlated better with those of exercise perfusion images than with the rest images. Measured washout rates of 9MPA from ischaemic myocardium were significantly slower than those from normal myocardium. The majority of areas with abnormal 9MPA distribution manifested wall motion abnormality, while all areas with normal tracer distribution presented normal wall motion. The detectability of myocardial ischaemia was improved by adding mid and delayed images in six cases. However, both washout and fill-in patterns were encountered in ischaemic lesions, rendering the interpretation of images difficult. In conclusion, the results of this study indicated that 9MPA has acceptable myocardial uptake, that its use is feasible for the detection of ischaemia and that the abnormal distribution of the tracer correlates well with wall motion abnormality reflecting metabolic disorders.

[IS&C image viewing and conversion software for Macintosh computer].

Although standardized data-transmission protocols such as DICOM are very beneficial, on-line transmission on images is often difficult to establish because of cost or security concerns. On the other hand, media-based data exchange is simple and adequate for the private use of medical images. We developed a viewing software program for medical images stored in magneto-optical disks with IS&C standard format. It displays images with gray-scale tone and stores them in Macintosh PICT format. The software is available for personal use from the first author.

[Schedule of stereotactic radiotherapy: a study considering the factors of repair and cell proliferation].

Stereotactic radiosurgery (SRS: stereotactic irradiation [STI] delivered in a single high dose) was initially developed by Leksell for non-malignant brain lesions, but there has been increasing interest in using it to treat small primary brain tumors or metastases. Recently, stereotactic radiotherapy (SRT: fractionated STI) has been recommended on the basis of radiobiological considerations for tumors in which both normal glial cells and tumor cells reside within the tumor margin. Strangely, the factors 'repair' and 'cell proliferation' have been neglected in the radiobiological evaluations of STI reported so far, mainly because of the complexity of the calculations. 'Half-time repair' which is the key value in the 'repair' factor may be larger for nervous tissue than for many other normal tissues because nerve cells have decreased ability to recover from damage. 'Cell proliferation' should be an important factor when the total radiation period is extended by applying SRT. In this study, we created models based on estimated 'half-time repair' and 'cell doubling time' and attempted to determine optimal SRT schedules. When repair and cell proliferation factors are also taken into consideration, the recommended SRT schedules would be 7 Gy x 7 fractions every other day for malignant tumors and 3.5 Gy x 12 fractions every other day for benign tumors. However, clinically, these schedules should be modified according to factors in individual cases, e. g., tumor size, presence of tumor necrosis, the patient's general condition, prognosis, and so on.

[Radiobiological considerations for stereotactic irradiation].

Stereotactic radiosurgery (SRS: stereotactic irradiation [STI] delivered in a single high dose) was initially developed by Leksell for non-malignant brain lesions, but there has been increasing interest in using it to treat small primary brain tumors or metastases. In more recent years, stereotactic radiotherapy (SRT: fractionated STI) has been developed, but radiobiological factors have not been sufficiently evaluated in relation to these techniques. Larson classified potential STI targets into four categories according to whether the target tissue is early- or late-responding and whether it is embedded within or only surrounded by normal tissue. We have actually calculated biologically effective doses for these categories to determine the indications for SRS and SRT, and to be able to choose suitable SRT schedules. Based on our calculations, theoretically SRS would be recommended for AVMs and benign tumors having distinct margins separating them from surrounding normal tissue and SRT would be recommended for low or high grade astrocytomas without clearly defined boundaries and metastasis. Recommended SRT schedules would be 49 Gy/7 fractions, 52 Gy/8 fractions or 54.9 Gy/9 fractions completed within 2 weeks. However, clinically, these indications and SRT schedules should be modified according to the many other factors involved in individual cases, such as tumor size, presence of tumor necrosis, the patient's general condition, prognosis, and so on.

[Is medical linac suitable for high-precision stereotactic irradiation?: investigations in geometrical accuracies of gantry and couch].

Linac-based radiosurgery has many advantages over the gamma knife, including low initial cost and no need of source replacement. On the other hand, most of the medical linacs currently in use were not originally designed to be applied for radiosurgery, and, therefore, careful quality assurance programs are required. In the gantry-head of a linac, a small CCD video camera is mounted in a position optically identical to that of the x-ray source. The video signal from the camera was digitalized to be evaluated for geometrical errors. A metal ball fixed to the stereotactic base frame via XYZ-sliding rods was used as a simulated target. Displacements of the target from the isocenter were measured during rotation of the gantry. Displacements in the gantry-rotation plane were satisfactorily small, while those perpendicular to it were maximal at gantry position angles of 0 degree and 180 degrees. This error night be caused by gravitational vending of the heavy gantry head. Although other major errors of the linac were within one millimeter, the center of coach rotation around the isocenter did not coincide with the center of gantry rotation, probably owing to gravitational vending. Special care should be taken when very small collimators are employed.

New system for linear accelerator radiosurgery with a gantry-mounted video camera.

PURPOSE: We developed a positioning method that does not depend on the positioning mechanism originally annexed to the linac and investigated the positioning errors of the system. METHODS AND MATERIALS: A small video camera was placed at a location optically identical to the linac x-ray source. A target pointer comprising a convex lens and bull's eye was attached to the arc of the Leksell stereotactic system so that the lens would form a virtual image of the bull's eye (virtual target) at the position of the center of the arc. The linac gantry and target pointer were placed at the side and top to adjust the arc center to the isocenter by referring the virtual target. Coincidence of the target and the isocenter could be confirmed in any combination of the couch and gantry rotation. In order to evaluate the accuracy of the positioning, a tungsten ball was attached to the stereotactic frame as a simulated target, which was repeatedly localized and repositioned to estimate the magnitude of the error. The center of the circular field defined by the collimator was marked on the film. RESULTS: The differences between the marked centers of the circular field and the centers of the shadow of the simulated target were less than 0.3 mm.

[Dose-volume histogram comparison among techniques of linac stereotactic radiosurgery].

The aim of this study was to analyze the three-dimensional dose distributions produced by various techniques used in current radiosurgery treatments by means of dose-volume histograms (DVH). Off-center ratio and tissue-peak ratio from 6-MV X-rays through additional cylindrical collimators were measured in advance for dose calculation. The use of the diamond detector and beam profile film dosimetry in small fields were certified. The impact of arc number, arc geometry, and field size on the dose distribution in a spherical phantom was evaluated through the use of DVH. These were calculated for a) four arc irradiations with collimator sizes of 5, 10, 20 and 30 mm in diameter, and b) for single-plane rotation, four or eleven multiple non-coplanar convergent arcs, dynamic rotation and precessional converging radio-therapy (PCR) with a field size of 20 mm. PCR is performed in the sitting position with the patient seated on a special treatment chair, which is rotating continuously while the gantry moves from the top to the center of the chair rotation. Most of the differences between techniques were found in a range of less than 40% of the maximum dose. Multiple non-coplanar convergent arcs with four or eleven arcs and PCR performed similar result in DVH, while single-plane rotation revealed almost unacceptably shallow dose falloffs. The DVH of dynamic rotation was between that of single-plane rotation and the other three methods.