Surface and build-up dose comparison between Elekta 6 MV flattening filter and flattening-filter-free beams using an advanced Markus ionization chamber and a solid water-equivalent phantom.

Using a plane-parallel advanced Markus ionization chamber and a stack of water-equivalent solid phantom blocks, percentage surface and build-up doses of Elekta 6 MV flattening filter (FF) and flattening-filter-free (FFF) beams were measured as a function of the phantom depth for field sizes ranging from 2 × 2 to 10 × 10 cm2 . It was found that the dose difference between the FF and the FFF beams was relatively small. The maximum dose difference between the FF and the FFF beams was 4.4% at a depth of 1 mm for a field size of 2 × 2 cm2 . The dose difference was gradually decreased while the field size was increased up to 10 × 10 cm2 . The measured data were also compared to published Varian FF and FFF data, suggesting that the percentage surface and build-up doses as well as the percentage dose difference between FF and FFF beams by our Elekta linac were smaller than those by the Varian linac.

Evaluation of In Vivo Volumetric Dosimetry for Prostate Cancer Using Electronic Portal Imaging Device.

PURPOSE: Volumetric modulated arc therapy (VMAT) is capable of acquiring projection images using electronic portal imaging device (EPID). Commercial EPID-based dosimetry software, dosimetry check (DC), allows in vivo dosimetry using projection images. The purpose of this study was to evaluate in vivo dosimetry for prostate cancer using VMAT. METHOD: VMAT plans were generated for eight patients with prostate cancer using treatment planning system (TPS), and patient quality assurances (QAs) were carried out with phantom. We analyzed five plans as phantom study and five plans as patient study. Projection images were acquired during VMAT delivery. DC converted acquired images into fluence images and used a pencil beam algorithm to calculate dose distributions delivered on the CT images of the phantom and the patients. We evaluated isocenter point doses and gamma analysis in both studies and dose indexes of planning target volume (PTV), bladder and rectum in patient study. RESULTS AND DISCUSSION: Dose differences at the isocenter were less than a criterion in both studies. Pass rates of the gamma analysis were less than a criterion by two plans in the phantom study. Dose indexes of reconstructed distribution were lower than original plans and standard deviations of PTV in reconstructed distribution were larger than original plans. The errors were caused by some issues, such as the commissioning of DC, variations in patient anatomy, and patient positioning. CONCLUSION: The method was feasible to non-invasively perform in vivo dose evaluation for prostate cancer using VMAT.

[Winston-lutz test and acquisition of flexmap using rotational irradiation].

PURPOSE: IGRT (image guided radiation therapy) is a useful technique for implementing precisely targeted radiation therapy. Quality assurance and quality control (QA/QC) medical linear accelerators with a portal imaging system (electronic portal imaging device: EPID) are the key to ensuring safe IGRT. The Winston-Lutz test (WLT) provides an evaluation of the MV isocenter, which is the intersection of radiation, collimator, and couch isocenters. A flexmap can indicate a displacement of EPID from the beam center axis as a function of gantry angles which can be removed from the images. The purpose of this study was to establish a novel method for simultaneously carrying out WLT and acquiring a flexmap using rotational irradiation. We also observed long-term changes in flexmaps over a period of five months. METHOD: We employed rotational irradiation with a rectangular field (30×30 mm). First, the displacement of EPID from the beam center axis, indicated by the ball bearing (BB) center, was evaluated using an in-house program. The location of the BB center was then modified according to WLT. Second, a second irradiation was used to acquire a flexmap. We performed this examination regularly and evaluated long-term changes in the flexmap. RESULTS AND DISCUSSION: It proved feasible to perform WLT and flexmap measurements using our proposed methods. The precision of WLT using rotational irradiation was 0.1 mm. In flexmap analysis, the maximum displacement from the mean value for each angle was 0.4 mm over five months. CONCLUSION: We have successfully established a novel method of simultaneously carrying out WLT and flexmap acquisition using rotational irradiation. Maximum displacement from the mean in each angle was 0.4 mm over five months.

Quality assurance of volumetric modulated arc therapy using Elekta Synergy.

PURPOSE. Recently, Elekta has supplied volumetric modulated arc therapy (VMAT) in which multi-leaf collimator (MLC) shape, jaw position, collimator angle, and gantry speed vary continuously during gantry rotation. A quality assurance procedure for VMAT delivery is described. METHODS AND MATERIALS. A single-arc VMAT plan with 73 control points (CPs) and 5-degree gantry angle spacing for a prostate cancer patient has been created by ERGO + + treatment planning system (TPS), where MLC shapes are given by anatomic relationship between a target and organs at risk and the monitor unit for each CP is optimized based on given dose prescriptions. Actual leaf and jaw positions, gantry angles and dose rates during prostate VMAT delivery were recorded in every 0.25 seconds, and the errors between planned and actual values were evaluated. The dose re-calculation using these recorded data has been performed and compared with the original TPS plan using the gamma index. RESULTS. Typical peak errors of gantry angles, leaf positions, and jaw positions were 3 degrees, 0.6 mm, and 1 mm, respectively. The dose distribution obtained by the TPS plan and the recalculated one agreed well under 2%-2 mm gamma index criteria. CONCLUSIONS. Quality assurance for prostate VMAT delivery has been performed with a satisfied result.

Verification of in-treatment tumor position using kilovoltage cone-beam computed tomography: a preliminary study.

Three-dimensional tumor position during rotational dose delivery was evaluated by acquiring in-treatment kilovoltage (kV) cone-beam CT (CBCT) to ensure treatment quality. The CBCT projection data of a phantom were acquired during rotational megavoltage (MV) dose delivery up to 6 Gy to evaluate image quality under MV beam irradiation. A lung tumor patient was treated with a total dose of 48 Gy in four fractions, each fraction including seven coplanar and noncoplanar beams, as well as a full-angle rotational beam. Tumor registration was performed between a planning CT image and a CBCT image immediately after patient setup. The patient couch was adjusted according to the registration results, and then the registration was repeated three times: immediately before treatment, during treatment, and immediately after treatment. The phantom image quality of the kV CBCT was not visually degraded up to the rotational MV dose of 6 Gy. Tumor position during rotational dose delivery was verified for the first time using kV CBCT.

[Motion analysis of target during stereotactic radiotherapy of lung tumors using volumetric modulated arc therapy].

Volumetric modulated arc therapy (VMAT) is a rotational intensity-modulated radiotherapy (IMRT) technique capable of acquiring projection images for cone-beam computed tomography (CBCT). Respiratory-correlated cone-beam computed tomography, namely 4D-CBCT, serves to assess the displacement of a tumor position between planning and treatment due to organ motion and respiration, and is important for more accurate radiation therapy. On the other hand, recently, a 320-detector row CT scanner, namely 4D-CT, has become available that allows axial volumetric scanning of a 16-cm-long range in a patient without table movement. The goal of our research is to establish a new method of verification during treatment in stereotactic body radiotherapy. In this study, we compare the movement of the tumor between "before treatment" using 4D-CT and "in treatment" using 4D-CBCT. Three patients (55-68 years of age) with lung tumors underwent CT scans for radiotherapy planning using 4D-CT scans to analyze the movement of the tumor before treatment. The patients were treated by VMAT while acquiring projection images. 4D-CBCT datasets were reconstructed from the projection images using in-house programs. The tumor positions in 4D-CT and 4D-CBCT were detected and the movement of the tumor between "before treatment" and "in treatment" was similar. The movement of the tumors during treatment was predictable from 4D-CT before treatment. Furthermore, 4D-CBCT clarified the tumor position during treatment and could reevaluate the actual tumor position and dose distribution. We have successfully shown the movement of the tumor between "before treatment" using 4D-CT and "in treatment" using 4D-CBCT.

[Motion analysis of target in stereotactic radiotherapy of lung tumors using 320-row multidetector CT].

Multi-detector computed tomography (MDCT) has rapidly evolved and is increasingly used for treatment simulation of thoracic and abdominal radiotherapy. A 320-detector row CT scanner has recently become available that allows axial volumetric scanning of a 16-cm-long range in a patient without table movement. Current radiotherapy techniques require a generous margin around the presumed gross tumor volume (GTV) to account for uncertainties such as tumor motion and set up error. Motion analysis is useful to evaluate the internal margin of a moving target due to respiration and to improve therapeutic precision. The purpose of this study is to propose a method using phase-only correlation to automatically detect the target and to assess the motion of the target in numerical phantoms and patients. Free-breathing scans using 320-detector row CT were acquired for 4 patients with lung tumor(s). The proposed method was feasible for motion analysis of all numerical phantoms and patients. The results reproduced the facts that the motion of tumors in the patients varied in orbits during the respiratory cycle and exhibited hysteresis. The maximum distance between peak exhalation and inhalation increased as the tumors approached the diaphragm. The proposed method detected the three-dimensional position of the targets automatically and analyzed the trajectories. The tumor motion due to respiration differed by region and was greatest for the lower lobe.