Collimator Angle Research Articles

Quality assurance of asymmetric jaw alignment using 2D diode array

A method using a 2D diode array is proposed to measure the junction gap (or overlap) and dose with high precision for routine quality assurance of the asymmetric jaw alignment. The central axis (CAX) of the radiation field was determined with a 15 × 15 cm(2) photon field at four cardinal collimator angles so that the junction gap (or overlap) can be measured with respect to the CAX. Two abutting fields having a field size of 15 cm (length along the axis parallel to the junction) × 7.5 cm (width along the axis perpendicular to the junction) were used to irradiate the 2D diode array (MapCHECK2) with 100 MU delivered at the photon energy of 6 MV. The collimator was slightly rotated at 15° with respect to the beam central axis to increase the number of diodes effective on the measurement of junction gap. The junction gap and dose measured in high spatial resolution were compared to the conventional methods using an electronic portal imaging device (EPID) and radiochromic film, respectively. In addition, the reproducibility and sensitivity of the proposed method to the measurements of junction gap and dose were investigated. The junction gap (or overlap) and dose measured by MapCHECK2 agreed well to those measured by the conventional methods of EPID and film (the differences ranged from -0.01 to 0 cm and from -1.34% to 0.6% for the gap and dose, respectively). No variation in the repeat measurements of the junction gap was found whereas the measurements of junction dose were found to vary in quite a small range over the days of measurement (0.21%-0.35%). While the sensitivity of the measured junction gap to the actual junction gap applied was the ideal value of 1 cm∕cm as expected, the sensitivity of the junction dose to the actual junction gap increased as the junction gap (or overlap) decreased (maximum sensitivity: 201.7%∕cm). The initial results suggest that the method is applicable for a comprehensive quality assurance of the asymmetric jaw alignment.

A comparative study of the peripheral doses from a linear accelerator with a multileaf collimator system

This study presents a comparison of peripheral doses (PDs) measured using an ionisation chamber with treatment planning system (TPS) data and a Monte Carlo (MC) simulation of a 6-MV photon beam. The ion chamber measurements and MC simulation produced similar results for all out-of-field distances and field sizes considered in this study. For the 0° and 90° collimation angles, the average local per cent dose differences between the MC and TPS calculations were 2.7 % (range: -2.4, +22.6) and -1.7 % (range: -12.2, +10.8), respectively. The corresponding differences between the MC calculations and the ion chamber measurements were 2.2 % (range: -2.4, 24.7) and -1.8 % (range: -17, 15.2) for all field sizes and depths, respectively. Whereas the PDs increased with field sizes, the variations with depth were negligible at large distances. The TPS calculations usually yielded higher PDs than ion chamber measurements at distances close to the field edge. In contrast, at the farther distances, the TPS results indicated lower doses than both the ion chamber and the MC data. TPS data are not sufficient for use in calculating the out-of-field doses. These results can be used to estimate non-target organ doses to patients.

SU‐E‐T‐390: Impact of Collimator Angles During VMAT Planning and Delivery

Purpose: To evaluate the dosimetric impact from collimator angles during the VMAT planning and delivery. Methods: An early stage lung cancer treatment with Pinnacle SmartArc™ planning system was evaluated, for both planning and delivery with dosimetric differences on an Elekta Synergy™ platform. Conventional planning process was utilizing 450 collimator angle to maximize the necessary PTV margins. A single 3590 arc with 10 MV photons were adopted. We compared three various collimator angles (00, 450 and 900) with generated planning and delivery variations; SunNuclear 3DVH™ module was also implemented to review the dose delivery discrepancy associated with the VMAT techniques corresponding to various collimator angle settings. Results: VMAT delivery with various collimator setting introduces dosimetric differences. PTV D95% dose coverage for all angles is comparable (00, 450, 900 = 93.3%, 92.9%, 92.7%) while 450 collimator introduces more leakage doses through the superior/inferior directions. OARs of 00 and 900 collimators have different distributions such as 1% of cord volume doses of 47.9 cGy and 40.7 cGy per fraction, but all within clinical tolerances. Total lung doses are within limits with V20 values of 17.2%, 17.8%, 16.7% for 00, 450, 900 collimator, respectively. Treatment MUs for all collimators are within 2% variation due to the similar control point distributions. Conclusion: 450 collimator may not the best choice for VMAT delivery with tumor size less than the MLC limitation. The convention was defined to suit the LINAC limitation with compromise at initial implementation. To avoid extra MLC leakage, 00 or 900 collimator may be a proper choice. Conformity with target shape or avoidance with the OAR locations with BEV optimization in initial planning stage is also important to validate the collimator angles. Currently there is no optimum collimator choice built in the planning system, user experience is also an influential factor.

SU‐E‐T‐673: Factors Influencing Normal Brain Dose for Single‐Isocenter Multi‐Target Radiosurgery

Purpose: To evaluate the effects of number of arcs, collimator angle, variable secondary collimator size, and optimization objectives on normal brain dose. Methods: Ten patients treated for multiple cranial metastases were retrospectively planned for single‐isocenter VMAT. All plans were optimized using a standardized objective function and were normalized such that 99% of the total target volume received 18 Gy. Multiple variables were utilized in planning, including numbers of arcs (2 versus 4), collimator angles (45° versus selected per beam to minimize area of normal brain exposed in the beams‐eye‐view), jaw tracking (fixed versus trailing MLC leaf), and a low dose constraint for healthy brain (mean dose 2 Gy versus no constraint). Plan quality was evaluated by conformity index, gradient index, mean absolute volume of normal brain receiving 1.8 Gy (V10), 4.5 Gy (V25), 12 Gy (V12Gy), and normal brain mean dose. Results: Conformity index, gradient index, and V12Gy were not significantly dependent on the factors considered. The low dose constraint reduced the brain mean dose by 56 cGy (13.7% reduction relative to no low dose constraint). Other factors did not have a significant effect on mean dose. The low dose constraint decreased V25 and V10 by 136 cc (28.8% reduction) and 128 cc (12.7% reduction), respectively. Collimator angle and jaw tracking each reduced V25 and V10 by up to 32 cc and 40 cc, respectively. Compared to 2 arcs, 4 arcs decreased V25 by 20 cc and increased V10 by 38 cc. Conclusion: Limiting the normal brain mean dose in the optimization objective function significantly reduces the low dose spill into the normal brain without changing target coverage. Jaw tracking and appropriate selection of collimator also reduces the low dose volume, but to a lesser extent.

SU-E-T-706: Effect of Gantry and Collimator Positional Errors On Volumetric Modulated Arc Therapy(VMAT)

Purpose: Gantry and Collimator positional errors occur during VMAT delivery. The purpose of this work is to understand the impact of such errors on VMAT dose distribution for selected head and neck cases. Methods: Five head and neck single arc (360 degree) VMAT treatment plans were created. Two types of systematic errors were simulated for an error magnitude of 1 degree on gantry and Collimator rotations respectively. For gantry, the systematic errors were, 1)Gantry angle is shifted in the same direction of VMAT arc by one degree 2)Gantry angle is shifted in the opposite direction of VMAT arc by one degree. In case of collimator, the systematic errors of 1 degree in the collimator angle in both positive and negative directions were simulated.The gantry and collimator angles were shifted for all the control points of VMAT arc. The dose was calculated and DVH was computed. Results: Conformity index (CI), D2, D98 and Dmean for PTV did not show significant variations for gantry and collimator errors in both positive and negative directions for all the 5 patients. For a positive gantry error, the average decrease in Dmean to the left parotid was found to be 80cGy and the same found to be increased for right parotid by 60cGy. However for both the directions of collimator error, the average increase in D0.01 for spinal cord was 100cGy. For a positive collimator error, the average increase in Dmean for left parotid was 90cGy and the same was decreased by 40cGy for right parotid. For a negative collimator error, the right parotid dose was increased and left parotid dose was decreased. Conclusion: Effect of one degree collimator error was found to be more significant compared to one degree Gantry error since collimator error increased the maximum dose to spinal cord and the mean dose to the parotids.

MO‐F‐108‐06: A Novel Analytic Approach to Assessing Rotation‐Induced Dosimetric Errors of Stereotactic Radiosurgery Cases

Purpose: To assess 3D rotational setup error effect on dose distribution, we derive a new set of beams such that the beams‐eye‐views of this new set of beams with respect to the planning CT are the same as those of the original plan for the patient, but with setup rotation errors incorporated. Methods: To assess the rotation effects on the dose distribution of stereotactic radiosurgery (SRS) cases, a methodology was developed and two sets of table, gantry, and collimator angles were obtained in analytical forms. Those new angles emulate the rotation effects of setup errors. Eight SRS cases were computed with a series of different combinations of patient rotation errors, ranging from (−5°, −5°, −5°) to (5°, 5°, 5°) (roll, pitch and yaw) with an increment of 1°. For each set of rotational errors, its corresponding equivalent beams were computed using our analytical solution and used for dose calculation. Results: The above new beams were implemented in a treatment planning system (TPS). Based on all the plans that accounted for different rotations, we have found that rotations have an insignificant effect on the minimum, maximum, mean doses, and V80% of the planning target volume when the rotations were relatively small. This was particularly true for the small and near‐spherical targets. They, however, did change V95% significantly when the rotations approached 5°. Our theory has been validated with SRS cases and proven to be practical and viable. Conclusions: We have derived analytical solutions to a new set of table, gantry and collimator angles for a given beam configuration as a function of patient rotation errors. Compared to the traditional method of rotational effect assessment by importing rotated CT images into TPS, our equivalent beam approach is simple and accurate. It is a promising technique for real time adaptive treatment planning.

SU‐E‐T‐378: IMRT Severity Scoring for TG‐100: Do You Really Know?

Purpose: Failure modes and effects analysis (FMEA) as defined in TG‐100 has become a popular concept throughout the radiotherapy community. This risk mitigation technique involves detailed process mapping, analysis, and ranking of potential errors by means of a subjective, ordinal scoring system. This study aims to reduce the subjectivity of FMEA severity scoring for IMRT delivery by providing quantitative values. Methods: First we created an IMRT delivery process map for physics‐applicable processes and identified 11 physical failure modes (FMs). To determine the magnitude of dose delivery errors for several of the physical FMs (i.e., the severity of the FM), FMs were induced and dosimetry measurements were performed on a Varian Clinac accelerator going out of clinical service. Treatment planning studies to simulate remaining FMs are to follow. The quantitative severity scores will be compared to recommended FMEA subjective scores. Results: We identified the following physical FMs to investigate: photon beam energy, symmetry, MLC position, MLC transmission and leakage, MLC rounded‐end and leaf offset, MLC tongue‐and‐groove, CT look‐up table, gantry angle, collimator angle, couch angle and displacement, and MU linearity. Within the PTV of a H&N IMRT phantom, the maximum dose delivery error of 3% absolute dose was seen for 2 mm systematic MLC offsets, 4% for 1.1% increased energy, and 4% for 3.5% symmetry error. Measurements will be compared to computational study results and then used for quantitative severity scoring determination. Conclusion: Current FMEA practice for radiotherapy requires quantitative data in order to make accurate assessments associated with clinical QA programs. This study has shown examples of error magnitudes induced by IMRT physical FMs that can be used to quantitate and rank FMEA severity scoring. Work supported by grants CA10953 and CA81647 (NCI, DHHS).

SU‐E‐T‐594: Software Application for Comparison and Verification of Radiotherapy Treatment Plans

Purpose: The physics check of a patient treatment plan is one of many that guard against treatment errors in radiation oncology departments. The process would be improved by adding a comparison of treatment plan and prescription parameters, but this would be inordinately time consuming, if not impossible, to do by eye, and would raise the potential for human error. The goal of this project was to develop a software application that would automatically compare treatment parameters from the treatment planning system (Pinnacle3) and the electronic record and verifying system (Mosaiq), supplementing the physics check without sacrificing speed or accuracy. Methods: Both Mosaiq and Pinnacle3 can generate .RTP files. These files contain relevant plan information, are delimited, and have comparable structures. MATLAB code was used to create a GUI‐based application that can run outside of the MATLAB environment. Using .RTP files as inputs, the application compares linac name, beam energy and modality, patient position, DICOM position, field names, gantry angles (checking that the field names contain the gantry angles), MU/fraction, SSD to reference point, beam weights, collimator angles, field sizes, and leaf positions. Program alerts user to any discrepancies and results are written to .txt files and an Excel spreadsheet (for leaf positions) that highlight any discrepancies. Results: The application is currently being rolled out in the Oncology Department for beta testing. During preliminary testing, the application flagged errors in cases with known discrepancies between treatment plan and prescription, indicating the applications effectiveness. Conclusion: The application adds another layer of safety for patients by providing a check for human error immediately before treatment. It focuses the user on patient care by ensuring he or she is cognizant of the current treatment plan. Finally, it produces digitally compact, easy‐to‐read records of the parameters of a given patients treatment.

SU-E-CAMPUS-T-05: Quality Evaluation of An Automatic VMAT Planning Method for Head and Neck Cancer Cases

Purpose: To evaluate the dosimetric quality of an automatic volumetric-modulated arc therapy (VMAT) planning method for Head and Neck (HN) cancer cases. Methods: Eighteen HN cancer cases are planned automatically by 2-arc VMAT. An OAR DVH prediction model automatically generates OAR dose sparing objectives for each patient case based on the patient's anatomical features. The model is trained by prior clinical IMRT plans. In addition, a pre-determined planning template was used for all the cases which specify the arc angles, collimator angles, jaw setting, etc, as well as the optimization priority for each objective, based on past VMAT planning experience. The automatic VMAT planning process requires only one round of optimization for each case. The dosimetric parameters for each OAR and PTV from the VMAT plans are compared with those in clinical IMRT plans. Results: The means (standard deviation in parenthesis) of the dosimetric parameters in the automatically generated VMAT and the clinical IMRT plans are (in Gy): parotid D50%: VMAT: 25.8 (8.6), IMRT: 25.7 (12.8), oral cavity D50%: VMAT: 33.2 (8.5), IMRT: 31.6 (9.5), larynx D50%: VMAT: 27.6 (13.3), IMRT: 26.3 (14.5), spinal cord + 5mm D2%: VMAT: 34.9 (2.2), IMRT: 36.2 (4.5), Brainstem D2%: VMAT: 18.9 (5.9), IMRT: 20.7(5.9). The dosimetric parameters for the OARs in the VMAT and IMRT plans have no significant differences. The only statistically significant difference is the dose homogeneity of the boost PTV (p = 0.0006), which is 5.2 Gy in VMAT plans vs. 7.1 Gy in IMRT plans. Conclusion: The automatic VMAT planning method can generate plans with dosimetric quality comparable to clinical IMRT plans generated by an expert. This study suggests the DVH prediction models learned from IMRT planning experience can be utilized to generate dose sparing objectives for VMAT planning. Partly supported by NIH/NCI and a Varian master research grant.

SU-E-T-137: Evaluation of Linac Mechanical Systems Using Statistical Process Control

Purpose: Measuring the accuracy of the mechanical readouts on a linear accelerator is a staple of monthly quality assurance in Radiation Oncology. This study uses statistical quality control to determine if the current specifications and the frequency of the checks are reasonable. Methods: Statistical quality control looks at a set of data and determines the limits of the data. Changes can be made to the system with an eye toward improving the process. Previous measurements are used to determine the limits which are used to determine if the specifications are achievable or if they could be tightened.The study analyzed monthly mechanical data measured by several physicists on 5 linacs over several years. We looked at the difference between digital readout and measured value in gantry, collimator, and table angles, table motion, and jaw readout. Cp, Cpk, range limits and confidence limits were calculated. Results: Cp values were greater than 1 indicating each system was capable of performing within specification. Cpk values were also greater than 1 showing that each distribution was within specification. Each system showed raw results well within specification. Upper range limits for each system were well under the specifications. Upper and lower confidence limits were also within the specifications. There were no readings outside of specifications, however there were differences exceeding the range limits and the confidence levels. Conclusion: The general mechanical specifications set forth by manufacturers and national societies are easily achievable by modern linacs. The measurements and processes are able to be controlled. Range and control limits can be used to identify potential problems before they occur. Modern linacs can reliably and accurately display mechanical readouts well within specifications over long periods of time which may lead to a recommendation to decrease the frequency of mechanical measurements.

SU‐E‐T‐416: Targeting Accuracy of a TrueBeam System for Multi‐Lesion Cranial Radiosurgery with Single Isocenter

Purpose: To investigate the targeting accuracy of a Varian TrueBeam System for delivering intracranial radiosurgery to multiple lesions using a single isocenter (ISO). Methods: The study was performed using hidden target (BB) tests along with a MV portal imager (EPID). Six BBs were inserted into a head phantom simulating the locations of multiple metastatic tumors. One BB acted as the common ISO. The distances from the ISO to the other BBs were 2.9 to 7.1 cm. Simulating the actual patient treatment in our clinic, the phantom was immobilized using a BrainLAB U‐shape frame and a head mask. CT image was acquired with 1 mm slice thickness. For a selected couch, gantry and collimator angle combination, a 3×3 cm MLC‐shaped field was created for each BB. The centers of BB and the field were aligned, and the distance from the plan was read out on the DRR. Before shooting the tests, CBCT was used to setup the phantom, and the translational/rotational mismatch was corrected iteratively until less than 1 mm and 0.1 degree respectively. The actual positions of the BBs relative to their corresponding field centers were measured on the portal images. The deviation between the measured and planned BB positions quantifies the geometric targeting error. The test was performed for various couch, gantry and collimator angle combinations (total 88 tests). Results: The mean±SD of the BB position deviations were 0.0±0.4 and 0.2±0.4 mm in the image transverse‐and longitudinal‐direction respectively; and 0.5±0.3 mm for the vector length. Six of 88 BB tests (6.8%) exceeded the vector length of 1 mm with a maximum of 1.2 mm Conclusion: The mechanical targeting accuracy of the TrueBeam System at our institution is acceptable for delivery of intracranial radiosurgery with a single isocenter for treatment of multiple targets. Application of this technique is being investigated.

Plasmon-Enhanced-Attosecond-Extreme Ultraviolet Source

A compact high repetition rate attosecond light source based on a standard laser oscillator combined with plasmonic enhancement is analyzed. At repetition rates of tens of MHz, we predict focusable pulses with durations of ≲300 as and a spherical wave front at collimation angles ≲5°. Plasmonic mode and guiding of the attosecond radiation determine the beam parameters. The beam is robust with respect to variations of driver pulse focus and duration.

Low-level radioactive waste measurement comparisons of three gamma-ray counting systems

Three different gamma-ray counting systems constructed by 1–3 HPGe detectors were used in this study to compare their system performance. One measurement scheme involved positioning a single HPGe detector on a movable cart with a 90° collimation angle to the observed item. The other two waste assay systems were configured with two or three HPGe detectors towards the sample drums, while the three-HPGe-detector counting system was in a shielded counter cavity. The measurement consistency of 38 low-level waste drums, system operating costs and acquisition times to achieve the same MDA for these counting systems were compared and discussed in this study.

EP-1305: Management of the interruptions in the conventional treatment of prostate cancer upon a database analysis

Purpose/Objective: The delivery of high quality stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments requires knowledge of the position of the isocentre to sub millimetre accuracy. The deviation between the radiation and mechanical isocentres must be less than 1 mm. The use of an add-on micromultileaf collimator (μMLC) in SRS and SRT is an additional challenge to the anticipated high-level geometric and dosimetric accuracy of the treatment. The aim of this work was to quantify the gantry excursions during rotation with and without an add-on μMLC attached to the gantry head. In addition, the shift in the position of the isocenter and its correlation to the kV beam centre of the cone-beam CT system was included in the study. Materials and Methods: The quantification of the gantry rotational performance was done using a pointer supported by an inhouse made rigid holder attached to the gantry head. The pointer positions were measured using a digital theodolite. The displacement of the isocentre due to an add-on μMLC of 50 kg was investigated. In case of the pointer measurement the μMLC was simulated by weights attached to the gantry head. A method of least squares was applied to determine the position and displacement of the mechanical isocentre. Additionally, the displacement of the radiation isocentre was measured using a Winston-Lutz phantom and the electronic portal image device (EPID) system. These measurements were based on eight MV photon beams irradiated onto the ball from the four cardinal angles and two opposed collimator angles. The measurements and analysis of the data were carried out automatically using software delivered by the manufacturer. Results: The displacement of the mechanical isocentre caused by a 50 kg heavy μMLC was found to be (-0.01±0.06, -0.10±0.03, -0.26±0.05) mm in lateral, longitudinal and vertical direction, respectively. Similarly, the displacement of the radiation isocentre was found to be (0.00±0.03, -0.08±0.06, -0.32±0.02) mm. Good agreement was found between the displacement of the two isocentres. A displacement of the kV cone-beam CT beam centre due to the attached weight of 50 kg could not be detected. By comparing the CW and CCW data the presence of the effect of backlash appears. The effect of backlash was found to be < 0.14 mm and < 0.10 mm in the lateral and vertical direction, respectively. Conclusions: General characteristics of the gantry arm excursions and displacements caused by an add-on μMLC have been reported. A 50 kg heavy add-on μMLC results in an isocentre displacement downward of 0.26 to 0.32 mm. We recommend that the beam centre of the kV cone-beam CT image system should be matched to the isocentre related to the weight of the μMLC. Consequently, the imperfections in isocentre localizations are transferred to the conventional radiotherapy where the clinical consequences of uncertainties in the sub millimetre regime are negligible.

Feasibility study of real‐time planning for stereotactic radiosurgery

3D rotational setup errors in radiotherapy are often ignored by most clinics due to inability to correct or simulate them accurately and efficiently. There are two types of rotation-related problems in a clinical setting. One is to assess the affected dose distribution in real-time if correction is not applied and the other one is to correct the rotational setup errors prior to the initiation of the treatment. Here, the authors present the analytical solutions to both problems. (1) To assess the real-time dose distribution, eight stereotactic radiosurgery (SRS) cases were used as examples. For each plan, two new sets of beams with different table, gantry, and collimator angles were given in analytical forms as a function of patient rotational errors. The new beams simulate the rotational effects of the patient during the treatment setup. By using one arbitrary set of beams, SRS plans were recomputed with a series of different combinations of patient rotational errors, ranging from (-5°, -5°, -5°) to (5°, 5°, 5°) (roll, pitch, and yaw) with an increment of 1° and compared with those without rotational errors. For each set of rotational errors, its corresponding equivalent beams were computed using the analytical solutions and then used for dose calculation. (2) To correct for the rotational errors, two new sets of table, gantry, and collimator angles were derived analytically to validate the previously published derivation. However, in the derivation, a novel methodology was developed and two sets of table, gantry, and collimator angles were obtained in analytical forms. The solutions provide an alternative approach to rotational error correction by rotating the couch, gantry, and collimator rather than the patient. For demonstration purpose, the above-derived new beams were implemented in a treatment planning system (TPS) to study the rotational effects on the SRS cases. For each case, the authors have generated ten additional plans that accounted for different rotations of the patient. They have found that rotations have an insignificant effect on the minimal, maximum, mean doses, and V80% of the planning target volume (PTV) when the rotations were relatively small. This was particularly true for the small and near-spherical targets. They, however, did change V95% significantly when the rotations approached 5°. The theory has been validated with clinical SRS cases and proven to be practical and viable. The preliminary results demonstrate that the rotational effects are patient-specific and depend on several important factors, such as the PTV size, the PTV location, and the beam configuration. The solutions given in this paper are of great potential values in clinical applications. They have derived the analytical solutions to a new set of table, gantry, and collimator angles for a given treatment beam configuration as a function of patient rotational errors. One solution was used to assess the dosimetric effects of an imperfect patient setup and the other one was used to correct for the setup errors without rotating the patient. Compared to the widely adopted method of rotation effect assessment by importing the rotational CT images into TPS, the equivalent beam approach is simple and accurate. The analytical solutions to correcting for rotational setup errors prior to treatment were also derived. Based on the initial clinical investigations, they firmly believe that clinically viable real-time treatment planning and adaptive radiation therapy are feasible with this novel method.

Gantry and isocenter displacements of a linear accelerator caused by an add‐on micromultileaf collimator

The delivery of high quality stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) treatments to the patient requires knowledge of the position of the isocenter to submillimeter accuracy. To meet the requirements the deviation between the radiation and mechanical isocenters must be less than 1 mm. The use of add-on micromultileaf collimators (μMLCs) in SRS and SRT is an additional challenge to the anticipated high-level geometric and dosimetric accuracy of the treatment. The aim of this work was to quantify the gantry excursions during rotation with and without an add-on μMLC attached to the gantry head. In addition, the shift in the position of the isocenter and its correlation to the kV beam center of the cone-beam CT system was included in the study. The quantification of the gantry rotational performance was done using a pointer supported by an in-house made rigid holder attached to the gantry head of the accelerator. The pointer positions were measured using a digital theodolite. To quantify the effect of an μMLC of 50 kg, the measurements were repeated with the μMLC attached to the gantry head. The displacement of the isocenter due to an add-on μMLC of 50 kg was also investigated. In case of the pointer measurement the μMLC was simulated by weights attached to the gantry head. A method of least squares was applied to determine the position and displacement of the mechanical isocenter. Additionally, the displacement of the radiation isocenter was measured using a ball-bearing phantom and the electronic portal image device system. These measurements were based on 8 MV photon beams irradiated onto the ball from the four cardinal angles and two opposed collimator angles. The measurements and analysis of the data were carried out automatically using software delivered by the manufacturer. The displacement of the mechanical isocenter caused by a 50 kg heavy μMLC was found to be (-0.01 ± 0.05, -0.10 ± 0.03, -0.26 ± 0.05) mm in lateral, longitudinal, and vertical direction, respectively. Similarly, the displacement of the radiation isocenter was found to be (0.00 ± 0.03, -0.08 ± 0.06, -0.32 ± 0.02) mm. Good agreement was found between the displacement of the two isocenters. A displacement of the kV cone-beam CT beam center due to the attached weight of 50 kg could not be detected. General characteristics of the gantry arm excursions and displacements caused by an add-on μMLC have been reported. A 50 kg heavy add-on μMLC results in a isocenter displacement downward of 0.26-0.32 mm. The authors recommend that the beam center of the kV cone-beam CT image system should be matched to the isocenter related to the weight of the μMLC. Consequently, the imperfections in isocenter localizations are transferred to the conventional radiotherapy where the clinical consequences of uncertainties in the submillimeter regime are negligible.

Effect of MLC leaf position, collimator rotation angle, and gantry rotation angle errors on intensity-modulated radiotherapy plans for nasopharyngeal carcinoma

The purpose of this study was to investigate the effect of multileaf collimator (MLC) leaf position, collimator rotation angle, and accelerator gantry rotation angle errors on intensity-modulated radiotherapy plans for nasopharyngeal carcinoma. To compare dosimetric differences between the simulating plans and the clinical plans with evaluation parameters, 6 patients with nasopharyngeal carcinoma were selected for simulation of systematic and random MLC leaf position errors, collimator rotation angle errors, and accelerator gantry rotation angle errors. There was a high sensitivity to dose distribution for systematic MLC leaf position errors in response to field size. When the systematic MLC position errors were 0.5, 1, and 2mm, respectively, the maximum values of the mean dose deviation, observed in parotid glands, were 4.63%, 8.69%, and 18.32%, respectively. The dosimetric effect was comparatively small for systematic MLC shift errors. For random MLC errors up to 2mm and collimator and gantry rotation angle errors up to 0.5°, the dosimetric effect was negligible. We suggest that quality control be regularly conducted for MLC leaves, so as to ensure that systematic MLC leaf position errors are within 0.5mm. Because the dosimetric effect of 0.5° collimator and gantry rotation angle errors is negligible, it can be concluded that setting a proper threshold for allowed errors of collimator and gantry rotation angle may increase treatment efficacy and reduce treatment time.

Cascaded two-photon spectroscopy of Yb atoms with a transportable effusive atomic beam apparatus

We present a transportable effusive atomic beam apparatus for cascaded two-photon spectroscopy of the dipole-forbidden transition (6s(2)(1)S0↔ 6s7s (1)S0) of Yb atoms. An ohmic-heating effusive oven is designed to have a reservoir volume of 1.6 cm(3) and a high degree of atomic beam collimation angle of 30 mrad. The new atomic beam apparatus allows us to detect the spontaneously cascaded two-photons from the 6s7s(1)S0 state via the intercombination 6s6p(3)P1 state with a high signal-to-noise ratio even at the temperature of 340 °C. This is made possible in our apparatus because of the enhanced atomic beam flux and superior detection solid angle.

Observations and Considerations on Patient X-ray Exposure in the Electrophysiology Lab.

To assess patient radiation during catheter ablation procedures and operator differences. From 84 patients (51 males, age 63 ± 10 years) undergoing complex catheter ablation by three experienced operators we collected: body mass index (BMI), procedure type and time, fluoroscopy time, dose area product (DAP), air kerma and X-ray system setting (cine, collimation and angiographic imaging angle). A new factor, fluoroscopy DAP-fluoroscopy time ratio, was introduced to compare operator differences. The results show the average procedure time was 179 (± 57) minutes (min), fluoroscopy time was 31 (± 21) min, DAP was 26.4 (± 19.6) Gy.cm(2) and air kerma was 0.26 (± 0.19) Gy. Procedure types were: pulmonary vein isolation (PVI) (52 %), redo PVI (11 %), pulmonary vein ablation catheter (PVAC) (14 %), ventricular tachycardia (VT) (8 %) and others (15 %). Inter-operator difference was observed in fluoroscopy and cine usage. Fluoroscopy DAP-time ratios showed a similar level of patient radiation dose rate by operator A and B (correlation: 0.89), and a significantly higher dose rate by operator C (correlation: 0.20, p<0.001; 0.26, p<0.01, to operator A and B). In conclusion, operators should be aware of patient radiation exposure levels and the influencing factors. Inter- and intra-operator differences can be measured and bench marked for improvement in X-ray efficiency and patient radiation reduction.

Investigation of dose accuracy using different measurement methods in intensity-modulated radiation therapy

In the study, 10 prostate IMRT plans were prepared to be treated lineer accelerator. These plans were transfered to the water equivalent solid phantom for quality control (QC) plans of each patient. QC plans, gantry, collimator and table angles of original and 0 degrees were created in two different ways. The measurements were performed using film and ion chambers with different volumes at 5 cm depth. Doses obtained with measurement methods, were compared by calculated doses in TPS.