Isocenter Deviation Research Articles

A novel end-to-end test for combined dosimetric and geometric treatment verification using a 3D-printed phantom.

The dosimetric and geometric accuracy are important components to ensure safe patient treatment in radiation therapy. Therefore, these components must be checked during quality control. This work presents a possible solution for the determination of the geometric isocenter deviation in the entire treatment chain. Additionally, the dose measurement of the established end-to-end test workflow measured in the same procedure as the geometric deviation is described. An in-house designed end-to-end test phantom went through the entire procedure of a standard patient treatment and the dosimetric and geometric accuracy were determined. At 3 linear accelerators (linac), the phantom was positioned either with cone beam computed tomography or with surface guidance. In this position, a Winston-Lutz test was performed and the deviations of the gantry, collimator and couch isocenter measurements to the phantom position were determined. Additionally, a dose measurement in the phantom was performed and compared to the dose predicted in the treatment planning system. To validate the results obtained with the in-house designed phantom, comparative measurements with commercial phantoms were performed. According to the performed end-to-end test, 2 out of the 3 linacs showed isocenter variations larger than 1 mm for collimator and gantry rotations and larger than 2 mm for couch rotations. With an isocenter variation of less than 1 mm for collimator and gantry rotations, 1 linac fulfilled the tolerance for stereotactic treatments without couch rotation. With couch rotation, an isocenter variation of less than 2 mm was detected at this linac, which fulfilled the tolerance for IMRT treatments. The mean dose deviation between measurement and treatment planning system was 1.82% ± 1.03%. The results acquired with the UMM phantom did not show statistically significant deviations to those acquired with relevant other commercial phantoms. The novel end-to-end test procedure allows for a combined dosimetric and geometric treatment evaluation. Besides the commonly performed dose end-to-end test the geometric isocenter deviation within a patient treatment workflow was evaluated and categorized for IMRT or SBRT.

Effect of isocenter deviation on volume modulated arc therapy plan results and gamma passing rate in the treatment of cervical cancer.

BackgroundIsocenter deviation, often induced by small displacements of both the device and the patient, is a common error seen in radiotherapy. In this study, we investigated the impact of isocenter deviation on the results of the volumetric modulated arc therapy (VMAT) plan and dosimetric verification gamma passing rate in the treatment of cervical cancer.MethodsThe clinical data of 15 patients with cervical cancer who were treated with VMAT were retrospectively collected and analyzed. In this study, the isocenter site modification method was adopted. The VMAT plan with isocenter deviation adjustment was set as the experimental group, while the original plan was set as the control group. The impact of isocenter deviation on the results of the VMAT plan and dosimetric verification gamma passing rate was analyzed. Applying gamma analysis with different test criterions, the impact of isocenter deviation on the gamma passing rates was evaluated, and the sensitivity of different test criterions in identifying isocenter deviation was also analyzed.ResultsThere was a significant difference in the average dose in the target area between experimental group and control group (P<0.05). In organs at risk (OAR) terms, isocenter deviations also caused significant differences in dose parameters between the two groups. Except that there was no significant difference in the rectal V40 between two groups when the isocenter deviation was greater than 3 mm in the y axis direction. With the increase in the isocenter deviation, there was a trend towards decreased gamma passing rates with different analysis criterions in the experimental group. The 2 mm/2% standard had the highest sensitivity for identifying isocenter deviation.ConclusionsIsocenter deviation has significant effects on the results of the volume rotation intensity modulation plan and dosimetric verification gamma passing rates in the treatment of cervical cancer. When the isocenter deviation was less than 3 mm, a higher gamma passing rate (>90%) could also be obtained under the condition of the 3 mm/3% test criterions. It is recommended that the 2 mm/2% test standard should be utilized in clinical practice.

Radiotherapy in oncological emergencies: fast-track treatment planning

Background and purposeTo report on our clinical experience with a newly implemented workflow for radiotherapy (RT) emergency treatments, which allows for a fast treatment application outside the regular working-hours, and its clinical applicability.MethodsTreatment planning of 18 emergency RT patients was carried out using diagnostic computed tomography (CT) without a dedicated RT simulation CT. The cone-beam CT (CBCT) deviations of the first RT treatment were analyzed regarding setup accuracy. Furthermore, feasibility of the “fast-track” workflow was evaluated with respect to dose deviations caused by different Hounsfield unit (HU) to relative electron density (rED) calibrations and RT treatment couch surface shapes via 3D gamma index analysis of exemplary treatment plans. The dosimetric uncertainty introduced by different CT calibrations was quantified.ResultsMean patient setup vs. CBCT isocenter deviations were (0.49 ± 0.44) cm (x), (2.68 ± 1.63) cm (y) and (1.80 ± 1.06) cm (z) for lateral, longitudinal and vertical directions, respectively. Three out of four dose comparisons between the emergency RT plan calculated on the diagnostic CT and the same plan calculated on the treatment planning CT showed clinically acceptable gamma passing rates, when correcting for surface artifacts. The maximum difference of rED was 0.054, while most parts of the CT calibration curves coincided well.ConclusionIn an emergency RT setting, the use of diagnostic CT data for treatment planning might be time-saving and was shown to be suitable for many cases, considering reproducibility of patient setup, accuracy of initial patient setup and accuracy of dose-calculation.

The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets-Quantification and possible corrections.

PurposeTo assess the impact of isocenter shifts due to linac gantry and table rotation during cranial stereotactic radiosurgery on D98, target volume coverage (TVC), conformity (CI), and gradient index (GI).MethodsWinston‐Lutz (WL) checks were performed on two Elekta Synergy linacs. A stereotactic quality assurance (QA) plan was applied to the ArcCHECK phantom to assess the impact of isocenter shift corrections on Gamma pass rates. These corrections included gantry sag, distance of collimator and couch axes to the gantry axis, and distance between cone‐beam computed tomography (CBCT) isocenter and treatment beam (MV) isocenter. We applied the shifts via script to the treatment plan in Pinnacle 16.2. In a planning study, isocenter and mechanical rotation axis shifts of 0.25 to 2 mm were applied to stereotactic plans of spherical planning target volumes (PTVs) of various volumes. The shifts determined via WL measurements were applied to 16 patient plans with PTV sizes between 0.22 and 10.4 cm3.ResultsArcCHECK measurements of a stereotactic treatment showed significant increases in Gamma pass rate for all three measurements (up to 3.8 percentage points) after correction of measured isocenter deviations. For spherical targets of 1 cm3, CI was most severely affected by increasing the distance of the CBCT isocenter (1.22 to 1.62). Gradient index increased with an isocenter‐collimator axis distance of 1.5 mm (3.84 vs 4.62). D98 (normalized to reference) dropped to 0.85 (CBCT), 0.92 (table axis), 0.95 (collimator axis), and 0.98 (gantry sag), with similar but smaller changes for larger targets. Applying measured shifts to patient plans lead to relevant drops in D98 and TVC (7%) for targets below 2 cm3 treated on linac 1.ConclusionMechanical deviations during gantry, collimator, and table rotation may adversely affect the treatment of small stereotactic lesions. Adjustments of beam isocenters in the treatment planning system (TPS) can be used to both quantify their impact and for prospective correction of treatment plans.

VACUUM AND THERMOPLASTIC MOULD-BASED IMMOBILIZATION SYSTEMS USED IN PATIENT UNDERGOING PELVIC RADIATION THERAPY: A COMPARATIVE STUDY

Introduction: The aim of the present study was to compare two immobilization systems for comparison of setup errors in targeted radiotherapy.&#x0D; Methods: Retrospective analysis was done for the patients undergoing radiotherapy from May 2012 to December 2018 at our institution. Immobilization was performed on 30 patients sessions (Vacuum cushion i.e., Vac-Lok™ = 15; Thermoplastic mould i.e., Pelvicast pelvic masks = 15). A total of 763 cone-beams were analysed. The target lesion location was verified by cone-beam computed tomography (CBCT) prior to each session, with displacements assessed by CBCT simulation prior to each treatment session. Systematic setup errors, random setup errors, isocenter deviations in the Medio-lateral (ML), Supero-inferior (SI), Antero-posterior (AP), Rotation (yaw) directions of the patient position was calculated.&#x0D; Results: On comparing the Vac-Lok™ and Pelvicast pelvic masks group with respect to Systematic and random error in the lateral, longitudinal, vertical and YAW direction, no statistically significant difference was seen except the random error in YAW direction (P=0.037, Unpaired t-test). There was no difference observed in comparing the isocentric deviation.&#x0D; Conclusion: It was inferred and concluded that using a vacuum cushion for pelvic radiotherapy provides no added benefit compared to using a thermoplastic mould. Thermoplastic mould is recommended for patients receiving pelvic radiotherapy to improve overall reproducibility.Keywords: Rotational therapy; Radiotherapy; Systematic, random error; Thermoplastic mould; Vacuum cushion.

A new concept for verifying the isocentric alignment of the proton-rotational gantry for radiation control.

The purpose of this study was to introduce the modified Winston-Lutz (mWL) test and to evaluate its feasibility. This is a new method to completely absorb the proton beam around the isocenter inside a phantom for radiation control. The mWL test was performed using a 14-cm-diameter acrylic Lucy 3D QA Phantom for a passive-scattering proton beam gantry. The energy of the unmodulated proton beam was adjusted such that the residual range was < 130mm, and the energy of the proton beam was completely lost around the isocenter. The radiation field was formed with a multi-leaf collimator at 8.6 × 8.6mm2 in the isocenter plane. The phantom was loaded with a 4-mm-diameter tungsten ball, and the EBT3 was set up at the isocenter. The proton beam was irradiated at gantry angles with 45° steps, and the isocenter deviation of the proton beam was measured and subsequently analyzed. Although the radiation field penumbra was blurred under the influence of scattered radiation due to placement in the phantom compared to the traditional WL test placed in the air, evaluation of the beam axis accuracy was possible. The results confirmed that the maximum total displacement was less than 0.9mm, and the specifications of the device were met. The mWL test is feasible and effective to reduce the building activation in proton beam treatment facilities. Thus, it can be considered a useful method that sufficiently satisfies the shielding calculation conditions.

A closer look at the conventional Winston-Lutz test: Analysis in terms of dose

To investigate whether the target-isocenter deviations reported by a conventional Winston-Lutz (WL) test actually reflect the shifts of the measured prescription isodose line with respect to the target. A conventional WL test uses a metallic ball as a target that aims at several fields. But this test does not report information on the accuracy of the delivery in terms of dose. A conventional WL test using a metallic pointer as a target (Pointer-WL test) has been recreated in the Eclipse treatment planning system over an acrylic phantom containing a radiochromic film (Dose-WL test). After Dose-WL test delivery, the shift of the 80% prescription isodose line with respect to the target center (d80%-center) was measured using film dosimetry. The Pointer-WL and Dose-WL tests were performed in 10 different sessions. The isocenter deviation reported by the Pointer-WL test was compared to the d80%-center vector, according to the three patient's directions (Left-Right or LR; Anterior-Posterior or AP; and Superior-Inferior or SI). The deviations (mean±SD) found for the Dose-WL tests (LR: 0.5±0.4mm; AP: 0.5±0.4mm; SI: 0.6±0.2mm) were in most cases less than 1mm, and they were significantly smaller (all p<0.05) than the maximum deviations reported by the Pointer-WL tests (LR: 1.3±0.3mm; AP: 1.2±0.4mm; SI: 1.1±0.3mm). The Dose-WL test described in this study allows estimating the spatial accuracy of the prescription isodose line.

Beam optics and isocenter property of SC200 proton therapy gantry

Studies on beam optics and the isocenter property in the gantry system for the SC200 (Superconducting Isochronous Cyclotron of 200 MeV in Hefei, China) are presented in this paper. The physical design of the isocentric gantry system is developed with the software TRANSPORT, which realizes a beam with a circular shape at the isocenter and a full width at half maximum of 4–10 mm. For stereotactic radiosurgery, the isocenter deviation of the gantry system should be less than 1 mm in diameter. In order to explore the property of the isocenter, an electromagnetic structure coupling analysis has been conducted given the self-gravity and the electromagnetic (EM) force of the magnets on the gantry beam line. The correlation between the isocenter property and the EM force has also been carefully studied. This paper puts forward two methods to obtain the isocenter deviation based on the characteristics of the nozzle installation structure and the calculation of the optical path after the nozzle, respectively. The results show that the maximum isocenter deviation is less than 0.33 mm with a safety factor of 1.5, and the deviation caused by the EM force is 0.05 mm. The latter result indicates that the impact of the EM force is negligible. This paper puts forward one possible way to realize real-time acquisition of the isocenter deviation in practical application. The gantry of SC200 is under construction at ASIPP.

A novel-integrated quality assurance phantom for radiographic and nonradiographic radiotherapy localization and positioning systems.

Various localization and positioning systems utilizing radiographic or nonradiographic methods have been developed to improve the accuracy of radiation treatment. Each quality assurance (QA) procedure requires its own phantom and is independent from each other, so the deviation between each system is unavailable. The purpose of this work is to develop and evaluate a single-integrated QA phantom for different localization and positioning systems. The integrated phantom was designed in three-dimensional (3D) CAD software and 3D printed. The phantom was designed with laser alignment marks, a raised letter "S" on the anterior surface for optical surface monitoring system registration, a core for radiofrequency (RF) tracking system alignment, eight internal fiducials for image alignment, and an isocentric bearing for Winston-Lutz test. Tilt legs and rotational stage were designed for rotational verification of optical surface mapping system and RF tracking system, respectively. The phantom was scanned using a CT scanner and a QA plan was created. This prototype phantom was evaluated against established QA techniques. The QA result between the proposed procedure and established QA technique are 1.12 ± 0.31 and 1.14 ± 0.31 mm, respectively, for RF tracking system and 0.18 ± 0.06 and 0.18 ± 0.05 mm for Winston-Lutz test. There is no significant difference for the QA results between the established QA and proposed procedure (P > 0.05, t test). The accuracy of rotational verification for surface mapping system and RF tracking system are less than 0.5 and 1° compared the predefined value. The isocenter deviation of each location system is around l mm. We have designed and evaluated a novel-integrated phantom for radiographic and nonradiographic localization and positioning systems for radiotherapy. With this phantom, we will reduce the variation in measurements and simplify the QA procedures.

SU-G-TeP2-04: Comprehensive Machine Isocenter Evaluation with Separation of Gantry, Collimator, and Table Variables

Purpose: To develop and demonstrate application of a method that characterizes deviation of linac x-ray beams from the centroid of the volumetric radiation isocenter as a function of gantry, collimator, and table variables. Methods: A set of Winston-Lutz ball-bearing images was used to determine the gantry radiation isocenter as the midrange of deviation values resulting from gantry and collimator rotation. Also determined were displacement of table axis from gantry isocenter and recommended table axis adjustment. The method, previously reported, has been extended to include the effect of collimator walkout by obtaining measurements with 0 and 180 degree collimator rotation for each gantry angle. Twelve images were used to characterize the volumetric isocenter for the full range of available gantry, collimator, and table rotations. Results: Three Varian True Beam, two Elekta Infinity and four Versa HD linacs at five institutions were tested using identical methodology. Varian linacs exhibited substantially less deviation due to head sag than Elekta linacs (0.4 mm vs. 1.2 mm on average). One linac from each manufacturer had additional isocenter deviation of 0.3 to 0.4 mm due to jaw instability with gantry and collimator rotation. For all linacs, the achievable isocenter tolerance was dependent on adjustment of collimator position offset, transverse position steering, and alignment of the table axis with gantry isocenter, facilitated by these test results. The pattern and magnitude of table axis wobble vs. table angle was reproducible and unique to each machine. Conclusion: This new method provides a comprehensive set of isocenter deviation values including all variables. It effectively facilitates minimization of deviation between beam center and target (ball-bearing) position. This method was used to quantify the effect of jaw instability on isocenter deviation and to identify the offending jaw. The test is suitable for incorporation into a routine machine QA program. Software development was performed by Radiological Imaging Technology, Inc.

Evaluation of the setup margins for cone beam computed tomography–guided cranial radiosurgery: A phantom study

The aim of this study is to evaluate the setup margins from the clinical target volume (CTV) to planning target volume (PTV) for cranial stereotactic radiosurgery (SRS) treatments guided by cone beam computed tomography (CBCT). We designed an end-to-end (E2E) test using a skull phantom with an embedded 6mm tungsten ball (target). A noncoplanar plan was computed (E2E plan) to irradiate the target. The CBCT-guided positioning of the skull phantom on the linac was performed. Megavoltage portal images were acquired after 15 independent deliveries of the E2E plan. The displacement 2-dimensional (2D) vector between the centers of the square field and the ball target on each portal image was used to quantify the isocenter accuracy. Geometrical margins on each patient׳s direction (left-right or LR, anterior-posterior or AP, superior-inferior or SI) were calculated. Dosimetric validation of the margins was performed in 5 real SRS cases: 3-dimesional (3D) isocenter deviations were mimicked, and changes in CTV dose coverage and organs-at-risk (OARs) dosage were analyzed. The CTV-PTV margins of 1.1mm in LR direction, and 0.7mm in AP and SI directions were derived from the E2E tests. The dosimetric analysis revealed that a 1-mm uniform margin was sufficient to ensure the CTV dose coverage, without compromising the OAR dose tolerances. The effect of isocenter uncertainty has been estimated to be 1mm in our CBCT-guided SRS approach.

ExacTrac Snap Verification: A New Tool for Ensuring Quality Control for Lung Stereotactic Body Radiation Therapy

The intrafraction verification provided by ExacTrac X-ray 6D Snap Verification (ET-SV) allows the tracking of potential isocenter displacements throughout patient position and treatment. The aims of this study were (1) to measure the intrafraction variations of the isocenter position (random errors); (2) to study the amplitude of the variation related to the fraction duration; and (3) to assess the impact of the table movement on positioning uncertainties. ET-SV uses images acquired before or during treatment delivery or both to detect isocenter displacement. Twenty patients treated with stereotactic body radiation therapy (SBRT) for lung tumors underwent SV before or during each beam. Noncoplanar beams were sometimes necessary. The time between the setup of the patient and each SV was noted, and values of deviations were compiled for 3 SV time groups: SV performed at ≤10 min (group 1), between 11 and 20 min (group 2), and ≥21 min (group 3). Random errors in positioning during the use of noncoplanar fields were noted. The mean isocenter deviation±SD was 2±0.5 mm (range, 1-8 mm). The average deviations±SD increased significantly from 1.6±0.5 mm to 2.1±0.8 mm and 2.2±0.6 mm for groups 1, 2, and 3 (P=.002), respectively. Percentages of deviation≥3 mm were 7.06%, 22.83%, and 28.07% and 1.08%, 4.15%, and 8.4% for ≥5 mm (P<.0001). For 11 patients, table rotation was necessary. The mean isocenter deviation±SD increased significantly from 1.9±0.5 mm before table rotation to 2.7±0.5 mm (P=.001) for the first beam treated after rotation. SV detects isocenter deviations, which increase in amplitude and frequency with the fraction duration, and enables intrafraction verification for SBRT (taking into account clinical condition and technical issues). SV gives accurate targeting at any time during irradiation and may raise confidence to escalate the dose. SV appears to be an important tool for ensuring the quality control of SBRT.

The Role of Intra- or Periprostatic Calcifications in Image-guided Radiotherapy for Prostate Cancer

The Role of Intra- or Periprostatic Calcifications in Image-guided Radiotherapy for Prostate Cancer

SU‐GG‐J‐37: Assessing Image Guided Radiation Therapy Targeting Accuracy: Coincidence Of Imaging System Isocenter With Treatment Machine Mechanical And Radiation Isocenters

Purpose: The purpose of this study is to introduce a method for quantifying the accuracy of linac and imaging system isocenters and demonstrate its use by measuring the targeting of two kilovoltage (kV) imaging systems, and one gantry mounted megavoltage Electronic Portal Imaging Device (EPID). One is the kV imaging system (OBI) integrated with the linear accelerator which supports radiographic and cone beam CT imaging. The other system is the BrainLab Exac Trac stand alone fixed to the treatment room ceiling and floor. Method and Materials: A cylindrical Lucite phantom was designed and fabricated to perform this study. The cylindrical phantom is 20 cm in diameter and 20 cm long. The phantom contains 13 radio‐opaque fiducial markers. A 5 mm spherical marker is positioned at the center of the cylinder. The phantom was imaged and the CT data sets were imported to two planning systems. Treatment beams were placed and both 3D data sets and isocenter information were transferred to the Linac delivery and imaging workstations. For each imaging system, phantom was initially positioned according to machine cross‐hair. Then images were acquired and measured shifts were applied to precisely position the phantom. Then, Winston‐Lutz test was performed for different gantry and collimator configuration. The radiation beam was collimated by a tray‐mounted 30 mm diameter circular collimator. Results: The radius of spherical volume in which all isocenter intersect or lie was measured. For EPID, OBI, CBCT, and Exac Trac, the radius is: 0.54 mm, 1.4 mm, 1.7 mm and 1.46 mm respectively. From the Winston‐Lutz analysis, the average isocenter deviation from all angles is 1.18 ± 0.28 mm. Conclusion: The error in mechanical, radiation and imaging isocenter is about one millimeter. Regardless what imaging system is used for patient setup, it is important to incorporate this systematic error in any planning margins.

2327: A Randomized Cross-Over Study Comparing two Immobilization Devices for the Treatment of Prostate Cancer Patients Using Megavoltage CT Scanning To Evaluate Setup Accuracy

2327: A Randomized Cross-Over Study Comparing two Immobilization Devices for the Treatment of Prostate Cancer Patients Using Megavoltage CT Scanning To Evaluate Setup Accuracy

SU‐FF‐J‐02: 3D Target Localization Using Cone‐Beam CT for Head and Neck IMRT Patients

Purpose: To evaluate clinical effectiveness of target localization using cone‐beam CT (CBCT) for H&amp;N IMRT treatment. Method and Materials: All H&amp;N patients treated with an IRB approved protocol were imaged on the first day of treatment and weekly thereafter with Varian On‐Board Imager. After the patient was aligned with skin markers, 2D kilovoltage portal images were acquired and compared to the planning DRRs for set‐up corrections (2D‐based correction). Then, CBCT images were acquired and registered with the planning CT by matching both soft tissue and bony landmarks, which generated the 3D‐based corrections. The difference between two correction measurements yielded the treatment isocenter variation using the 3D/3D matching method. Dose distributions with different isocenter deviations were re‐constructed on the planning CT. The treatment DVHs of PTVs and critical structures were compared with those in the original plan. Results: A total of 12 CBCT were analyzed for 3 patients during their first 4‐weeks of treatment. Compared to set‐up variations, anatomical shrinkage/deformation was negligible. The mean isocenter deviations after 2D‐based corrections were 1.1mm in AP direction (max 3.0mm), 2.6mm in lateral direction (max 5.0mm), and 3.8mm in SI direction (max 6.0mm). For the worst isocenter deviation, the reconstructed mean PTV dose was reduced by 3.6% from the planned mean value. For two patients, the mean brainstem dose increased from 33% to 49.5% and from 36.9% to 66.8%, respectively, in worst cases after 2D‐based correction. Other critical structures that received significantly higher doses were cord (max dose changed from 41.8% to 64%), eye (mean dose changed from 37.5% to 83.9%) and parotid (mean dose changed from 58% to 72.5%). Conclusion: For H&amp;N IMRT treatment, CBCT image‐guidance improves localization accuracy compared to 2D‐based technique and should be the method of choice for target localization when tight margins are applied.Partially supported by Varian research grant.

A randomized trial evaluating rigid immobilization for pelvic irradiation

Background Accuracy and reproducibility of the patient’s position is fundamental to the successful delivery of radiation therapy. In recent years, a number of pelvic immobilization techniques have been developed. Few have been evaluated in randomized trials, and many of these studies have produced contradictory and inconclusive results. Purpose To assess whether the use of rigid immobilization devices improve the accuracy and reproducibility of prostate irradiation to a clinically useful degree. Methods and materials A total of 100 patients receiving radical irradiation for either prostate or bladder cancer were randomized to be treated with or without the use of rigid immobilization (RI) devices. Of these, 96 patients were suitable for analysis. The control group consisted of the patients being simulated and treated in the prone position with no immobilization devices. Patients randomized to the immobilized arm were simulated and treated prone using a customized Uvex cast of the pelvis as well as ankle- and shoulder-stabilizing devices. Weekly orthogonal port films (PFs) were obtained for each patient. Using previously specified bone landmarks, we measured variations in the isocenter position on each PF compared with the simulation film. The assessors were unaware of the treatment assignment. Patient comfort, skin toxicity, and treatment times were recorded. Results The average simulation-to-treatment deviation of the isocenter position was 8.5 mm in the control group and 6.2 mm in the immobilization group ( p < 0.001). In the control arm, 30.9% of port films had isocenter deviations >10 mm compared with 10.6% in the immobilized arm ( p = 0.001). For the control group, average deviations in the anteroposterior, right-left, and superior-inferior directions were 5.2 mm, 3.2 mm, and 4.3 mm, respectively, compared with 2.9 mm, 2.1 mm, and 3.9 mm for the immobilized group ( p = <0.001, p < 0.001, p = 0.55). The RTOG skin reaction was greater with in patients with a cast (28% having Grade 2 toxicity vs. 10% in the control arm), although this was not statistically significant ( p = 0.68). Patients in both groups found the treatment position comfortable: 90% in the immobilized group and 87% in the control group scored the treatment position either reasonably or very comfortable. Treatment times were very similar between the two groups: the average treatment time was 15.5 min in the control group vs. 16.1 min in the immobilized group ( p = 0.82). Conclusions The use of rigid immobilization improves the accuracy of treatment delivery for the prone position, especially in the anteroposterior direction. Of clinical importance, the number of major deviations >10 mm (that is, that would result in a geographic miss) was reduced from 31% to 11%.

Commissioning and clinical implementation of a sliding gantry CT scanner installed in an existing treatment room and early clinical experience for precise tumor localization.

The primary objective of the present study is to demonstrate that a unique computed tomography (CT)-linear accelerator combination can be used to reduce uncertainties caused by organ motion and setup inaccuracy. The acceptance, commissioning, and clinical implementation of a sliding gantry CT scanner installed in an existing linear accelerator room are reported in this paper. A Siemens CT scanner was installed directly opposite to an existing accelerator. The scanner is movable on a pair of horizontal rails mounted parallel to the longitudinal axis of the treatment couch replaced with a carbon fiber tabletop. Acceptance and commissioning of the CT scanner were verified with phantom studies. For clinical implementation, quality assurance (QA) procedures have been instituted to ensure the integrity of the CT gantry axis alignment and the accuracy of its movement using a phantom designed in house. A clinical example employing the CT-Linac combination to correct the isocenter positioning caused by organ motion and setup inaccuracy was presented for a prostate irradiation. Dose calculations were performed to study the effects on tumor coverage without the adjustments of the isocenter. A summary of the isocenter adjustments for the first 30 patients is also presented. The geometric accuracy of the CT scanner is < or =1 mm. An isocenter deviation of > or =2 mm from the original plan can be detected. For the clinical example of a prostate patient, the average movement of the prostate gland was found to be approximately 3mm in the anterior-posterior (AP/PA) direction and 5 mm in the cephalic-caudal direction. Variations in the isocenter position may result in underdosage of the PTV if correction is not made for the change in the isocenter position. Our experience with the first 30 patients indicates that while the left-right adjustment of the isocenter is minimal, in the AP/PA direction, about 33% of treatments required an adjustment of 3-5 mm, and about 18% required a 5.1-mm to 10-mm adjustment. In the caudal-cephalic direction, about 26% required an adjustment of 3-5 mm, and 8% required a 5.1-mm to 10-mm adjustment. Retrofitting a CT scanner in an existing linear accelerator room requires careful planning and well-coordinated efforts from all personnel involved. Special QA procedures are needed to ensure the mechanical integrity and imaging accuracy of the CT scanner. A CT scan of the patient prior to irradiation provides valuable information on organ motion. Any deviations from treatment plan can be corrected before dose delivery. Significant deviation from the planning isocenter may occur due to daily variations in the rectal filling. The CT-Linac combination has significant implications for the treatment of prostate cancer.

Isocenter accuracy in frameless stereotactic radiotherapy using implanted fiducials

Purpose The stereotactic radiotherapy (SRT) system verifies isocenter accuracy in patient space. In this study, we evaluate isocenter accuracy in frameless SRT using implanted cranial gold markers. Methods and materials We performed frameless SRT on 43 intracranial tumor patients between August 1997 and December 2000. The treatment technique was determined by the tumor shape and volume, and by the location of critical organs. The coordinates of anterior-posterior and lateral port film were inputted to ISOLOC software, which calculated ( 1) the couch moves translation distance required to bring the target point to the isocenter, and ( 2) the intermarker distance comparisons between the CT study and the treatment machine films. We evaluated the isocenter deviation based on the error between orthogonal film target coordinates and isocenter coordinates. Results The mean treatment isocenter deviations ( x, y, z) were −0.03, 0.14, and −0.04 mm, respectively. The systematic component isocenter standard deviations were 0.28, 0.31, and 0.35 mm (1 SD), respectively, and the random component isocenter standard deviations were 0.53, 0.52, and 0.50 mm (1 SD), respectively. Conclusion The isocenter accuracy in the frameless SRT–implanted fiducial system is highly reliable and is comparable to that of other stereotactic radiosurgery systems.

A technique of intensity-modulated radiosurgery (IMRS) for spinal tumors.

This study is to demonstrate the feasibility of spinal radiosurgery using an image-guided intensity-modulated radiosurgical (IMRS) procedure. A dedicated Novalis shaped beam surgery unit equipped with a built-in micro-multileaf collimator (mMLC) with a single 6 MV photon beam was used. Each patient was simulated in the supine position using an AcQsim CT simulator with infrared sensitive markers for localization. A variety of different treatment plans were developed, but the most common plan was the use of seven coplanar intensity-modulated beams to minimize radiation to critical organs such as the spinal cord and kidneys. An automatic localization device based on infrared and video cameras was used to guide the initial patient setup. Two keV x-ray imaging systems were used to identify potential deviations from the planned isocenter. A total of 25 patients with spinal tumors have been treated using this procedure with a single prescription dose ranging from 6 to 12 Gy. The final verification images indicated that the average isocenter deviation from the planned isocenter was within 2 mm. The phantom verification of isocenter doses indicated that the average deviation of measured isocenter doses from the planned isocenter doses for all patients treated with intensity-modulated beams was less than 2%. Film dose measurement in a phantom study demonstrated good agreement of above 50% isodose lines between the planned and measured results. Preliminary experience shows that precision delivery of high dose radiation could be administered to the planned target volume while the dose to the critical organs is kept within tolerable limits.