Assessment of intrafractional movement and internal motion in radiotherapy of rectal cancer using megavoltage computed tomography.

PURPOSE: The aim of this study was to provide estimates of setup and internal margins of patients treated for rectal carcinoma using helical tomotherapy and to assess possible margin adaptations. Using helical tomotherapy, highly conformal dose distributions can be created, and the integrated megavoltage computed tomography (MVCT) modality allows very precise daily patient positioning. In clinical protocols, however, margins originating from traditional setup procedures are still being applied. This work investigates whether this modality can aid in redefining treatment margins. METHODS AND MATERIALS: Ten patients who were treated with tomotherapy underwent MVCT scanning before and after 10 treatments. Using automatic registration the necessary setup margin was investigated by means of bony landmarks. Internal margins were assessed by delineating and describing the mesorectal movement. RESULTS: Based on bony landmarks, movement of patients during treatments was limited to 2.45 mm, 1.99 mm, and 1.09 mm in the lateral, longitudinal, and vertical direction, respectively. Systematic errors were limited to <1 mm. Measured movement of the mesorectal space was -1.6 mm (+/- 4.2 mm) and 0.1 mm (+/- 4.0 mm) for left and right lateral direction. In the antero-posterior direction, mean shifts were -2 mm (+/- 6.8 mm) and -0.4 mm (+/- 3.8 mm). Mean shifts in the cranio-caudal direction were respectively -3.2 mm (+/- 5.6 mm) and -3.2 mm (+/- 6.8 mm). CONCLUSIONS: The use of the integrated MVCT on the tomotherapy system can minimize the setup margin for rectal cancer, and can also be used to adequately describe the internal margin allowing for direct treatment margin adaptation.

The long- and short-term variability of breathing induced tumor motion in lung and liver over the course of a radiotherapy treatment.

PURPOSE: To evaluate the short and long-term variability of breathing induced tumor motion. MATERIALS AND METHODS: 3D tumor motion of 19 lung and 18 liver lesions captured over the course of an SBRT treatment were evaluated and compared to the motion on 4D-CT. An implanted fiducial could be used for unambiguous motion information. Fast orthogonal fluoroscopy (FF) sequences, included in the treatment workflow, were used to evaluate motion during treatment. Several motion parameters were compared between different FF sequences from the same fraction to evaluate the intrafraction variability. To assess interfraction variability, amplitude and hysteresis were compared between fractions and with the 3D tumor motion registered by 4D-CT. Population based margins, necessary on top of the ITV to capture all motion variability, were calculated based on the motion captured during treatment. RESULTS: Baseline drift in the cranio-caudal (CC) or anterior-poster (AP) direction is significant (ie. >5 mm) for a large group of patients, in contrary to intrafraction amplitude and hysteresis variability. However, a correlation between intrafraction amplitude variability and mean motion amplitude was found (Pearson's correlation coefficient, r = 0.72, p < 10-4). Interfraction variability in amplitude is significant for 46% of all lesions. As such, 4D-CT accurately captures the motion during treatment for some fractions but not for all. Accounting for motion variability during treatment increases the PTV margins in all directions, most significantly in CC from 5 mm to 13.7 mm for lung and 8.0 mm for liver. CONCLUSION: Both short-term and day-to-day tumor motion variability can be significant, especially for lesions moving with amplitudes above 7 mm. Abandoning passive motion management strategies in favor of more active ones is advised.

Assessment of secondary patient motion induced by automated couch movement during on-line 6 dimensional repositioning in prostate cancer treatment.

BACKGROUND AND PURPOSE: The purpose of this study is to assess retrospectively secondary patient motion induced by 6D patient setup correction. MATERIALS AND METHODS: For 104 patients, treated with Novalis, 6D setup correction prior to treatment was performed by ExacTrac5.0/NovalisBody in combination with the Robotic Tilt Module mounted underneath the Exact Couch top. This 6D correction might induce additional setup errors due to patient reaction against the rotations. To evaluate induced secondary motion, the 6D setup correction is verified and evaluated with respect to the tolerance limits. RESULTS: The majority of measured secondary motions are found within the tolerance limits. Detected secondary motions are mostly found in longitudinal shifts and lateral rotations, and mainly found in only 1 dimension during the same verification. The verifications indicate that the patient population can be divided into a group that hardly moves and a group that moves throughout all 6D setup corrections. The patient's behavior can be predicted by the evaluation of the first five fractions as none of the patients demonstrate a learning curve during the treatment. CONCLUSIONS: 6D setup correction does not induce secondary motion for the majority of the patients and can therefore be applied for all treatment indications.

An assessment of the use of skin flashes in helical tomotherapy using phantom and in-vivo dosimetry.

BACKGROUND AND PURPOSE: In helical tomotherapy the nature of the optimizing and planning systems allows the delivery of dose on the skin using a build-up compensating technique (skin flash). However, positioning errors or changes in the patient's contour can influence the correct dosage in these regions. This work studies the behavior of skin-flash regions using phantom and in-vivo dosimetry. MATERIALS AND METHODS: The dosimetric accuracy of the tomotherapy planning system in skin-flash regions is checked using film and TLD on phantom. Positioning errors are induced and the effect on the skin dose is investigated. Further a volume decrease is simulated using bolus material and the results are compared. RESULTS: Results show that the tomotherapy planning system calculates dose on skin regions within 2 SD using TLD measurements. Film measurements show drops of dose of 2.8% and 26% for, respectively, a 5mm and 10mm mispositioning of the phantom towards air and a dose increase of 9% for a 5mm shift towards tissue. These measurements are confirmed by TLD measurements. A simulated volume reduction shows a similar behavior with a 2.6% and 19.4% drop in dose, measured with TLDs. CONCLUSION: The tomotherapy system allows adequate planning and delivery of dose using skin flashes. However, exact positioning is crucial to deliver the dose at the exact location.

Treating patients with Dynamic Wave Arc: First clinical experience.

BACKGROUND AND PURPOSE: Dynamic Wave Arc (DWA) is a system-specific noncoplanar arc technique that combines synchronized gantry-ring rotation with D-MLC optimization. This paper presents the clinical workflow, quality assurance program, and reports the geometric and dosimetric results of the first patient cohort treated with DWA. METHODS AND MATERIALS: The RayStation TPS was clinically integrated on the Vero SBRT platform for DWA treatments. The first 15 patients treated with DWA represent a broad range of treatment sites: breast boost, prostate, lung SBRT and bone metastases, which allowed us to explore the potentials and assess the limitations of the current DWA site-specific template solution. For the DWA verification a variety of QA equipment was used, from 3D diode array to an anthropomorphic end-to-end phantom. The geometric accuracy of each arc was verified with an independent orthogonal fluoroscopy method. RESULTS: The average beam-on delivery time was 3min, ranging from 1.22min to 8.82min. All patient QAs passed our institutional clinical criteria of gamma index. For both EBT3 film and Delta4 measurements, DWA planned versus delivered dose distributions presented an average agreement above 97%. An overall mean gantry-ring geometric deviation of -0.03°±0.46° and 0.18°±0.26° was obtained, respectively. CONCLUSION: For the first time, DWA has been translated into the clinic and used to treat various treatment sides. DWA has been successfully added to the noncoplanar rotational IMRT techniques arsenal, allowing additional flexibility in dose shaping while preserving dosimetrically robust delivery.

Breast conserving treatment for breast cancer: dosimetric comparison of different non-invasive techniques for additional boost delivery.

BACKGROUND: Today it is unclear which technique for delivery of an additional boost after whole breast radiotherapy for breast conserved patients should be state of the art. We present a dosimetric comparison of different non-invasive treatment techniques for additional boost delivery. METHODS: For 10 different tumor bed localizations, 7 different non-invasive treatment plans were made. Dosimetric comparison of PTV-coverage and dose to organs at risk was performed. RESULTS: The Vero system achieved an excellent PTV-coverage and at the same time could minimize the dose to the organs at risk with an average near-maximum-dose (D2) to the heart of 0.9 Gy and the average volume of ipsilateral lung receiving 5 Gy (V5) of 1.5%. The TomoTherapy modalities delivered an average D2 to the heart of 0.9 Gy for the rotational and of 2.3 Gy for the static modality and an average V5 to the ipsilateral lung of 7.3% and 2.9% respectively. A rotational technique offers an adequate conformity at the cost of more low dose spread and a larger build-up area. In most cases a 2-field technique showed acceptable PTV-coverage, but a bad conformity. Electrons often delivered a worse PTV-coverage than photons, with the planning requirements achieved only in 2 patients and with an average D2 to the heart of 2.8 Gy and an average V5 to the ipsilateral lung of 5.8%. CONCLUSIONS: We present advices which can be used as guidelines for the selection of the best individualized treatment.

Breast conserving treatment for breast cancer: dosimetric comparison of sequential versus simultaneous integrated photon boost.

BACKGROUND: Breast conserving surgery followed by whole breast irradiation is widely accepted as standard of care for early breast cancer. Addition of a boost dose to the initial tumor area further reduces local recurrences. We investigated the dosimetric benefits of a simultaneously integrated boost (SIB) compared to a sequential boost to hypofractionate the boost volume, while maintaining normofractionation on the breast. METHODS: For 10 patients 4 treatment plans were deployed, 1 with a sequential photon boost, and 3 with different SIB techniques: on a conventional linear accelerator, helical TomoTherapy, and static TomoDirect. Dosimetric comparison was performed. RESULTS: PTV-coverage was good in all techniques. Conformity was better with all SIB techniques compared to sequential boost (P = 0.0001). There was less dose spilling to the ipsilateral breast outside the PTVboost (P = 0.04). The dose to the organs at risk (OAR) was not influenced by SIB compared to sequential boost. Helical TomoTherapy showed a higher mean dose to the contralateral breast, but less than 5 Gy for each patient. CONCLUSIONS: SIB showed less dose spilling within the breast and equal dose to OAR compared to sequential boost. Both helical TomoTherapy and the conventional technique delivered acceptable dosimetry. SIB seems a safe alternative and can be implemented in clinical routine.

Dosimetric comparison of different treatment modalities for stereotactic radiosurgery of arteriovenous malformations and acoustic neuromas.

PURPOSE: We investigated the influence of beam modulation on treatment planning by comparing four available stereotactic radiosurgery (SRS) modalities: Gamma-Knife-Perfexion, Novalis-Tx Dynamic-Conformal-Arc (DCA) and Dynamic-Multileaf-Collimation-Intensity-Modulated-radiotherapy (DMLC-IMRT), and Cyberknife. MATERIAL AND METHODS: Patients with arteriovenous malformation (n = 10) or acoustic neuromas (n = 5) were planned with different treatment modalities. Paddick conformity index (CI), dose heterogeneity (DH), gradient index (GI) and beam-on time were used as dosimetric indices. RESULTS: Gamma-Knife-Perfexion can achieve high degree of conformity (CI = 0.77 ± 0.04) with limited low-doses (GI = 2.59 ± 0.10) surrounding the inhomogeneous dose distribution (D(H) = 0.84 ± 0.05) at the cost of treatment time (68.1 min ± 27.5). Novalis-Tx-DCA improved this inhomogeneity (D(H) = 0.30 ± 0.03) and treatment time (16.8 min ± 2.2) at the cost of conformity (CI = 0.66 ± 0.04) and Novalis-TX-DMLC-IMRT improved the DCA CI (CI = 0.68 ± 0.04) and inhomogeneity (D(H) = 0.18 ± 0.05) at the cost of low-doses (GI = 3.94 ± 0.92) and treatment time (21.7 min ± 3.4) (p<0.01). Cyberknife achieved comparable conformity (CI = 0.77 ± 0.06) at the cost of low-doses (GI = 3.48 ± 0.47) surrounding the homogeneous (D(H) = 0.22 ± 0.02) dose distribution and treatment time (28.4min±8.1) (p<0.01). CONCLUSIONS: Gamma-Knife-Perfexion will comply with all SRS constraints (high conformity while minimizing low-dose spread). Multiple focal entries (Gamma-Knife-Perfexion and Cyberknife) will achieve better conformity than High-Definition-MLC of Novalis-Tx at the cost of treatment time. Non-isocentric beams (Cyberknife) or IMRT-beams (Novalis-Tx-DMLC-IMRT) will spread more low-dose than multiple isocenters (Gamma-Knife-Perfexion) or dynamic arcs (Novalis-Tx-DCA). Inverse planning and modulated fluences (Novalis-Tx-DMLC-IMRT and CyberKnife) will deliver the most homogeneous treatment. Furthermore, Linac-based systems (Novalis and Cyberknife) can perform image verification at the time of treatment delivery.

A complementary dual-modality verification for tumor tracking on a gimbaled linac system.

BACKGROUND AND PURPOSE: For dynamic tracking of moving tumors, robust intra-fraction verification was required, to assure that tumor motion was properly managed during the course of radiotherapy. A dual-modality verification system, consisting of an on-board orthogonal kV and planar MV imaging device, was validated and applied retrospectively to patient data. METHODS AND MATERIALS: Real-time tumor tracking (RTTT) was managed by applying PAN and TILT angular corrections to the therapeutic beam using a gimbaled linac. In this study, orthogonal X-ray imaging and MV EPID fluoroscopy was acquired simultaneously. The tracking beam position was derived from respectively real-time gimbals log files and the detected field outline on EPID. For both imaging modalities, the moving target was localized by detection of an implanted fiducial. The dual-modality tracking verification was validated against a high-precision optical camera in phantom experiments and applied to clinical tracking data from a liver and two lung cancer patients. RESULTS: Both verification modalities showed a high accuracy (<0.3mm) during validation on phantom. Marker detection on EPID was influenced by low image contrast. For the clinical cases, gimbaled tracking showed a 90th percentile error (E90) of 3.45 (liver), 2.44 (lung A) and 3.40 mm (lung B) based on EPID fluoroscopy and good agreement with XR-log file data by an E90 of 3.13, 1.92 and 3.33 mm, respectively, during beam on. CONCLUSION: Dual-modality verification was successfully implemented, offering the possibility of detailed reporting on RTTT performance.

Initial assessment of tumor tracking with a gimbaled linac system in clinical circumstances: a patient simulation study.

PURPOSE: To have an initial assessment of the Vero Dynamic Tracking workflow in clinical circumstances and quantify the performance of the tracking system, a simulation study was set up on 5 lung and liver patients. METHODS AND MATERIALS: The preparatory steps of a tumor tracking treatment, based on fiducial markers implanted in the tumor, were executed allowing pursuit of the tumor with the gimbaled linac and monitoring X-rays acquisition, however, without activating the 6 MV beam. Data were acquired on workflow time-efficiency, tracking accuracy and imaging exposure. RESULTS: The average time between the patient entering the treatment room and the first treatment field was about 9 min. The time for building the correlation model was 3.2 min. Tracking errors of 0.55 and 0.95 mm (1σ) were observed in PAN/TILT direction and a 2D range of 3.08 mm. A skin dose was determined of 0.08 mGy/image, with a source-to-skin distance of 900 mm and kV exposure of 1 mAs. On average 1.8 mGy/min kV skin dose was observed for 1 Hz monitoring. CONCLUSION: The Vero tracking solution proved to be fully functional and showed performance comparable with other real-time tracking systems.

Implementation of HybridArc treatment technique in preoperative radiotherapy of rectal cancer: dose patterns in target lesions and organs at risk as compared to helical Tomotherapy and RapidArc.

PURPOSE: HybridArc is a novel treatment technique blending aperture-enhanced optimized arcs with discrete IMRT-elements, allowing selection of arcs with a set of static IMRT-beams. This study compared this new technique to helical Tomotherapy, and RapidArc, in preoperative radiotherapy of rectal cancer. MATERIAL AND METHODS: Twelve rectal cancer patients treated consecutively with Tomotherapy Hi-Art II system were simulated with HybridArc and RapidArc. Treatment plans were designed to deliver homogeneous dose of 46.0Gy to mesorectum and draining lymph nodes, with a simultaneous-integrated-boost to the primary tumor up to a total dose of 55.2Gy. Planning objectives were 95% of prescribed dose to 95% of PTVs, while minimizing the volume of small bowel receiving more than 15Gy (V15) and the mean bladder dose. Dose gradient towards simultaneous-integrated-boost (GI), calculated by dividing the volume receiving more then 52.4Gy (95% of PTV55.2Gy)to the volume of PTV55.2Gy, was kept below 1.5. Mean beam-on time and amount of MUs were also analyzed. RESULTS: PTV swere adequately covered by all plans. Significant advantage was found for Tomotherapy in sparing small bowel (V15 = 112.7cm(3)SD73.4cm(3)) compared to RapidArc (133.4cm(3)SD75.3cm(3)) and HybridArc (143.7cm(3)SD74.4cm(3)) (p < 0.01). The mean bladder dose was better with RapidArc (20.6GySD2.2Gy) compared to HybridArc (24.2Gy SD4.3Gy) and Tomotherapy (23.0GySD4.7Gy) (p < 0.01). The mean beam-on time was significantly lower (p < 0.01) for HybridArc (2.7min SD0.8) and RapidArc (2.5min SD0.5) compared to Tomotherapy (11.0min SD0.7). The total amount of MUs was significantly (p < 0.01) lower for RapidArc (547SD44)compared to HybridArc (949 SD153). CONCLUSIONS: HybridArc is a feasible solution for preoperative RT with a simultaneous-integrated-boost in rectal cancer patients. It achieved similar PTV coverage with significant lower beam-on time, but less efficient in sparing small bowel and bladder compared to Tomotherapy and RapidArc. The added value of HybridArc is that the treatment modality can be implemented on every LINAC equipped with Dynamic-Conform-Arc and IMRT treatment techniques, while maintaining the same QA-schemes.

Clinical evaluation of a robotic 6-degree of freedom treatment couch for frameless radiosurgery.

PURPOSE: To evaluate the added value of 6-degree of freedom (DOF) patient positioning with a robotic couch compared with 4DOF positioning for intracranial lesions and to estimate the immobilization characteristics of the BrainLAB frameless mask (BrainLAB AG, Feldkirchen, Germany), more specifically, the setup errors and intrafraction motion. METHODS AND MATERIALS: We enrolled 40 patients with 66 brain metastases treated with frameless stereotactic radiosurgery and a 6DOF robotic couch. Patient positioning was performed with the BrainLAB ExacTrac stereoscopic X-ray system. Positioning results were collected before and after treatment to assess patient setup error and intrafraction motion. Existing treatment planning data were loaded and simulated for 4DOF positioning and compared with the 6DOF positioning. The clinical relevance was analyzed by means of the Paddick conformity index and the ratio of prescribed isodose volume covered with 4DOF to that obtained with the 6DOF positioning. RESULTS: The mean three-dimensional setup error before 6DOF correction was 1.91 mm (SD, 1.25 mm). The rotational errors were larger in the longitudinal (mean, 0.23°; SD, 0.82°) direction compared with the lateral (mean, -0.09°; SD, 0.72°) and vertical (mean, -0.10°; SD, 1.03°) directions (p < 0.05). The mean three-dimensional intrafraction shift was 0.58 mm (SD, 0.42 mm). The mean intrafractional rotational errors were comparable for the vertical, longitudinal, and lateral directions: 0.01° (SD, 0.35°), 0.03° (SD, 0.31°), and -0.03° (SD, 0.33°), respectively. The mean conformity index decreased from 0.68 (SD, 0.08) (6DOF) to 0.59 (SD, 0.12) (4DOF) (p < 0.05). A loss of prescribed isodose coverage of 5% (SD, 0.08) was found with the 4DOF positioning (p < 0.05). Half a degree for longitudinal and lateral rotations can be identified as a threshold for coverage loss. CONCLUSIONS: With a mask immobilization, patient setup error and intrafraction motions need to be evaluated and corrected for. The 6DOF patient positioning with a 6DOF robotic couch to correct translational and rotational setup errors improves target positioning with respect to treatment isocenter, which is in direct relation with the clinical outcome, compared with the 4DOF positioning.

Setup accuracy of the Novalis ExacTrac 6DOF system for frameless radiosurgery.

PURPOSE: Stereotactic radiosurgery using frame-based positioning is a well-established technique for the treatment of benign and malignant lesions. By contrast, a new trend toward frameless systems using image-guided positioning techniques is gaining mainstream acceptance. This study was designed to measure the detection and positioning accuracy of the ExacTrac/Novalis Body (ET/NB) for rotations and to compare the accuracy of the frameless with the frame-based radiosurgery technique. METHODS AND MATERIALS: A program was developed in house to rotate reference computed tomography images. The angles measured by the system were compared with the known rotations. The accuracy of ET/NB was evaluated with a head phantom with seven lead beads inserted, mounted on a treatment couch equipped with a robotic tilt module, and was measured with a digital water level and portal films. Multiple hidden target tests (HTT) were performed to measure the overall accuracy of the different positioning techniques for radiosurgery (i.e., frameless and frame-based with relocatable mask or invasive ring, respectively). RESULTS: The ET/NB system can detect rotational setup errors with an average accuracy of 0.09° (standard deviation [SD] 0.06°), 0.02° (SD 0.07°), and 0.06° (SD 0.14°) for longitudinal, lateral, and vertical rotations, respectively. The average positioning accuracy was 0.06° (SD 0.04°), 0.08° (SD 0.06°), and 0.08° (SD 0.07°) for longitudinal, lateral and vertical rotations, respectively. The results of the HTT showed an overall three-dimensional accuracy of 0.76 mm (SD 0.46 mm) for the frameless technique, 0.87 mm (SD 0.44 mm) for the relocatable mask, and 1.19 mm (SD 0.45 mm) for the frame-based technique. CONCLUSIONS: The study showed high detection accuracy and a subdegree positioning accuracy. On the basis of phantom studies, the frameless technique showed comparable accuracy to the frame-based approach.

Short course radiotherapy with simultaneous integrated boost for stage I-II breast cancer, early toxicities of a randomized clinical trial.

BACKGROUND: TomoBreast is a unicenter, non-blinded randomized trial comparing conventional radiotherapy (CR) vs. hypofractionated Tomotherapy (TT) for post-operative treatment of breast cancer. The purpose of the trial is to compare whether TT can reduce heart and pulmonary toxicity. We evaluate early toxicities. METHODS: The trial started inclusion in May 2007 and reached its recruitment in August 2011. Women with stage T1-3N0M0 or T1-2N1M0 breast cancer completely resected by tumorectomy (BCS) or by mastectomy (MA) who consented to participate were randomized, according to a prescribed computer-generated randomization schedule, between control arm of CR 25x2 Gy/5 weeks by tangential fields on breast/chest wall, plus supraclavicular-axillary field if node-positive, and sequential boost 8x2 Gy/2 weeks if BCS (cumulative dose 66 Gy/7 weeks), versus experimental TT arm of 15x2.8 Gy/3 weeks, including nodal areas if node-positive and simultaneous integrated boost of 0.6 Gy if BCS (cumulative dose 51 Gy/3 weeks). Outcomes evaluated were the pulmonary and heart function. Comparison of proportions used one-sided Fisher's exact test. RESULTS: By May 2010, 70 patients were randomized and had more than 1 year of follow-up. Out of 69 evaluable cases, 32 were assigned to CR (21 BCS, 11 MA), 37 to TT (20 BCS, 17 MA). Skin toxicity of grade ≥1 at 2 years was 60% in CR, vs. 30% in TT arm. Heart function showed no significant difference for left ventricular ejection fraction at 2 years, CR 4.8% vs. TT 4.6%. Pulmonary function tests at 2 years showed grade ≥1 decline of FEV1 in 21% of CR, vs. 15% of TT and decline of DLco in 29% of CR, vs. 7% of TT (P = 0.05). CONCLUSIONS: There were no unexpected severe toxicities. Short course radiotherapy of the breast with simultaneous integrated boost over 3 weeks proved feasible without excess toxicities. Pulmonary tests showed a slight trend in favor of Tomotherapy, which will need confirmation with longer follow-up of patients. TRIAL REGISTRATION: [corrected] ClinicalTrials.gov NCT00459628.

Computer-aided analysis of star shot films for high-accuracy radiation therapy treatment units.

As mechanical stability of radiation therapy treatment devices has gone beyond sub-millimeter levels, there is a rising demand for simple yet highly accurate measurement techniques to support the routine quality control of these devices. A combination of using high-resolution radiosensitive film and computer-aided analysis could provide an answer. One generally known technique is the acquisition of star shot films to determine the mechanical stability of rotations of gantries and the therapeutic beam. With computer-aided analysis, mechanical performance can be quantified as a radiation isocenter radius size. In this work, computer-aided analysis of star shot film is further refined by applying an analytical solution for the smallest intersecting circle problem, in contrast to the gradient optimization approaches used until today. An algorithm is presented and subjected to a performance test using two different types of radiosensitive film, the Kodak EDR2 radiographic film and the ISP EBT2 radiochromic film. Artificial star shots with a priori known radiation isocenter size are used to determine the systematic errors introduced by the digitization of the film and the computer analysis. The estimated uncertainty on the isocenter size measurement with the presented technique was 0.04 mm (2σ) and 0.06 mm (2σ) for radiographic and radiochromic films, respectively. As an application of the technique, a study was conducted to compare the mechanical stability of O-ring gantry systems with C-arm-based gantries. In total ten systems of five different institutions were included in this study and star shots were acquired for gantry, collimator, ring, couch rotations and gantry wobble. It was not possible to draw general conclusions about differences in mechanical performance between O-ring and C-arm gantry systems, mainly due to differences in the beam-MLC alignment procedure accuracy. Nevertheless, the best performing O-ring system in this study, a BrainLab/MHI Vero system, and the best performing C-arm system, a Varian Truebeam system, showed comparable mechanical performance: gantry isocenter radius of 0.12 and 0.09 mm, respectively, ring/couch rotation of below 0.10 mm for both systems and a wobble of 0.06 and 0.18 mm, respectively. The methodology described in this work can be used to monitor mechanical performance constancy of high-accuracy treatment devices, with means available in a clinical radiation therapy environment.

Geometric accuracy of a novel gimbals based radiation therapy tumor tracking system.

PURPOSE: VERO is a novel platform for image guided stereotactic body radiotherapy. Orthogonal gimbals hold the linac-MLC assembly allowing real-time moving tumor tracking. This study determines the geometric accuracy of the tracking. MATERIALS AND METHODS: To determine the tracking error, an 1D moving phantom produced sinusoidal motion with frequencies up to 30 breaths per minute (bpm). Tumor trajectories of patients were reproduced using a 2D robot and pursued with the gimbals tracking system prototype. Using the moving beam light field and a digital-camera-based detection unit tracking errors, system lag and equivalence of pan/tilt performance were measured. RESULTS: The system lag was 47.7 ms for panning and 47.6 ms for tilting. Applying system lag compensation, sinusoidal motion tracking was accurate, with a tracking error 90% percentile E(90%)<0.82 mm and similar performance for pan/tilt. Systematic tracking errors were below 0.14 mm. The 2D tumor trajectories were tracked with an average E(90%) of 0.54 mm, and tracking error standard deviations of 0.20 mm for pan and 0.22 mm for tilt. CONCLUSIONS: In terms of dynamic behavior, the gimbaled linac of the VERO system showed to be an excellent approach for providing accurate real-time tumor tracking in radiation therapy.

The effect of tomotherapy imaging beam output instabilities on dose calculation.

A radiotherapy treatment plan is based on an anatomical 'snapshot' of the patient acquired during the preparation stage using a kVCT (kilovolt computed tomography) scanner. Anatomical changes will occur during the treatment course, in some cases requiring a new treatment plan to deliver the prescribed dose. With the introduction of 3D volumetric on-board imaging devices, it became feasible to use the produced images for dose recalculation. However, the use of these on-board imaging devices in clinical routine for the calculation of dose depends on the stability of the images. In this study the validation of tomotherapy MVCT (megavolt computed tomography) produced images, for the purpose of dose recalculation by the Planned Adaptive software, has been performed. To investigate the validity of MVCT images for dose calculation, a treatment plan was created based on kVCT-acquired images of a solid water phantom. During a period of 4 months, MVCT images of the phantom have been acquired and were used by the planned adaptive software to recalculate the initial kVCT-based dose on the MVCT images. The influence of the adapted IVDTs (image value-to-density tables) has been investigated as well as the effect of image acquisition with or without preceding airscan. Output fluctuations and/or instabilities of the imaging beam result in MV images of different quality yielding different results when used for dose calculation. It was shown that the output of the imaging beam is not stable, leading to differences of nearly 3% between the original kV-based dose and the recalculated MV-based dose, for solid water only. MVCT images can be used for dose calculation purposes bearing in mind that the output beam is liable to fluctuations. The acquisition of an IVDT together with the MVCT image set, that is going to be used for dose calculation, is highly recommended.

An in-house developed resettable MOSFET dosimeter for radiotherapy.

The purpose of this note is to report the feasibility and clinical validation of an in-house developed MOSFET dosimetry system and describe an integrated non-destructive reset procedure. Off-the-shelf MOSFETs are connected to a common PC using an 18 bit/analogue-input and 16 bit/output data acquisition card. A reading algorithm was developed defining the zero-temperature-coefficient point (ZTC) to determine the threshold voltage. A wireless interface was established for ease of use. The reset procedure consists of an internal circuit generating a local heating induced by an electrical current. Sensitivity has been investigated as a function of bias voltage (0-9 V) to the gate. Dosimetric properties have been evaluated for 6 MV and 15 MV clinical photon beams and in vivo benchmarking was performed against thermoluminescence dosimeters (TLD) for conventional treatments (two groups of ten patients for each energy) and total body irradiation (TBI). MOSFETS were pre-irradiated with 20 Gy. Sensitivity of 0.08 mV cGy(-1) can be obtained for 200 cGy irradiations at 5 V bias voltage. Ten consecutive measurements at 200 cGy yield a SD of 2.08 cGy (1.05%). Increasing the dose in steps from 5 cGy to 1000 cGy yields a 1.00 Pearson correlation coefficient and agreement within 2.0%. Dose rate dependence (160-800 cGy min(-1)) was within 2.5%, temperature dependence within 2.0% (25-37 degrees C). A strong angular dependence has been observed for gantry incidences exceeding +/-30 degrees C. Dose response is stable up to 50 Gy (saturation occurs at approximately 90 Gy), which is used as threshold dose before resetting the MOSFET. An average measured-over-calculated dose ratio within 1.05 (SD: 0.04) has been obtained in vivo. TBI midplane-dose assessed by entrance and exit dose measurements agreed within 1.9% with ionization chamber in phantom, and within 1.0% with TLD in vivo. An in-house developed resettable MOSFET-based dosimetry system is proposed. The system has been validated and is currently used for in vivo entrance dose measurement in clinical routine for simple (open field) treatment configurations.