Standard Linear Accelerator Research Articles

Stereotactic radiotherapy of pancreatic cancer: a systematic review on pain relief.

Locally advanced pancreatic carcinoma (LAPC) has a poor prognosis and the purpose of treatment is survival prolongation and symptom palliation. Radiotherapy has been reported to reduce pain in LAPC. Stereotactic RT (SBRT) is considered as an emerging radiotherapy technique able to achieve high local control rates with acceptable toxicity. However, its role in pain palliation is not clear. To review the impact on pain relief with SBRT in LAPC patients, a literature search was performed on PubMed, Scopus, and Embase (January 2000–December 2017) for prospective and retrospective articles published in English. Fourteen studies (479 patients) reporting the effect of SBRT on pain relief were finally included in this analysis. SBRT was delivered with both standard and/or robotic linear accelerators. The median prescribed SBRT doses ranged from 16.5 to 45 Gy (median: 27.8 Gy), and the number of fractions ranged from 1 to 6 (median: 3.5). Twelve of the 14 studies reported the percentage of pain relief (in patients with pain at presentation) with a global overall response rate (complete and partial response) of 84.9% (95% CI, 75.8%–91.5%), with high heterogeneity (Q2 test: P<0.001; I2=83.63%). All studies reported toxicity data. Acute and late toxicity (grade ≥3) rates were 3.3%–18.0% and 6.0%–8.2%, respectively. Reported gastrointestinal side effects were duodenal obstruction/ulcer, small bowel obstruction, duodenal bleeding, hemorrhage, and gastric perforation. SBRT achieves pain relief in most patients with pancreatic cancer with an acceptable gastrointestinal toxicity rate. Further prospective studies are needed to define optimal dose/fractionation and the best systemic therapies modality integration to reduce toxicity and improve the palliative outcome. Finally, the quality of life and, particularly, pain control should be considered as an endpoint in all future trials on this emerging treatment technique.

Osteoradionecrosis of the Skull Base in Nasopharyngeal Carcinoma: Incidence and Risk Factors

Nasopharyngeal carcinoma (NPC) is a type of malignancy with a high prevalence in southern China and Southeast Asia. The primary treatment modality is radiation therapy (RT). Osteoradionecrosis (ORN) of the skull base remains one of the most serious complications after RT, affecting survival time and quality of life. Thus far, skull base ORN has been seldom reported and can be difficult to distinguish and easy to misdiagnose. In this retrospective study, we report the incidence of skull base ORN and analyze its associated factors in an attempt to decrease the occurrence of ORN. From January 2001 to December 2012, a total of 1348 patients who received diagnoses of NPC received 1 course of RT. Complete medical records were reviewed, and the patients were examined by magnetic resonance imaging and nasopharyngeal endoscopy during follow-up after primary treatment. Patients with other tumors of the head and neck, a history of RT, failure to complete RT, and those lost to follow-up were excluded. Treatment was delivered with external beam RT using standard linear accelerators. A total of 1348 patients with NPC were enrolled in this study after 1 course of RT; among these patients, 14 received diagnoses of skull base ORN. The incidence of skull base ORN was 1.04%. The average latency interval from the completion of RT to the diagnosis of skull base ORN was 45.57months. Skull base ORN after 1 course of RT was associated with the T stage; total radiation dose to the nasopharynx, including the skull base in the radiation field; and anemia. The occurrence of skull base ORN was associated with primary tumors with advanced T stages, high doses of nasopharynx RT, and radiation fields that included the skull base. These factors may be used as predictors for the incidence of skull base ORN.

A high resolution 2D array detector system for small-field MRI-linac applications

A monolithic silicon small-field array detector is proposed for relative dosimetry applications in hybrid MRI-linac systems. The detector has high sampling resolution, with 512 active elements arranged with 2 mm pitch over a 46 mm × 46 mm detection area. Experimental measurements were performed in a custom-designed permanent magnet device that is compatible with a standard clinical linear accelerator. It can be configured in both inline and perpendicular magnetic-field-to-photon-beam orientations and produces magnetic field strengths 0.95 T and 1.20 T, respectively. Monte Carlo simulation data, obtained using the GEANT4 toolkit, are presented to supplement experimental data. Beam profiles show agreement to EBT3 film within 0.5 mm for FWHM and penumbral width measurement of small square fields (width ranging from 0.75 cm to 2.25 cm), in both inline and perpendicular magnetic field orientations. The detector can be used to accurately resolve normalised beam profiles in magnetic fields. The impact of electron return effects (ERE) in a small air gap surrounding the detector was also quantified. For the perpendicular orientation, a reduced profile intensity was observed for an increasing air gap width above the detector (10% at 2 mm) due to ERE. In the inline orientation, a very small increase in response relative to the zero field case was observed with an air gap above the detector (2% at 2 mm). Calibration of the device in a magnetic field will therefore be necessary; the zero field calibration is non-transferable. The MagicPlate-512 provides a high-resolution real time alternative to accurately measure normalised beam profiles in magnetic fields, and is expected to be a suitable array detector for use in magnetic field environments typical of MRI-linacs.

First Experience With Markerless Online 3D Spine Position Monitoring During SBRT Delivery Using a Conventional LINAC

The purpose was to report our initial experience with online markerless 3-dimensional (3D) spine position monitoring. We used template matching plus triangulation of fluoroscopic kilovoltage images acquired with the gantry-mounted imager during flattening filter-free volumetric modulated arc spine stereotactic body radiation therapy delivery on a conventional linear accelerator. Kilovoltage images were acquired at 7 frames per second and streamed to a stand-alone computer.Two-dimensional templates (1/°) containing the clinical target volume were generated from planning computed tomography (CT) data before the first fraction and matched to the (prefiltered) kilovoltage images during treatment.Each 2-dimensional registration was triangulated with multiple previous registrations, resulting in the 3D spine position offset from the planned position in realtime during treatment. If the offset was more than a certain threshold, the treatment was manually stopped and a cone beam CT scan was acquired to reposition the patient. During irradiation of 10 fractions in 3 patients, images were analyzed at an average rate of 1.0 to 1.3 frames per second; all other frames were excluded from the analysis because of limitations in processing speed. As a result of the start-up period of triangulation and poorer image quality at the start of treatment (lateral imaging angles), the first 3D position was determined after an average of 4.9seconds. On the basis of the position results, we interrupted the treatment beam 2 times for different patients. In all cases the spine position results corresponded well with the CT-cone beam CT match values used for subsequent repositioning. For the first time, we have determined the spine position during stereotactic body radiation therapy delivery on a standard linear accelerator using the gantry-mounted kilovoltage imager. This has the potential to increase confidence in the treatment, and the need for 2 treatment interruptions demonstrates the benefit of monitoring during irradiation. However, software improvements are needed to increase processing speed.

The first clinical implementation of real-time image-guided adaptive radiotherapy using a standard linear accelerator

PurposeUntil now, real-time image guided adaptive radiation therapy (IGART) has been the domain of dedicated cancer radiotherapy systems. The purpose of this study was to clinically implement and investigate real-time IGART using a standard linear accelerator. Materials/methodsWe developed and implemented two real-time technologies for standard linear accelerators: (1) Kilovoltage Intrafraction Monitoring (KIM) that finds the target and (2) multileaf collimator (MLC) tracking that aligns the radiation beam to the target. Eight prostate SABR patients were treated with this real-time IGART technology. The feasibility, geometric accuracy and the dosimetric fidelity were measured. ResultsThirty-nine out of forty fractions with real-time IGART were successful (95% confidence interval 87–100%). The geometric accuracy of the KIM system was −0.1 ± 0.4, 0.2 ± 0.2 and −0.1 ± 0.6 mm in the LR, SI and AP directions, respectively. The dose reconstruction showed that real-time IGART more closely reproduced the planned dose than that without IGART. For the largest motion fraction, with real-time IGART 100% of the CTV received the prescribed dose; without real-time IGART only 95% of the CTV would have received the prescribed dose. ConclusionThe clinical implementation of real-time image-guided adaptive radiotherapy on a standard linear accelerator using KIM and MLC tracking is feasible. This achievement paves the way for real-time IGART to be a mainstream treatment option.

The First Clinical Implementation of Real-time Adaptive Radiation Therapy Using a Standard Linear Accelerator

Until now, real-time adaptive radiotherapy has been the domain of expensive dedicated cancer radiotherapy systems. The purpose of this study was to clinically implement and investigate real-time adaptive radiotherapy using a standard linear accelerator. The hypothesis is that real-time adaptive radiotherapy using a standard linear accelerator is feasible. We led an international consortium of universities and hospitals that developed two real-time image guidance technologies for standard linear accelerators: (1) Kilovoltage Intrafraction Monitoring (KIM) which finds the target position in real-time during radiotherapy and (2) multileaf collimator (MLC) tracking which aligns the radiation beam to the moving target. Together, KIM and MLC tracking enable real-time adaptive radiotherapy using a standard linear accelerator without any additional hardware. These two technologies have been clinically implemented independently, but have not been used together to treat patients. To test the real-time radiotherapy technology, prostate SABR patients enrolled on the TROG 15.01 SPARK (NCT02397317) clinical trial were treated. The feasibility hypothesis was tested using maximum likelihood estimates assuming a binomial distribution of success. The geometric accuracy of the real-time adaptive technology was measured by kV/MV triangulation. The dosimetric impact was assessed by dose reconstruction. Ten of 10 clinical treatments with real-time adaptation using the KIM and MLC tracking technologies were successful. The hypothesis test yielded a probability estimate of 1.0, with 95% confidence intervals of [0.69, 1.0]. For the fraction with the largest observed motion, up to 9mm, the geometric accuracy of the KIM system was -0.19±0.49, 0.17±0.26 and 0.08±0.64 mm in the LR, SI, and AP directions, respectively. The dose reconstruction results showed that real-time adaptation faithfully reproduced the planned dose in the presence of intrafraction motion. For the largest motion fraction, with real-time adaptation 98% of the CTV received the prescription dose; without real-time adaptation only 94% of the CTV would have received the prescription dose. With real-time tracking the rectal V70% increased from 10% planned to 12% delivered; without real-time adaptation the rectal V70% would have been 23%. The clinical implementation of real-time adaptive radiotherapy using a standard linear accelerator using KIM and MLC tracking is feasible. This milestone paves the way for this technology to be tested more broadly, enabling more accurate radiotherapy on widely available linear accelerators, and ushering in the era of real-time radiotherapy as a mainstream treatment option.

Dosimetric comparison of photon beam profile characteristics for different treatment parameters

AbstractPurposeTo deliver radiation doses with higher accuracy, radiation treatment through megavoltage photon beams from linear accelerators, is accepted widely for treating malignancies. Before calibrating the linear accelerators, it is essential to make a complete analysis of all photon beam profile parameters. The main objective of this exploration was to investigate the 6 and 15 MV photon beam profile characteristics to improve the accuracy of radiation treatment plans.MethodsIn this exploration, treatment parameters like depth, field size and beam energy were varied to observe their effect on dosimetric characteristics of beam profiles in a water phantom, generated by linear accelerator Varian Clinac.ResultsThe results revealed thatDmaxandDmindecreased with increasing depth but increased with increasing field sizes. Both left and right penumbras increased with increasing depth, field size and energy. Homogeneity increased with field size but decreased with depth. Symmetry had no dependence on depth, energy and field size.ConclusionAll the characteristics of photon beam dosimetry were analysed and the characteristics like homogeneity and symmetry measured by an ion chamber in a water phantom came within clinically acceptable level of 3 and 103%, respectively, thus fulfilled the requirements of standard linear accelerator specifications. This exploration can be extended to the determination of beam profile characteristics of electron and photon beams of other energies at various depths and field sizes for designing optimum treatment plans.

MA13.02 First-In-Human Clinical Experience with Real-Time Tumor Targeting via MLC Tracking for Stereotactic Radiotherapy of Lung Cancer

MLC tracking is an emerging technology to improve tumor targeting and reduce normal tissue irradiation during radiotherapy. The purpose of this work is to present the early clinical experience from the first-in-human trial of real-time tumor targeting via MLC tracking for stereotactic ablative body radiotherapy (SABR) of lung cancer. Full ethics approval through an Australian ethics board has been received for recruitment of 20 patients with stage 1 lung cancer or lung metastases into the MLC tracking clinical trial (NCT02514512). To date, seven recruited patients have each had three electromagnetic beacons inserted near the tumor. An MLC tracking SABR plan was generated with planning target volume (PTV) expanded 5mm from end-exhale tumor volume (GTV). For comparison a conventional motion-encompassing SABR plan was generated with PTV expanded 5mm from a 4DCT-derived internal target volume. Treatment was delivered using a standard linear accelerator using in-house developed software to continuously adapt the MLC motion based on the Calypso beacons’ movement. Tumor motion, treated volume and reconstructed delivered dose were compared between MLC tracking and conventional motion-encompassing treatment planning. All seven patients have been treated successfully with MLC tracking (29 successful fractions). The MLC tracking PTV for all patients has been smaller than with ITV based planning (range 12% to 41% reduction, or 2 to 18 cm3 with MLC tracking). Subsequent reductions in normal lung dose were observed. Tumor motion was seen to vary in motion range from the planning 4DCT during treatment; significantly, larger motion was observed during treatment that exceeded standard PTV boundaries. Reconstruction of delivered treatments confirmed the accurate delivery of MLC tracking, with 100% prescribed dose delivered to the GTV. The first treatments with MLC tracking have been successfully performed in seven lung cancer patients. Reductions in treated volumes were observed, which translated to reductions in delivered lung dose.

IPEM topical report 1: guidance on implementing flattening filter free (FFF) radiotherapy

Flattening filter free (FFF) beams are now commonly available with new standard linear accelerators. These beams have recognised clinical advantages in certain circumstances, most notably the reduced beam-on times for high dose per fraction stereotactic treatments. Therefore FFF techniques are quickly being introduced into clinical use. The purpose of this report is to provide practical implementation advice and references for centres implementing FFF beams clinically. In particular UK-specific guidance is given for reference dosimetry and radiation protection.

Intensity Modulated Radiation Therapy With Simultaneous Integrated Boost in Patients With Brain Oligometastases: A Phase 1 Study (ISIDE-BM-1)

To investigate the maximum tolerated dose of intensity modulated radiationtherapy simultaneous integrated boost whole-brain radiation therapy for palliative treatment of patients with <5 brain metastases using a standard linear accelerator. The whole brain plus 3-mm margin was defined as the planning target volume (PTVwb), whereas each brain metastasis, defined as the contrast-enhancing tumor on MRI T1 scans, plus a 3-mm isotropic margin, was defined as metastases PTV (PTVm). Radiation therapy was delivered in 10 daily fractions (2weeks). Only the dose to PTVm was progressively increased in the patient cohorts (35Gy, 40Gy, 45Gy, 50Gy), whereas the PTVwb was always treated with 30Gy (3Gy per fraction) in all patients. The dose-limiting toxicity was evaluated providing that 3months of follow-up had occurred after the treatment of a 6-patient cohort. Thirty patients were enrolled in the study (dose PTVm: 35Gy, 8 patients; 40Gy, 6 patients; 45Gy, 6 patients; 50Gy, 10 patients). The number of treated brain metastases was 1 in 18 patients, 2 in 5 patients, 3 in 6 patients, and 4 in 1 patient. Three patients experienced dose-limiting toxicity: 1 patient at dose level 2 presented grade 3 (G3) skin toxicity; 1 patient at dose level 4 presented G3 neurologic toxicity; and 1 patient at the same level showed brain hemorrhage. Most patients showed G1 to 2 acute toxicity, in most cases skin (n=19) or neurologic (n=10). Twenty-seven were evaluable for response: 6 (22%) stable disease, 18 (67%) partial response, and 3 (11%) complete response. Median survival and 1-year overall survival were 12months and 53%, respectively. No patient showed late toxicity. In this first prospective trial on the use of intensity modulated radiation therapy simultaneous integrated boost delivered with a standard linear accelerator in patients with brain oligometastases, a boost dose up to 50Gy in 10 fractions was tolerable according to the study design.

The first patient treatment of electromagnetic-guided real time adaptive radiotherapy using MLC tracking for lung SABR

Background and purposeReal time adaptive radiotherapy that enables smaller irradiated volumes may reduce pulmonary toxicity. We report on the first patient treatment of electromagnetic-guided real time adaptive radiotherapy delivered with MLC tracking for lung stereotactic ablative body radiotherapy. Materials and methodsA clinical trial was developed to investigate the safety and feasibility of MLC tracking in lung. The first patient was an 80-year old man with a single left lower lobe lung metastasis to be treated with SABR to 48Gy in 4 fractions. In–house software was integrated with a standard linear accelerator to adapt the treatment beam shape and position based on electromagnetic transponders implanted in the lung. MLC tracking plans were compared against standard ITV-based treatment planning. MLC tracking plan delivery was reconstructed in the patient to confirm safe delivery. ResultsReal time adaptive radiotherapy delivered with MLC tracking compared to standard ITV-based planning reduced the PTV by 41% (18.7–11cm3) and the mean lung dose by 30% (202–140cGy), V20 by 35% (2.6–1.5%) and V5 by 9% (8.9–8%). ConclusionAn emerging technology, MLC tracking, has been translated into the clinic and used to treat lung SABR patients for the first time. This milestone represents an important first step for clinical real-time adaptive radiotherapy that could reduce pulmonary toxicity in lung radiotherapy.

Total body irradiation-an attachment free sweeping beam technique.

IntroductionA sweeping beam technique for total body irradiation in standard treatment rooms and for standard linear accelerators (linacs) is introduced, which does not require any accessory attached to the linac. Lung shielding is facilitated to reduce the risk of pulmonary toxicity. Additionally, the applicability of a commercial radiotherapy planning system (RTPS) is examined.Material and MethodsThe patient is positioned on a low couch on the floor, the longitudinal axis of the body in the rotational plane of the linac. Eight arc fields and five additional fixed beams are applied to the patient in supine and prone position respectively. The dose distributions were measured in a solid water phantom and in an Alderson phantom. Diode detectors were calibrated for in-vivo dosimetry. The RTPS Oncentra was employed for calculations of the dose distribution.ResultsFor the cranial 120 cm the longitudinal dose profile in a slab phantom measured with ionization chamber varies between 94 and 107 % of the prescription dose. These values were confirmed by film measurements and RTPS calculations. The transmittance of the lung shields has been determined as a function of the thickness of the absorber material. Measurements in an Alderson phantom and in-vivo dosimetry of the first patients match the calculated dose.Discussion and conclusionA treatment technique with clinically good dose distributions has been introduced, which can be applied with each standard linac and in standard treatment rooms. Dose calculations were performed with a commercial RTPS and should enable individual dose optimization.

MO-FG-BRA-06: Electromagnetic Beacon Insertion in Lung Cancer Patients and Resultant Surrogacy Errors for Dynamic MLC Tumour Tracking

Purpose: To assess endo-bronchial electromagnetic beacon insertion and to quantify the geometric accuracy of using beacons as a surrogate for tumour motion in real-time multileaf collimator (MLC) tracking of lung tumours. Methods: The LIGHT SABR trial is a world-first clinical trial in which the MLC leaves move with lung tumours in real time on a standard linear accelerator. Tracking is performed based on implanted electromagnetic beacons (CalypsoTM, Varian Medical Systems, USA) as a surrogate for tumour motion. Five patients have been treated and have each had three beacons implanted endo-bronchially under fluoroscopic guidance. The centre of mass (C.O.M) has been used to adapt the MLC in real-time. The geometric error in using the beacon C.O.M as a surrogate for tumour motion was measured by measuring the tumour and beacon C.O.M in all phases of the respiratory cycle of a 4DCT. The surrogacy error was defined as the difference in beacon and tumour C.O.M relative to the reference phase (maximum exhale). Results: All five patients have had three beacons successfully implanted with no migration between simulation and end of treatment. Beacon placement relative to tumour C.O.M varied from 14 to 74 mm and in one patient spanned two lobes. Surrogacy error was measured in each patient on the simulation 4DCT and ranged from 0 to 3 mm. Surrogacy error as measured on 4DCT was subject to artefacts in mid-ventilation phases. Surrogacy error was a function of breathing phase and was typically larger at maximum inhale. Conclusion: Beacon placement and thus surrogacy error is a major component of geometric uncertainty in MLC tracking of lung tumours. Surrogacy error must be measured on each patient and incorporated into margin calculation. Reduction of surrogacy error is limited by airway anatomy, however should be taken into consideration when performing beacon insertion and planning. This research is funded by Varian Medical Systems via a collaborative research agreement.

SU‐F‐T‐169: A Periodic Quality Assurance Program for a Spot‐Scanning Proton Treatment Facility

Purpose:To develop daily and monthly quality assurance (QA) programs in support of a new spot‐scanning proton treatment facility using a combination of commercial and custom equipment and software. Emphasis was placed on efficiency and evaluation of key quality parameters.Methods:The daily QA program was developed to test output, spot size and position, proton beam energy, and image guidance using the Sun Nuclear Corporation rf‐DQA™3 device and Atlas QA software. The program utilizes standard Atlas linear accelerator tests repurposed for proton measurements and a custom jig for indexing the device to the treatment couch. The monthly QA program was designed to test mechanical performance, image quality, radiation quality, isocenter coincidence, and safety features. Many of these tests are similar to linear accelerator QA counterparts, but many require customized test design and equipment. Coincidence of imaging, laser marker, mechanical, and radiation isocenters, for instance, is verified using a custom film‐based device devised and manufactured at our facility. Proton spot size and position as a function of energy are verified using a custom spot pattern incident on film and analysis software developed in‐house. More details concerning the equipment and software developed for monthly QA are included in the supporting document. Thresholds for daily and monthly tests were established via perturbation analysis, early experience, and/or proton system specifications and associated acceptance test results.Results:The periodic QA program described here has been in effect for approximately 9 months and has proven efficient and sensitive to sub‐clinical variations in treatment delivery characteristics.Conclusion:Tools and professional guidelines for periodic proton system QA are not as well developed as their photon and electron counterparts. The program described here efficiently evaluates key quality parameters and, while specific to the needs of our facility, could be readily adapted to other proton centers.

Real-Time 3D Image Guidance Using a Standard LINAC: Measured Motion, Accuracy, and Precision of the First Prospective Clinical Trial of Kilovoltage Intrafraction Monitoring–Guided Gating for Prostate Cancer Radiation Therapy

Kilovoltage intrafraction monitoring (KIM) is a new real-time 3-dimensional image guidance method. Unlike previous real-time image guidance methods, KIM uses a standard linear accelerator without any additional equipment needed. The first prospective clinical trial of KIM is underway for prostate cancer radiation therapy. In this paper we report on the measured motion accuracy and precision using real-time KIM-guided gating. Imaging and motion information from the first 200 fractions from 6 patient prostate cancer radiation therapy volumetric modulated arc therapy treatments were analyzed. A 3-mm/5-second action threshold was used to trigger a gating event where the beam is paused and the couch position adjusted to realign the prostate to the treatment isocenter. To quantify the in vivo accuracy and precision, KIM was compared with simultaneously acquired kV/MV triangulation for 187 fractions. KIM was successfully used in 197 of 200 fractions. Gating events occurred in 29 fractions (14.5%). In these 29 fractions, the percentage of beam-on time, the prostate displacement was >3 mm from the isocenter position, reduced from 73% without KIM to 24% with KIM-guided gating. Displacements >5 mm were reduced from 16% without KIM to 0% with KIM. The KIM accuracy was measured at <0.3 mm in all 3 dimensions. The KIM precision was <0.6 mm in all 3 dimensions. Clinical implementation of real-time KIM image guidance combined with gating for prostate cancer eliminates large prostate displacements during treatment delivery. Both in vivo KIM accuracy and precision are well below 1 mm.

Clinical outcome of stereotactic body radiotherapy for primary and oligometastatic lung tumors: a single institutional study with almost uniform dose with different five treatment schedules.

BackgroundTo evaluate clinical outcomes of stereotactic body radiotherapy (SBRT) for localized primary and oligometastatic lung tumors by assessing efficacy and safety of 5 regimens of varying fraction size and number.MethodsOne-hundred patients with primary lung cancer (n = 69) or oligometastatic lung tumors (n = 31), who underwent SBRT between May 2003 and August 2010, were included. The median age was 75 years (range, 45–88). Of them, 98 were judged to have medically inoperable disease, predominantly due to chronic illness or advanced age. SBRT was performed using 3 coplanar and 3 non-coplanar fixed beams with a standard linear accelerator. Fraction sizes were escalated by 1 Gy, and number of fractions given was decreased by 1 for every 20 included patients. Total target doses were between 50 and 56 Gy, administered as 5–9 fractions. The prescribed dose was defined at the isocenter, and median overall treatment duration was 10 days (range, 5–22).ResultsThe median follow-up was 51.1 months for survivors. The 3-year local recurrence rates for primary lung cancer and oligometastasis was 6 % and 3 %, respectively. The 3-year local recurrence rates for tumor sizes ≤3 cm and >3 cm were 3 % and 14 %, respectively (p = 0.124). Additionally, other factors (fraction size, total target dose, and BED10) were not significant predictors of local control. Radiation pneumonia (≥ grade 2) was observed in 2 patients. Radiation-induced rib fractures were observed in 22 patients. Other late adverse events of greater than grade 2 were not observed.ConclusionWithin this dataset, we did not observe a dose response in BED10 values between 86.4 and 102.6 Gy. SBRT with doses between 50 and 56 Gy, administered over 5–9 fractions achieved acceptable tumor control without severe complications.

SU‐E‐T‐416: Experimental Evaluation of a Commercial GPU‐Based Monte Carlo Dose Calculation Algorithm

Purpose:A new commercial GPU‐based Monte Carlo dose calculation algorithm (GPUMCD) developed by the vendor Elekta™ to be used in the Monaco Treatment Planning System (TPS) is capable of modeling dose for both a standard linear accelerator and for an Elekta MRI‐Linear accelerator (modeling magnetic field effects). We are evaluating this algorithm in two parts: commissioning the algorithm for an Elekta Agility linear accelerator (the focus of this work) and evaluating the algorithm's ability to model magnetic field effects for an MRI‐linear accelerator.Methods:A beam model was developed in the Monaco TPS (v.5.09.06) using the commissioned beam data for a 6MV Agility linac. A heterogeneous phantom representing tumor‐in‐lung, lung, bone‐in‐tissue, and prosthetic was designed/built. Dose calculations in Monaco were done using the current clinical algorithm (XVMC) and the new GPUMCD algorithm (1 mm3 voxel size, 0.5% statistical uncertainty) and in the Pinnacle TPS using the collapsed cone convolution (CCC) algorithm. These were compared with the measured doses using an ionization chamber (A1SL) and Gafchromic EBT3 films for 2×2 cm2, 5×5 cm2, and 10×10 cm2 field sizes.Results:The calculated central axis percentage depth doses (PDDs) in homogeneous solid water were within 2% compared to measurements for XVMC and GPUMCD. For tumor‐in‐lung and lung phantoms, doses calculated by all of the algorithms were within the experimental uncertainty of the measurements (±2% in the homogeneous phantom and ±3% for the tumor‐in‐lung or lung phantoms), except for 2×2 cm2 field size where only the CCC algorithm differs from film by 5% in the lung region. The analysis for bone‐in‐tissue and the prosthetic phantoms are ongoing.Conclusion:The new GPUMCD algorithm calculated dose comparable to both the XVMC algorithm and to measurements in both a homogeneous solid water medium and the heterogeneous phantom representing lung or tumor‐in‐lung for 2×2 cm2‐10×10 cm2 field sizes.Funding support was obtained from Elekta.

TH-AB-303-09: Gated Prostate Radiotherapy: Accuracy and Dosimetric Results From First Clinical Study with Kilovoltage Intrafraction Monitoring

Purpose: The purpose of this study is to investigate the localisation accuracy and dosimetric impact of a new real-time IGRT system, kilovoltage intrafraction monitoring (KIM), in a gated prostate cancer radiotherapy clinical trial. Methods: KIM uses kV fluoroscopy to monitor, in real-time, the 3D position of radio-opaque markers implanted into the prostate target. The real-time target position is used to guide the treatment: if the prostate moves outside the tolerance (displacement exceeding 3mm for 5 seconds) the beam is paused and the patient is repositioned. Localisation accuracy is calculated offline by comparing using triangulation when markers are visible in both kV and MV images (ground truth) with the KIM determined positions. Dosimetric accuracy is calculated for fractions with gating events by comparing reconstructed delivered dose (with or without simulated gating events) against planned doses. Results: Seven patients have been recruited to the KIM study, with 4 patients completed treatment. Localisation accuracy of KIM from the 126 fractions to date is 0.1±0.5, 0.3±0.3, and −0.5±0.4 mm in the LR, SI and AP directions, respectively. Prostate gating and couch shift correction was applied in 15 fractions, which reduced the mean prostate position error from 3.9±1.4 mm (simulated no gating) to 2.0±1.2 mm. Delivered target doses (PTV D95%) were closer to planned with gating −1.4% compared to simulated no gating 3.2%. Delivered OAR doses were also closer to planned with gating compared to simulated no gating, mean (range) differences were respectively: rectum V65% −3.5 (−9.4, 5.7) & −5.0 (−11.2,18.7); and bladder V65% 3.2 (−0.9, 25.3) & 4.7 (−1.0, 30.5). Conclusion: KIM gating for prostate radiotherapy has been clinically implemented with sub-millimeter accuracy and improved agreement between the planned and delivered dose distributions. The KIM technology has wide-scale applicability as it is implemented on a standard linear accelerator with little modification. Varian Collaborative Research Agreement; Cancer Australia; NHMRC Australia

Dosimetric feasibility of magnetic resonance imaging‐guided tri-cobalt 60 preoperative intensity modulated radiation therapy for soft tissue sarcomas of the extremity

PurposeThe purpose of this study was to investigate the dosimetric differences of delivering preoperative intensity modulated radiation therapy (IMRT) to patients with soft tissue sarcomas of the extremity (ESTS) with a teletherapy system equipped with 3 rotating 60Co sources and a built-in magnetic resonance imaging and with standard linear accelerator (LINAC)‐based IMRT. Methods and materialsThe primary study population consisted of 9 patients treated with preoperative radiation for ESTS between 2008 and 2014 with LINAC-based static field IMRT. LINAC plans were designed to deliver 50 Gy in 25 fractions to 95% of the planning target volume (PTV). Tri-60Co system IMRT plans were designed with ViewRay system software. ResultsTri-60Co‐based IMRT plans achieved equivalent target coverage and dosimetry for organs at risk (long bone, skin, and skin corridor) compared with LINAC-based IMRT plans. The maximum and minimum PTV doses, heterogeneity indices, and ratio of the dose to 50% of the volume were equivalent for both planning systems. One LINAC plan violated the maximum bone dose constraint, whereas none of the tri-60Co plans did. ConclusionsUsing a tri-60Co system, we were able to achieve equivalent dosimetry to the PTV and organs at risk for patients with ESTS compared with LINAC-based IMRT plans. The tri-60Co system may be advantageous over current treatment platforms by allowing PTV reduction and by elimination of the additional radiation dose associated with daily image guidance, but this needs to be evaluated prospectively.

Prospective study on dosimetric comparison of helical tomotherapy and 3DCRT for craniospinal irradiation – A single institution experience

AimThis prospective study aims to assess feasibility of helical tomotherapy (HT) for craniospinal irradiation (CSI) and perform dosimetric comparison of treatment plans for both HT and 3D conformal radiotherapy (3DCRT). BackgroundCSI is a challenging procedure. Large PTV size requires field matching due to technical limitations of standard linear accelerators, which cannot irradiate such volumes as a single field. HT could help to avoid these limitations as irradiation of long fields is possible without field matching. Materials and methodsThree adults were enrolled from 2009 to 2010. All patients received radiochemotherapy. Treatment plans in prone position for 3DCRT and in supine position for HT were generated. The superior plan was used for patients’ irradiation. Plans were compared with the application of DVH, Dx parameters – where x represents a percentage of the structure volume receiving a normalized dose and homogeneity index (HI). ResultsAll patients received HT irradiation. The treatment was well tolerated. The HT plans resulted in a better dose coverage and uniformity in the PTV: HI were 5.4, 7.8, 6.8 for HT vs. 10.3, 6.6, 10.4 for 3DCRT. For most organs at risk (OARs), the D(V80) was higher for HT than for 3DCRT, whereas D(V5) was lower for HT. ConclusionsHT is feasible for CSI, and in comparison with 3DCRT it improves PTV coverage. HT reduces high dose volumes of OARs, but larger volumes of normal tissue receive low radiation dose. HT requires further study to establish correlations between dosimetrical findings and clinical outcomes, especially with regard to late sequelae of treatment.