Beam Portals Research Articles

Moment Capacity of Bolted-Side Plate for the Eaves Joint of Nested Tapered Box Beam Portal Frame

The joints of cold-formed steel (CFS) portal frames with box sections are primarily formed either through stiffened welded connections or by bolted end plates, which typically involve costly full penetration butt welds. In New Zealand, CFS portal frames with nested tapered box beam (NTBB) sections are popular, and the joints of these portal frames are also formed through a bolted end plate system. In literature, it was found that welds in these joints are prone to cracking in the heat-affected zone (HAZ) under bearing opening moments (tension). Therefore, this paper focuses on proposing an alternative connection system (bolted-side plate) for eaves joints without any weld, aiming to reduce the overall cost of construction and increase construction speed. The performance (strength and stiffness) of these joints is mainly dependent on the geometric details of the bolted-side plate, which is limited in Australia/New Zealand standards (AS/NZ 4600). Herein, a non-linear finite element (FE) model was developed for the NTBB portal frame and validated against the experimental test results from the literature. Firstly, the validated FE model was used to investigate the feasibility of using bolted-side plates for NTBB portal frames and found that the bolted-side plate can increase the ultimate capacity of NTBB portal frame by 9%. A parametric study was then conducted by considering different geometric details of the bolted-side plates under both closing (gravity) and opening (wind uplift) moments. Finally, unified design equations for the moment capacity of bolted-side plates were proposed based on the results of the parametric study, and their accuracy was assessed through reliability analysis.

External beam patient dose verification based on the integral quality monitor (IQM®) output signals

Background: The Integral Quality Monitor (IQM®) can essentially measure the integral fluence through a segment and provide real-time information about the accuracy of radiation delivery based on comparisons of measured segment signals and pre-calculated reference values. However, the present IQM chamber cannot calculate the dose in the patient. Aim: This study aims to make use of IQM field output signals to calculate the number of monitor units (MUs) delivered through an arbitrary treatment field in order to convert Monte Carlo (MC)-generated dose distributions in a patient model into absolute dose. Methods: XiO and Monaco treatment planning systems (TPSs) were used to define treatment beam portals for cervix and esophagus conformal radiotherapy as well as prostate intensity-modulated radiotherapy for the translation of patient and beam setup information from DICOM to DOSXYZnrc. The planned beams were simulated in a patient model built from actual patient CT images and each simulated integral field/segment was weighted with its MUs before summation to get the total dose in the plan. The segment beam weights (MUs) were calculated as the ratio of the open-field IQM measured signal and the calculated signal per MU extracted from chamber sensitivity maps. These are the actual MUs delivered not just MUs set. The beam weighting method was evaluated by comparing weighted MC doses with original planned doses using profile and isodose comparisons, dose difference maps, γ analysis and dose-volume histogram (DVH) data. Results: γ pass rates of up to 98% were found, except for the esophagus plan where the γ pass rate was below 45%. DVH comparisons showed good agreement for most organs, with the largest differences observed in low-density lung. However, these discrepancies can result from differences in dose calculation algorithms or differences in MUs used for dose weighting planned by the TPS and MUs calculated using IQM field output signals. To test this, a 4-field box DOSXYZnrc MC simulation weighted with planned (XiO) MUs was compared with the same simulation weighted with IQM-based MUs. Dose differences of up to 5% were found on the isocentre slice. For XiO versus MC, up to 7% dose differences were found, indicating additional error due to limitations of XiO’s superposition algorithm. Dose differences between MC Monaco and MC EGSnrc were less than 3%. Conclusions: The most valuable comparison was MC versus MC as it eliminated algorithm discrepancies and evaluated dose differences precisely according to beam weighting. For XiO TPS, care must be taken as dose differences may also arise due to limitations in XiO’s planning software, not merely due to differences in MUs. Overall, the IQM was successfully used to compute beam dose weights to accurately reconstruct the patient dose using unweighted MC beams. Our technique can be used for pre-treatment QA provided each segment output is known and an accurate linac source model is available.

O9. Dosimetry effects caused by bilateral and unilateral hip prostheses: A Monte Carlo study in megavoltage photon radiotherapy

Introduction Hip prostheses (HPs) are used in hip augmentation to replace diseased hip joints. However, high-density and high-atomic-number inserts may cause dose perturbations in the target volume and in tissue-HP interface regions. This study evaluates the dosimetric effect of various HPs during prostate radiotherapy using Monte Carlo (MC) simulations. Materials and Methods BEAMnrc and DOSXYZnrc MC user-codes were respectively used to model an Elekta Precise linac head and to calculate absorbed dose distributions in a CT-based phantom. Unilateral and bilateral HP contours were delineated in the pelvis CT dataset using MCSHOW and then converted into stainless steel (SS316L), titanium (Ti6Al4V) and ultra-high-molecular-weight polyethylene (UHMWPE) using an IDL code. Four, five and six conformal photon field plans of 6, 10, 15 and 20 MV were used. Results The dose within and beyond the metallic inhomogeneity drops significantly due to beam attenuation. For bilateral prostheses, the maximum isocenter dose reduction was 23%, 17% and <1% for SS316L, Ti6Al4V and UHMWPE, respectively. For unilateral HP, the respective dose reductions were 19%, 12% and <1%. The 6-field plan produced the best target coverage and least dose perturbation. Up to 38% dose enhancement was found at the bone-steel proximal interface due to radiation backscattered from the metal surface. Backscatter perturbation was more pronounced than forward scatter. Conclusions Our software enabled the addition of HPs in the CT dataset of a patient without HPs, allowing simulations to be performed without metal artefacts. Plans with more oblique beam portals can compensate for dose attenuated in fields passing through HPs. Shadowing and interface effects are density-dependent and greatest for SS316L. The attenuation effect of metallic HPs can be minimized (to below 5%) by using a 6-field plan at high energies. However, non-coplanar beams may be necessary to avoid HPs whilst giving sufficient target dose. Hip prostheses (HPs) are used in hip augmentation to replace diseased hip joints. However, high-density and high-atomic-number inserts may cause dose perturbations in the target volume and in tissue-HP interface regions. This study evaluates the dosimetric effect of various HPs during prostate radiotherapy using Monte Carlo (MC) simulations. BEAMnrc and DOSXYZnrc MC user-codes were respectively used to model an Elekta Precise linac head and to calculate absorbed dose distributions in a CT-based phantom. Unilateral and bilateral HP contours were delineated in the pelvis CT dataset using MCSHOW and then converted into stainless steel (SS316L), titanium (Ti6Al4V) and ultra-high-molecular-weight polyethylene (UHMWPE) using an IDL code. Four, five and six conformal photon field plans of 6, 10, 15 and 20 MV were used. The dose within and beyond the metallic inhomogeneity drops significantly due to beam attenuation. For bilateral prostheses, the maximum isocenter dose reduction was 23%, 17% and <1% for SS316L, Ti6Al4V and UHMWPE, respectively. For unilateral HP, the respective dose reductions were 19%, 12% and <1%. The 6-field plan produced the best target coverage and least dose perturbation. Up to 38% dose enhancement was found at the bone-steel proximal interface due to radiation backscattered from the metal surface. Backscatter perturbation was more pronounced than forward scatter. Our software enabled the addition of HPs in the CT dataset of a patient without HPs, allowing simulations to be performed without metal artefacts. Plans with more oblique beam portals can compensate for dose attenuated in fields passing through HPs. Shadowing and interface effects are density-dependent and greatest for SS316L. The attenuation effect of metallic HPs can be minimized (to below 5%) by using a 6-field plan at high energies. However, non-coplanar beams may be necessary to avoid HPs whilst giving sufficient target dose.

SU‐E‐T‐137: Attenuation of Carbon Fiber IGRT Couch Top for SBRT By Using a MapCheck 2 and Ion Chamber

Purpose:It is common to use posterior oblique beams in intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT) for regular and stereotactic body radiotherapy (SBRT). Beam attenuation by the treatment couch is not negligible when the couch is in the beam portal. In this study, we have correlated relative dose versus beam angle through a MapCheck 2TM diode array and an ionization chamber measurement.Methods:A Varian TrueBeam linear accelerator equipped with the image‐guided radiation therapy (IGRT) carbon fiber couch was used for delivery of radiation with different photon energies of 6MV, 15MV and 6MV FFF. A MapCHECK 2TM diode array with a MapPhan was employed for the measurement at three field sizes (3 by 3, 5 by 5, and 10 by 10 cm). The independent measurement was performed with an ionization chamber placed at the center of an acrylic cylindrical phantom at the isocenter. The measured data with the MapCHECK 2TM diode array were averaged over the central 2×2 cm diodes for each measurement. The couch attenuation was deduced from the angular dependence with and without the couch involvement.Results:For the MapCHECK 2TM with MapPhan used only the obvious attenuation was observed at the beam angle between 5 and 10 degree from the angular dependence using a 6MV photon beam. The similar Result was obtained for the MapCHECK 2TM /MapPhan with the couch involvement. The couch attenuation was then deduced from the difference of these two sets of data measurements. Maximum couch attenuation was found up to 4.2% which was verified with the ion chamber measurement.Conclusion:A maximum attenuation of 4.2% was found for the carbon fiber couch top using the MapCHECK 2TM/MapPhan, which is consistent with the ion chamber measurement.

Analysis of low dose level volumes in intensity modulated radiotherapy and 3-D conformal radiotherapy

Purpose: To analyze the low dose volume regions in four facets: Analysis 1 : low dose volume regions were compared between 3-dimensional conformal radiotherapy (3DCRT) and intensity modulated radiotherapy (IMRT) plans for each case; Analysis 2 : the effect on low dose volume in 3DCRT as the number of fields are increased; Analysis 3 : the same effect in IMRT, and Analysis 4: above two analysis were inter-compared between the two modalities. Methods : For this work 18 patients were taken for which both 3DCRT and IMRT plans with varying number of beams were planned using Anisotropic Analytical Algorithm (AAA) in Eclipse 10 Version Treatment Planning System with two to nine beams and five to nine beams, respectively. The plans were analyzed on the basis of conformity index and dose to the critical structures. Results: In Analysis 1 , 5 Gy volume region was greater for IMRT than for 3DCRT but the 10 Gy, 15 Gy and 20 Gy volume regions were smaller for IMRT plans. In Analysis 2 and 3 shows that as the number of fields increases the low dose volume regions also increases. In Analysis 4 , the effect pronounced due to increase in the beam portals is similar in 3DCRT as well as IMRT. Also it was seen that with same number of beams for both IMRT and 3DCRT, low dose volume region is higher in 3DCRT than IMRT. Conclusion : Low dose volume regions are a major concern in 3DCRT and IMRT plans with multiple beams because of its risk of secondary cancer incidence. This study concludes that by increasing number of beams low dose volume regions increases in 3DCRT and IMRT. It shows that the low dose volume regions are slightly higher in IMRT than 3DCRT, however with proper optimization the volume of tissue receiving 10 Gy, 15 Gy and 20 Gy can be reduced. ----------------------- Cite this article as : Ekambaram V, Velayudham R. Analysis of low dose level volumes in intensity modulated radiotherapy and 3-D conformal radiotherapy. Int J Cancer Ther Oncol 2014; 2 (3):02032. DOI : 10.14319/ijcto.0203.2

Clinical application of in vivo treatment delivery verification based on PET/CT imaging of positron activity induced at high energy photon therapy

The purpose of this study was to investigate in vivo verification of radiation treatment with high energy photon beams using PET/CT to image the induced positron activity. The measurements of the positron activation induced in a preoperative rectal cancer patient and a prostate cancer patient following 50 MV photon treatments are presented. A total dose of 5 and 8 Gy, respectively, were delivered to the tumors. Imaging was performed with a 64-slice PET/CT scanner for 30 min, starting 7 min after the end of the treatment. The CT volume from the PET/CT and the treatment planning CT were coregistered by matching anatomical reference points in the patient. The treatment delivery was imaged in vivo based on the distribution of the induced positron emitters produced by photonuclear reactions in tissue mapped on to the associated dose distribution of the treatment plan. The results showed that spatial distribution of induced activity in both patients agreed well with the delivered beam portals of the treatment plans in the entrance subcutaneous fat regions but less so in blood and oxygen rich soft tissues. For the preoperative rectal cancer patient however, a 2 ± (0.5) cm misalignment was observed in the cranial-caudal direction of the patient between the induced activity distribution and treatment plan, indicating a beam patient setup error. No misalignment of this kind was seen in the prostate cancer patient. However, due to a fast patient setup error in the PET/CT scanner a slight mis-position of the patient in the PET/CT was observed in all three planes, resulting in a deformed activity distribution compared to the treatment plan. The present study indicates that the induced positron emitters by high energy photon beams can be measured quite accurately using PET imaging of subcutaneous fat to allow portal verification of the delivered treatment beams. Measurement of the induced activity in the patient 7 min after receiving 5 Gy involved count rates which were about 20 times lower than that of a patient undergoing standard 18F-FDG treatment. When using a combination of short lived nuclides such as 15O (half-life: 2 min) and 11C (half-life: 20 min) with low activity it is not optimal to use clinical reconstruction protocols. Thus, it might be desirable to further optimize reconstruction parameters as well as to address hardware improvements in realizing in vivo treatment verification with PET/CT in the future. A significant improvement with regard to 15O imaging could also be expected by having the PET/CT unit located close to the radiation treatment room.

Image-guided radiation therapy for carcinoma of gallbladder: implication on margin for set-up errors

AbstractPurposeA retrospective study was undertaken to analyse set-up variations in patients being treated with post-operative radiation therapy for carcinoma of gall bladder by image-guided radiotherapy (IGRT) using cone-beam computed tomography (CBCT) scans and paired kilovoltage beam portals (kVps).Materials and methodsThree consecutive patients receiving post-operative radiation therapy for carcinoma of gall bladder were studied. A total of 32 imaging studies were performed. The immobilisation system was an all-in-one system along with a thermoplastic mask, with knees either resting on the knee rest or in a vacuum cushion. The CBCT scans and kVps were reviewed in an off-line mode. The surrogate markers used for matching during co-registration were 12th rib, coeliac trunk, vertebral bodies and canal. Individual readings were used to calculate mean shifts (m); the mean of these means (M) was calculated to arrive at the systematic error in each direction and its standard deviation (Σ) was calculated. The margins for set-up error (SM) were then calculated.ResultsThere were a total of 32 readings of which 21 were CBCTs and 11 were kVps. The mean shifts in each direction for each patient were 0·06, 0·25 and 0·15 cm in vertical, longitudinal and lateral directions, respectively. The resultant planning target volume margins calculated were 0·24, 0·9 and 0·47 cm in vertical, longitudinal and lateral directions.ConclusionsIGRT for upper abdominal malignancies using CBCT and kVps is a useful method to keep the margins for set-up error low. The use of surrogates for matching should be relevant to the target volume. Good immobilisation system helps in keeping the margins low.

Effect of tissue heterogeneity on an in vivo range verification technique for proton therapy

It was proposed recently that time-resolved dose measurements during proton therapy treatment by passively scattered beams may be used for in vivo range verification. The method was shown to work accurately in a water tank. In this paper, we further evaluated the potential of the method for more clinically relevant situations where proton beams must pass through regions with significant tissue heterogeneities. Specifically, we considered prostate treatment where the use of anterior or anterior- oblique fields was recently proposed in order to reduce rectal dose by taking advantage of the sharp distal fall-off of the Bragg peak. These beam portals pass through various parts of pubic bone and potential air cavities in the bladder and bowels. Using blocks of materials with densities equivalent to bone, air, etc, arranged in the water tank in relevant configurations, we tested the robustness of the method against range shifting and range mixing. In the former, the beam range is changed uniformly by changes in tissue density in the beam path, while in the latter, variations in tissue heterogeneities across the beam cross section causes the mixing of beam energies downstream, as often occurs when the beam travels along the interface of materials with significantly different densities. We demonstrated that in the region of interest, the method can measure water-equivalent path length with accuracy better than ±0.5 mm for pure range shifting and still reasonable accuracy for range mixing between close beam energies. In situations with range mixing between significantly different beam energies, the dose rate profiles may be simulated for verifying the beam range. We also found that the above performances can be obtained with very small amount of dose (<0.5 cGy), if silicon diodes are used as detectors. This makes the method suitable for in vivo range verification prior to each treatment delivery.

Dosimetric explanations of fatigue in head and neck radiotherapy: An analysis from the PARSPORT Phase III trial

BackgroundAn unexpected finding from the phase III parotid sparing radiotherapy trial, PARSPORT (ISRCTN48243537, CRUK/03/005), was a statistically significant increase in acute fatigue for those patients who were treated with intensity-modulated radiotherapy (IMRT) compared to standard conventional radiotherapy (CRT). One possible explanation was the difference in dose to central nervous system (CNS) structures due to differing beam portals. Using data from the trial, a dosimetric analysis of individual CNS structures was performed. MethodDosimetric and toxicity data were available for 67 patients (27 CRT, 40 IMRT). Retrospective delineation of the posterior fossa, brainstem, cerebellum, pituitary gland, pineal gland, hypothalamus, hippocampus and basal ganglia was performed. Dosimetry was reviewed using summary statistics and dose–volume atlases. ResultsA statistically significant increase in maximum and mean doses to each structure was observed for patients who received IMRT compared to those who received CRT. Both maximum and mean doses were significantly higher for the posterior fossa, brainstem and cerebellum for the 42 patients who reported acute fatigue of Grade 2 or higher (p⩽0.01) compared to the 25 who did not. Dose–volume atlases of the same structures indicated that regions representing larger volumes and higher doses to each structure were consistent with a higher incidence of acute fatigue. There was no association between the dose distribution and acute fatigue for the other structures tested. ConclusionsThe excess fatigue reported in the IMRT arm of the trial may, at least in part, be attributed to the dose distribution to the posterior fossa, cerebellum and brainstem. Future studies that modify dose delivery to these structures may allow us to test the hypothesis that radiation-induced fatigue is avoidable.

TH-E-BRA-10: The 1.5 T MRI Accelerator for MRI during Radiation Delivery: Status Report

Purpose: We have developed and built a prototype ring based accelerator around a 1.5 T MRI system for MRI guided radiation therapy. Methods: The design is a closed bore cylindrical 1.5 T MRI with a ring mounted 6 MV accelerator in the transversal mid-plane with a 7 mm non-rotating MLC without back-up collimators and flatness filter. The ring allows continuous rotation in both directions. The magnetic and RF interference between the MRI and the accelerator is overcome by active shielding and redesign of the Faraday cage respectively. The beam passage through the magnet and gradient coils is through a circumferential beam portal which is homogeneous and contains the equivalent of approximately 10 cm of aluminium. Results: After installation of the ring gantry around the MRI at zero field, radiation from the rotating accelerator was shown and arbitrary segments can be delivered. Recently the MRI is brought up to 1.5T and performance testing at 1.5 T is in progress. Radiation delivery was shown. For static gantry positions MRI can be performed, without striking image deterioration, substantiation is work in progress. Gantry rotation in both directions can be performed, but this does affect the MRI B0 field. These distortions are mapped using a standard field camera and a combination of active and passive corrections as presented by Amthor et al. (2011) seems feasible and is ongoing. Conclusion: The MRI accelerator is close to full functionality, that is, diagnostic 1.5T MRI during IMRT for delivering stereotactic precision radiation under MRI guidance. Amthor T. and Overweg J. Compensation of magnetic field perturbations for combined MRI/radiotherapy system. Eposter 526 in proc ESMRMB 2011Leipzig, Germany. Work is supported by Elekta, U.K. and Philips, Best, The Netherlands

SU-E-T-605: RapidArc Combined with DIBH Technique for Thoracic Esophageal Carcinoma: The Potential Value of Target Immobilization and Reduced Lung Density in Dose Escalation

To compare the dosimetric benefits of Rapidarc (RA) combined with deep inspiration breath-hold (DIBH) with those of other standard techniques, including free breathing (FB) during fixed-field intensity modulated radiation therapy (IMRT) and dual arc RA, in the treatment of patients with thoracic esophageal carcinoma (EC). Ten patients with EC underwent computed tomography (CT) scans under 2 respiration conditions: free-breathing (FB) and DIBH. These scans were used to generate 3-dimensional conformal treatment plans. For breath-hold scans, the patients were brought to reproducible respiration levels using active breathing control (ABC) maneuvers. Planning target volumes (PTVs) for FB plans included a 0.5 cm margin for setup plus a 1 cm margin equal to the extent of tumor motion for respiration. PTVs for DIBH plans included a 0.5 cm margin for setup error and a 0.5 cm margin for residual uncertainty in tumor position. Using a dose level of 60 Gy to the PTV, three treatment plans were generated: IMRT-FB, RA-FB and RA-ABC, and the target and normal tissue volumes were compared, as were the dosimetry parameters. On average, the DIBH technique resulted in increased lung volumes compared with FB techniques. There was no significant differences in gross tumor volume between the two breathing states (p > 0.05); but PTV and heart volume were larger for FB than for DIBH (p < 0.05). The overall CI and HI for the RA-ABC plan was slightly inferior to those of the IMRT- FB and RA-FB plans (p < 0.05 each). With DIBH, the heart was partly out of the beam portals and the average mean heart dose was reduced. Compared with conventional FB, RA combined with DIBH significantly reduced cardiac and pulmonary doses without compromising the target coverage and may reduce treatment toxicity, enabling dose escalation in future prospective studies of patients with EC.

Impact of MLC leaf width on the quality of the dose distribution in partial breast irradiation

Partial-breast irradiation (PBI) aims to limit the target volume for radiotherapy in women with early breast cancer after partial mastectomy to the region at highest risk of local recurrence, the tumor bed. Multileaf collimators are used to achieve conformal radiation beam portals required for PBI. Narrower leaf widths are generally assumed to allow more conformal shaping of beam portals around irregularly shaped target volumes. The aim was to compare 5-mm and 10-mm leaf widths for patients previously treated using PBI and assess subsequent planning target volume (PTV) coverage and organ at risk (OAR) doses for 16 patients. Several plans (5-mm leaf width or 10-mm leaf width) were generated for each patient using the original treated plan as the basis for attempts at further optimization. Alternating between different leaf widths found no significant difference in terms of overall PTV coverage and OAR doses between treatment plans. Optimization of the original treated plan allowed a small decrease in ipsilateral breast dose, which was offset by a lower PTV minimum. No significant dosimetric difference was found to support an advantage of 5-mm over 10-mm leaf width in this setting.

SU-E-T-571: EPID Assisted Dosimetric Evaluation of Treatment Planning Using Helical or 4D CT in Stereotactic Radiotherapy of Lung Cancer

Purpose: To investigate potential dosimetric discrepancy due to the choice of CT scanning protocols (helical versus 4DCT) for treatment planning in stereotactic body radiotherapy of lungcancer. Method and Materials: For each lungcancer patient treated with SBRT, both free‐breathing helical CT and 4DCT images were obtained. A conventional PTV (PTVconv) was created by applying an anisotropic margin of 5mm in the axial plane and 10mm in the longitudinal (craniocaudal) plane to the GTV outlined on the helical CT. Similarly, a PTV4D was created by applying a uniform margin of 5 mm to the internal target volume (ITV) contoured on the average intensity image set derived from the 4DCT. Both approaches are accepted by RTOG protocols (0813 and 0915). In this study, two SBRT plans were created for each patient using identical beam portals except the aperture of each beam is adjusted according to the PTVconv or PTV4D, respectively. The volume difference of both PTVs and target coverage were compared between two plans. On a daily basis, the position of the GTV was monitored real‐time for each field using an EPID in cine mode. The dose distribution was recalculated based on geometrical information obtained from the cine images. Results: It was observed that the volume of PTVconv is 16% smaller than PTV4D. Subsequently the SBRT plan using PTVconv provides suboptimal dose coverage to the tumor than PTV4D. The V100 of PTV4D decreased from 95% to 82% and D95 of ITV decreased from 47Gy to 42Gy. The cine EPIDimages have confirmed tumor excursion with respiratory motion is within the predefined ITV boundary. However if the plan is based on apertures designed using PTVconv, there could be geometrical misses resulting in target under dosage. Conclusion: It is recommended to perform 4D CT and use PTV4D for treatment planning for SBRTlungcancer.

SU‐C‐214‐01: Design and Evaluation of a Low Megavoltage Imaging Beam from a Prototype Waveguide

Purpose: To design and evaluate an optimised radiotherapy imaging beam, based on a prototype Elekta waveguide, with a novel coupling device (rotovane) allowing for a wide, continuously variable energy range. Methods: A waveguide test piece consisting of a buncher, rotovane and short relativistic section was used to investigate the performance of the novel continuously variable coupling device, and in particular investigate imaging at low megavoltage energies. An optimised imaging system utilising the lowest electron energy of the waveguide (1.4 MeV) was designed using BEAMnrc and implemented experimentally. Characterisation of the imaging beam was conducted experimentally and via Monte Carlo simulations. Cone beam computed tomography images (CBCT) were acquired using a Caesium Iodide imaging panel of humanoid anthropomorphic phantoms and a Catphan phantom. The contrast to noise ratio was assessed for CBCT and compared to kilo‐voltage and megavoltage imaging systems. Results: The optimised imaging beam target assembly consisted of an electron window, 5 mm carbon electron absorber and 2.5 mm of Aluminium filtration. The x‐ray beam had an average photon energy of 220 keV and a half value layer of 5.9 mm of Copper. Images with the same contrast to noise ratio as a 100 kVp CBCT system (XVI, Elekta) were obtained in doses less than 2 cGy; this is 11x higher than XVI but 150x lower than a megavoltage imaging system. Qualitatively, kilo‐voltage equivalent images of head and pelvis phantoms were obtained in doses between 1 and 8 cGy. Conclusions: The prototype waveguide section was capable of producing electron energies from 1.4 to 9 MeV, suitable for imaging and therapy. The waveguide technology has the potential for producing near kilo‐voltage equivalent images in acceptable dose from the therapy beam portal without the need for add‐on x‐ray systems. Additionally such a system could provide for enhanced target tracking during radiotherapy treatment.This work is supported by Elekta Ltd and The Institute of Cancer Research. Work of the ICR radiotherapy physics group is partially supported by Cancer Research UK under programme grant C46/A3970

Characterization of dose impact on IMRT and VMAT from couch attenuation for two Varian couches

In intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT), the use of posterior oblique beams has become common. Beam attenuation by the treatment couch is not negligible when the couch is in the beam portal. In this study, we established the relationship of relative dose vs. beam angle for two Varian 21EX linacs, one equipped with the Exact couch (standard couch) with sliding side support rails, and the other equipped with the Exact image‐guided radiation therapy (IGRT) carbon fiber couch. Measurements were performed using an ion chamber placed at the center of an acrylic cylindrical phantom positioned at the linac isocenter for 6 MV and 18 MV photon beams. Measurements were performed at three different field sizes (3×3,5×5, and 10×10 cm2, and were repeated with the phantom positioned at different longitudinal locations on the couches. To evaluate beam attenuation by the standard couch in a clinical setting, two test IMRT plans and two test VMAT plans on the standard couch were delivered. The plans were generated with the sliding rails at the “in” position and delivered with the rails at both “in” and “out” positions. The dose difference to the ion chamber was determined. For oblique fields with 6 MV photons, the standard couch attenuated the radiation beam by up to 26.8%, while the carbon fiber IGRT couch attenuated the beam by up to 4.1%. In the clinical evaluation, the highest dose difference between rails set at the “in” and “out” positions was 2.6% in the IMRT case and 2.1% in the VMAT case. The magnitude of potential dose difference has been quantified and could be used for a quick estimation of dose difference due to couch attenuation in IMRT and VMAT.PACS number: 87.56.‐v

Cardiac and pulmonary dose reduction for tangentially irradiated breast cancer, utilizing deep inspiration breath-hold with audio-visual guidance, without compromising target coverage

Background and purpose. Cardiac disease and pulmonary complications are documented risk factors in tangential breast irradiation. Respiratory gating radiotherapy provides a possibility to substantially reduce cardiopulmonary doses. This CT planning study quantifies the reduction of radiation doses to the heart and lung, using deep inspiration breath-hold (DIBH). Patients and methods. Seventeen patients with early breast cancer, referred for adjuvant radiotherapy, were included. For each patient two CT scans were acquired; the first during free breathing (FB) and the second during DIBH. The scans were monitored by the Varian RPM™ respiratory gating system. Audio coaching and visual feedback (audio-visual guidance) were used. The treatment planning of the two CT studies was performed with conformal tangential fields, focusing on good coverage (V95>98%) of the planning target volume (PTV). Dose-volume histograms were calculated and compared. Doses to the heart, left anterior descending (LAD) coronary artery, ipsilateral lung and the contralateral breast were assessed. Results. Compared to FB, the DIBH-plans obtained lower cardiac and pulmonary doses, with equal coverage of PTV. The average mean heart dose was reduced from 3.7 to 1.7 Gy and the number of patients with >5% heart volume receiving 25 Gy or more was reduced from four to one of the 17 patients. With DIBH the heart was completely out of the beam portals for ten patients, with FB this could not be achieved for any of the 17 patients. The average mean dose to the LAD coronary artery was reduced from 18.1 to 6.4 Gy. The average ipsilateral lung volume receiving more than 20 Gy was reduced from 12.2 to 10.0%. Conclusion. Respiratory gating with DIBH, utilizing audio-visual guidance, reduces cardiac and pulmonary doses for tangentially treated left sided breast cancer patients without compromising the target coverage.

MicroRT—Small animal conformal irradiator

A novel small animal conformal radiation therapy system has been designed and prototyped: MicroRT. The microRT system integrates multimodality imaging, radiation treatment planning, and conformal radiation therapy that utilizes a clinical 192Ir isotope high dose rate source as the radiation source (teletherapy). A multiparameter dose calculation algorithm based on Monte Carlo dose distribution simulations is used to efficiently and accurately calculate doses for treatment planning purposes. A series of precisely machined tungsten collimators mounted onto a cylindrical collimator assembly is used to provide the radiation beam portals. The current design allows a source-to-target distance range of 1-8 cm at four beam angles: 0 degrees (beam oriented down), 90 degrees, 180 degrees, and 270 degrees. The animal is anesthetized and placed in an immobilization device with built-in fiducial markers and scanned using a computed tomography, magnetic resonance, or positron emission tomography scanner prior to irradiation. Treatment plans using up to four beam orientations are created utilizing a custom treatment planning system-microRTP. A three-axis computer-controlled stage that supports and accurately positions the animals is programmed to place the animal relative to the radiation beams according to the microRTP plan. The microRT system positioning accuracy was found to be submillimeter. The radiation source is guided through one of four catheter channels and placed in line with the tungsten collimators to deliver the conformal radiation treatment. The microRT hardware specifications, the accuracy of the treatment planning and positioning systems, and some typical procedures for radiobiological experiments that can be performed with the microRT device are presented.

Intra- and Interfractional Variations for Prone Breast Irradiation: An Indication for Image-Guided Radiotherapy

Intra- and interfractional errors for breast cancer patients undergoing breast irradiation in the prone position were analyzed. To assess intrafractional error resulting from respiratory motion, four-dimensional computed tomography scans were acquired for 3 prone and 3 supine patients, and the respiratory motion was compared for the two positions. To assess the interfractional error caused by daily set-up variations, daily electronic portal images of one of the treatment beams were taken for 15 prone-positioned patients. Portal images were then overlaid with images from the planning system that included the breast contour and the isocenter, treatment beam portal, and isocenter. The shift between the planned and actual isocenter was recorded for each portal image, and descriptive statistics were collected for each patient. The margins were calculated using the 2Sigma + 0.7sigma recipe, as well as 95% confidence interval based on the pooled standard deviation of the datasets. Respiratory motion of the chest wall is drastically reduced from 2.3 +/- 0.9 mm in supine position to -0.1 +/- 0.4 mm in prone position. The daily set-up errors vary in magnitude from 0.0 cm to 1.65 cm and are patient dependent. The margins were defined by considering only the standard deviation to be 1.1 cm, and 2.0 cm when the systematic errors were considered using the 2Sigma + 0.7sigma recipe. Prone positioning of patients for breast irradiation significantly reduces the uncertainty introduced by intrafractional respiratory motion. The presence of large systematic error in the interfractional variations necessitates a large clinical target volume-to-planning target volume margin and indicates the importance of image guidance for partial breast irradiation in the prone position, particularly using imaging modality capable of identifying the lumpectomy cavity.

7001 ORAL Long-term outcome of 403 patients treated with ruthenium brachytherapy for choroidal melanoma

Background and Purpose: Previously, we reported on development of an optically guided system for 3D conformal intracranial radiotherapy using multiple noncoplanar fixed fields. In this paper we report on the extension of our system for stereotactic fractionated radiotherapy to include intensity modulated static ports.Methods and Materials: A 3D treatment plan with maximum beam separation is developed in the stereotactic space established by an optically guided system. Gantry angles are chosen such that each beam has a unique entrance and exit pathway, avoids the critical structures, and has a minimal beam's eye view projection. Once, a satisfactory treatment plan is found using this geometric approach an inverse treatment plan is developed using the beam portals established previously. The purpose of adding inverse planing is two fold, on the one hand it allows further reduction of margins around the PTV, while on the other hand it affords the possibility of conformal avoidance of critical structures that are close to or abut the PTV.Results: The use of the optically guided system in conjunction with intensity modulated noncoplanar radiotherapy treatment planning using fixed fields allows the generation of highly conformal treatment plans that exhibit smaller 90, 70, and 50% of prescription dose isodose volumes, improved PITV ratios, comparable or improved EUD, smaller NTDmean for the critical structures, and an inhomogeneity index that is within generally accepted limits.Conclusion: Because optically guided technology improves the accuracy of patient localization relative to the linac isocenter and allows real-time monitoring of patient position, the planning target volume needs to be corrected only for the limitations of image resolution. Intensity modulated static beam radiotherapy planning then provides the user the ability to further reduce margins on the PTV and to conform very closely to this smaller target volume, and enhances the normal tissue sparing, and high degree of conformality possible with 3D conformal radiotherapy. In addition, since optically guided technology affords improved patient localization and online monitoring of patient position during treatment delivery it allows for safe and efficient delivery of intensity modulated radiotherapy.

TU‐D‐AUD‐08: Monte Carlo Kernel‐Based Convolution‐Superposition Dose Calculation for Intensity‐Modulated Arc Therapy (IMAT)

Purpose: Dose calculations for intensity‐modulated arc therapy (IMAT) have traditionally been performed by approximating continuous arcs with multiple static beams. Not only that the calculated dose may not truthfully represent the delivered dose, but also the large number of beams used to approximate an arc requires very long calculation time. We propose to use our homegrown Monte Carlo (MC)/superposition dose calculation algorithm to compute doses for rotational delivery. Method and Materials: We first generate static IMRT plans with Pinnacle3 P3 IMRT module utilizing 36 equi‐spaced beams. The resulting intensity maps are inputted to the continuous intensity map optimization (CIMO) IMAT leaf sequencer. CIMO outputs deliverable arcs, each represented by aperture shapes spaced at 10‐degree intervals. The apertures are then interpolated at 1‐degree intervals. Dose calculations for both 36‐field plan and 351‐field plan (which closely represents the actual delivery) are performed using our homegrown MC‐based dose calculator. The time needed for calculating dose for the two plans are compared and the differences in dose distributions are also examined. Results and Conclusion: Four IMAT plans, a prostate, a head and neck, an orbit, and a brain, were selected for the study. Using history‐based sampling, the dose computation time only increased slightly for the 351‐beam plans. Each case took approximately 30 minutes to compute on a single CPU running Mac OSX. No significant differences in plan quality between the 36‐beam and the 351‐beam plans were observed. However, finger‐like artifacts at the lower isodose levels became evident when using 36 beams to approximate an arc. The dose calculation time using our MC‐based dose calculator is largely independent of the number of beam portals. For the cases studied, using 36 beams to approximate an arc delivery is sufficient, but more accurate calculation with finer angular spacing is achievable without a significant increase in computation time.