bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐J‐60: Daily Cone Beam CT (CBCT) Reduces the Setup Margins Needed for External Beam Radiotherapy of Brain Tumors
Authors: Yu N , Gajdos S , Koyfman S , Chao S , Suh J , Xia P
Published: August 2010

Purpose: To identify the margin needed to account for setup error in brain tumor radiotherapy based on CBCT data, and quantify the reduction of the required margin with the use of daily CBCT. Methods and Materials: Patients who underwent EBRT for brain tumors and had daily CBCT imaging were included on this IRB approved study. All patients were immobilized with thermoplastic masks. Prior to each treatment, a CBCT was acquired and the images were co‐registered with the planning CT. Positional errors in the three orthogonal directions were recorded. Using Van‐Herk's formula, we modeled the necessary setup margin, as a component of the PTV expansion, for all patients. Assuming the use of CBCT corrected for systematic error, we then used data from verification CBCTs taken after the initial shifts were corrected to quantify the margin needed to account for residual random error, and compared these results. Results: A total of 2008 CBCTs, in 76 patients were included in this analysis. The mean interfraction shifts for all treatments were 1.2 ± 1.5, 1.5 ± 1.5 and 1.7 ± 1.3 mm in the medial‐lateral, superoinferior, and anteroposterior dimensions, respectively. Without the use of daily CBCT, the necessary setup margins would be 4.0, 4.7, and 5.1 mm in the three dimensions, respectively. Verification CBCTs were acquired for 65 fractions in 21 patients. The standard deviations of the residual errors were 1.1, 1.0 and 0.8 mm, in the three dimensions, respectively. The margin required to account for the residual random error was thereby reduced to 0.8, 0.7 and 0.6 mm, respectively. Conclusions: Image‐guided radiotherapy using daily CBCT for brain tumors significantly reduces the margins needed to account for setup error.
SU‐GG‐J‐63: Evaluation of Treatment Shifts for Prostate and Rectum Patients Undergoing CBCT Guided IGRT Using KV CBCT
Authors: Tomic N , Vuong T , Niazi T , Bahoric B , Liang L , DeBlois F , Seuntjens J , Devic S
Published: August 2010

Purpose: We present results of a pilot study established to investigate protocol to access inter‐fraction setup errors for patients with rectal and prostate cancer receiving CBCT based IGRT. We also report on radiation skin doses to patients from a typical CBCT acquisition for daily patient positioning using previously established 2D reference radiochromic film dosimetry protocol. Method and Materials: Patients receiving radical course of IGRT based treatment using CBCT were eligible. Pre‐treatment 3D imaging of 20 patients in treatment position was acquired using kV CBCT on‐board imaging (OBI) system (v. 1.4, Varian Medical Systems, Palo Alto). Scans were preformed using Pelvis CBCT protocol. Reconstructed CBCT images obtained prior to treatment were co‐registered with planning CT data set using bony landmarks for rectal cancer patients and prostate soft tissue visibility and prostate contours for prostate cancer patients. For dose measurements we used 2D dosimetry protocol that employs XR‐QA radiochromic film. Results: Our measurements show that average displacements over investigated rectal cancer patient population was 3.68±1.51 mm in vertical, 3.18±2.0 mm in lateral and 1.59±1.34 mm in longitudinal direction with the maximum observed shift in lateral direction of 11 mm. Average displacements over prostate cancer population showed 2.29±1.69 mm in lateral, 3.36±2.06 mm in vertical, and 3.36±2.06 mm in longitudinal direction with the maximum shift observed in vertical direction of 18 mm. Measured anterior dose during CBCT amounts to 1.60 cGy in larger and up to 3.17 cGy in smaller patients, while the lateral dose was ranging from 1.63 cGy in larger and up to 2.68 cGy in smaller patients. Conclusion: Our investigation showed that 3D conformai therapy with reduced margin in all 3 directions can be used for prostate and rectal cancer patients undergoing daily CBCT based IGRT. However, this benefit must be weighted against dose delivered to patients during CBCT.
SU‐GG‐J‐64: Cone Beam Computed Tomography Dose Measurements. Head & Neck Collimators in Elekta Synergy®
Authors: Juan‐Senabre X , López‐Tarjuelo J , Santos‐Serra A , Calzada‐Feliu S , Sanfeliu‐Lucas E
Published: August 2010

Purpose: Image Guided Radiotherapy employs kV and MV images to improve treatment quality. On board cone beam computed tomography (CBCT) is widely used. Several authors have reported absorbed dose measurements using different approaches which could seem unclear if compared with diagnostic radiology literature. Our objectives are to establish a coherent set of measurements which could be used as a reference along linac life and to present our data for head and neck collimators. Method and Materials: An Elekta Synergy equipped with 15×15, S10 and S20 collimators for CBCT has been used. Measurements have been performed with a Farmer chamber in a PMMA CTDI head phantom placed at the isocenter. Since diagnostic radiology equipment is calibrated in air kerma (Kair), this has been the chosen magnitude. For that reason, ionization chamber has been calibrated against a multipurpose Barracuda diagnostic detector. After that, we have measured central air kerma (Kair,c) and peripheral air kerma (Kair,p) and derived a cone beam dose index as CBDI=(l/3) Kair,c +(2/3) Kair,p. Head & neck CBCT acquisition protocol comprised an arc of 210°, 100 kV and 36.1 mAs. Results: CBDI for 15×15 was 1.158 ± 0.010 mGy. CBDI for S10 was 1.268 ± 0.011 mGy. CBDI for S20 was 1.255 ± 0.011 mGy. Conclusion: It is possible to find a reproducible set up to measure dose for CBCT. Although inconsistencies could be found in the recent IGRT literature, it should be considered that CT regulations stated dose measurements in a PMMA phantom. Besides, diagnostic radiology measurements should be performed in Kair units so CBDI is an index built upon air kerma measured inside PMMA.
SU‐GG‐J‐68: Significant Dosimetric Impact of Variable FSDs during Prostate IMRT: The Role of CBCT
Authors: Booth J , Eade T , Forde E
Published: August 2010

Purpose: The modulated beam fluence used for IMRT treatments requires a volumetric approach to check for variations in daily patient contours. Cone Beam CT (CBCT) can be used to compare the body contour at the time of treatment with the planning CT. This study investigates the random and systematic changes in FSDs detected with CBCT off central axis. Method and Materials: FSD structures +/− 1 and 2cm from the body were created. CBCTs were scheduled for fractions 1–3 and then weekly. Each CBCT was reviewed and a variation of > 1 cm in FSDs required a repeat CBCT the following day. For three consecutive variations > 1cm, patients were resimulated and the dosimetry evaluated. Results: 19 patients with 198 CBCT images over 640 treatment fractions were analysed. Overall 68% of CBCT body contours were 0–1 cm, 28% 1–2 cm and 4% > 2 cm. The deviations occurred most commonly in the postero‐lateral direction, and generally coincided with a sharp increase in fluence along the rectal/prostate interface. Six patients (32%) did not require repeat CBCT. In the remaining 13 patients, 20 further CBCTs were performed and 4 patients (21%) required re‐simulation. Three of these four patients had a decrease in their FSDs resulting in a decrease in D95 coverage of the PTV of between 4 and 18% per fraction. Of the 198 CBCT images, there was only one variation in FSDs >1cm on the central axis. Conclusion: A CBCT obtained during a patient's treatment enables a simple, off‐axis method to estimate FSD variations. In our cohort 21% of patients were found to have a significant change in their body contour with the potential to impact on their delivered dose. Manual central axis FSD measurement was not be sufficient for this cohort to highlight patients with significant body contour changes.
SU‐GG‐J‐72: Study of Constant Beam Shaping Method in Tumor Tracking Radiotherapy during Respiration
Authors: Kim S , Yoon M , Lee S , Lee S , Shin D , Park S , Ahn S
Published: August 2010

Purpose: To evaluate the tumor tracking radiotherapy without consideration for the deformation of moving tumor. Method: Four dimensional computed tomography (4D CT) corresponding to every 10% phase intervals were acquired for 4 lung and 4 liver cancer patients treated in our institution and the initial treatment planning was established for the end of inhalation phase (phase 1). Results: As a simulation of deformation‐free tumor tracking radiotherapy, the beam center of initial plan was moved to the tumor center in other phases and the constant tumor shape acquired from phase 1 was assumed in all phases. The feasibility of the proposed method was analyzed based on various dosimetric tools such as equivalent uniform dose (EUD), homogeneity index (HI), coverage index (COV), etc. In photon radiation treatment, the movement‐induced dose reduction was not that critical showing 0.5%, 17.3% and 2.8% as the average variation of EUD, HI, COV, respectively. This difference become greater in proton treatment, which shows the change of 0.3%, 40.5%, 2.2% compared to initial EUD, HI, COV, respectively. Conclusion:These results suggest that proton is more sensitive than photon in tumor movement and the deformation of tumor may be disregarded in tumor tracking radiotherapy for photon treatment.
SU‐GG‐J‐75: Neutron‐Induced Electronic Failures around a High‐Energy Linear Accelerator
Authors: Kry S , Johnson J , White R , Howell R , Kudchadker R , Gillin M
Published: August 2010

Purpose: After a new in‐vault CT‐on‐rails system repeatedly malfunctioned following use of a high‐energy radiotherapy beam, we investigated the presence and impact of neutron radiation on this electronic system, as well as potential neutron shielding and neutron evaluation options. Methods: We first determined the CT scanner's failure rate as a function of the number of 18 MV monitor units (MUs) delivered. We then reexamined the failure rate with both 2.7‐cm‐thick and 7.6‐cm‐thick borated polyethylene (BPE) covering the linac head for neutron shielding. To further examine shielding options, as well as to explore which neutrons were relevant to the scanner failure, Monte Carlo was used to calculate the neutron fluence and spectrum in the bore of the CT scanner. Simulations included BPE covering the CT scanner itself as well as covering the linac head. Results: We found that the CT scanner had a 57% chance of failure after the delivery of 200 MUs. While the addition of neutron shielding to the accelerator head reduced this risk of failure, the benefit was minimal and even 7.6 cm of BPE was still associated with a 29% chance of failure after the delivery of 200 MU. This shielding benefit was achieved regardless of whether the linac head or CT scanner was shielded. Additionally, it was determined that fast neutrons were primarily responsible for the electronic failures. Conclusions: As illustrated by the CT‐on‐rails system in the current study, physicists should be aware that electronic systems may be highly sensitive to neutron radiation. Physicists should therefore monitor electronic systems that have not been evaluated for potential neutron sensitivity. This is particularly relevant as electronics are increasingly common in the therapy vault and may exhibit increased sensitivity in newer electronic systems.
SU‐GG‐J‐84: ADiver: A Respiratory Physiology Based Motion Phantom for Evaluating Radiotherapy Motion Management
Authors: Lam K , Hadley S , Balter J
Published: August 2010

Purpose: Most phantoms for evaluating breathing‐motion management execute motions from a pre‐defined function or a stored profile. We have developed a phantom (aDiver) that models human respiratory physiology so that it responds to its respiration history. This is valuable in evaluating motion management methods that modify the breathing pattern of patients. Method and Materials: aDiver is based on a servo‐motor controlled linear actuator. Software is downloaded to the micro‐controller which drives the actuator to the programmed position. We developed the software so that multiple segments of a breathing cycle can be combined to construct profiles of different amplitudes, shapes and periods. It calculates the real‐time concentration of carbon dioxide according to the executed profile. It assumes that the metabolic output rate of carbon dioxide is constant and there is perfect mixing of the different gas components. At the beginning of each inhalation, it will determine the inhalation depth based on the current concentration of carbon dioxide. aDiver accepts an input signal to hold the breath immediately and an input to execute a deep inhalation. It can also set a panic output when the carbon dioxide concentration is above a set level. Results: aDiver can ‘breathe’ practically indefinitely, responding to input to hold the breath, breathing deeper after a breath hold, responding to input to take a deep breath, and setting the panic signal when needed. We have demonstrated that it can produce a regular breathing profile at tidal volume for respiratory gating, it can take deep breaths and hold the breath for Active Breathing Control (ABC) repeatedly, and it can hold the breath in every (or every other) breathing cycle for Audio‐gated Radiotherapy (ART) or Gated Breathing Synchronizer (GBS). Conclusion: aDiver simulates the breathing pattern of a human under different commands automatically. Supported in part by NIH P01‐CA59827.
SU‐GG‐J‐86: Impact of Hysteresis on Breathing Motion and Its Distribution
Authors: Zhao T , White B , Mutic S , Lu W , Bradley J , Parikh P , Low D
Published: August 2010

Purpose: To study the impact of hysteresis on breathing motion and its distribution by investigating the angle of hysteresis breathing motion with respect to non‐hysteresis breathing motion. Method and Materials: Hysteresis in breathing motion is defined as the variation between motion trajectories during inhalation and exhalation. A free‐breathing lung motion model, x= x0+αv+βf, where v and f denote tidal volume and air flow respectively, decomposes the breathing motion into a non‐hysteresis component αv, which is purely due to air filling, and a hysteresis component βf. The angle between α and β would provide information on the interplay of hysteresis motion and non‐hysteresis motion. An angle less than 90° would suggest a positive impact of hysteresis motion on non‐hysteresis motion and an angle more than 90° would suggest a negative impact of hysteresis motion on non‐hysteresis motion. 49 patient datasets were acquired in Cinè mode, α and β were obtained by linear least‐square fitting the registered motions to the tidal volumes and the air flows measured by spirometry. Result: Hysteresis motion was found to offset non‐hysteresis motion at anterior portions of left lungs and anterior portions of right lungs. However, at the posterior portions of left lungs and anterior portions of right lungs, hysteresis motion was found to increase non‐hysteresis motion. Hysteresis motion was most likely to take a maximum likelihood angle at 42.1° or 149.5° in the left lungs, and 41.8° or 138.2° in the right lungs with respect to the non‐hysteresis motion. Conclusion: Left lungs and right lungs have distinctly characteristic distributions of the angles between α and β, indicating that different patterns of hysteresis distributions in the left lungs and the right lungs might come from intrinsic properties of both sides of lungs respectively. This work supported in part by NIHR01CA096679 and NIHR01CA116712
SU‐GG‐J‐89: Optimizing Principal Component Models for Interfraction Variation in Lung Cancer
Authors: Hugo G , Badawi A , Weiss E , Brown A , Orton M
Published: August 2010

Purpose: To optimize principal component models for modeling interfraction changes in lung cancer by investigating information and structure requirements. Method and Materials: In 16 patients, weekly CT sessions consisting of three repeat intrafraction scans were acquired with active breathing control at end of normal inspiration. The gross tumor volume (GTV) and lungs were delineated on the first week image by a physician, and propagated to all other images using deformable image registration. Principal components analysis (PCA) was used to model the target and lung variability during treatment. Four PCA models were generated for each patient: 1) individual models for the GTV and each lung from one image per week; 2) a composite model of all structures from the weekly images; 3) individual models using all images (weekly plus repeat intrafraction images); 4) composite model with all images. Dominant modes representing 95% of the total variability were used to reconstruct the observed anatomy. Residual reconstruction error between the model‐reconstructed and observed anatomy was calculated to compare the accuracy of the models. Results: Two to seven eigenmodes were necessary to capture 95% variability over all patients. An average of 3.1 and 4.9 modes were required for the weekly plus repeat models, for the GTV and composite models, respectively. The weekly model required one less mode in 40% of patients. For the composite model, the average reconstruction error was 0.7 +/− 0.2 mm, which increased to 1.1 +/− 0.4 mm when the weekly model was used. Using the four dominant modes from the weekly model, 90% of the error was less than 3 mm for 86% of patients and less than 2 mm for 50 % of patients. Conclusion: Simple composite structure models using the four principal modes and weekly imaging is feasible for reproducing observed anatomy. Supported by NIH R01CA116249.
SU‐GG‐J‐91: Electromagnetic Transponders Indicate Prostate Size Increase Followed by Decrease during and after a Course of External Beam Radiation Therapy
Authors: Butler W , Merrick G , Reed J , Kurko B , Murray B
Published: August 2010

Purpose: Real‐time image guidance enables more accurate radiation therapy by tracking target movement. This study used transponder positions to follow changes in prostate volume that may be a source of dosimetric and target inaccuracy. Materials and Methods: Twenty‐four men with biopsy‐proven T1c‐T3a prostate cancer each had three electromagnetic transponders implanted transperineally. Their coordinates were recorded by the Calypso® system, and the perimeter of the triangle formed by the transponders was used to calculate prostate volumes at sequential time points throughout the course of radiation therapy to a dose of 81 Gy in 1.8 Gy fractions. Following treatment, the transponder positions were determined once each month for six months. Results: There was a significant decrease in mean prostate volume of 10.9 % from the first to the final day of radiation therapy. The volume loss did not occur monotonically, but increased in most patients (75 %) during the first several weeks to a median maximum on day 7. The volume increased by a mean of 6.1 % before decreasing by a mean maximum difference of 18.4 % to nadir (p < 0.001 for both increase and decrease). Glandular shrinkage was asymmetric with the apex to right base dimension varying more than twice that of the lateral dimension. For all dimensions, the mean change was < 0.5 cm. Following the course of radiation therapy, there was no further significant change over the ensuing six months. Conclusion: Real‐time transponder positions indicated a volume increase during the initial days of radiation therapy and then significant and asymmetric shrinkage by the final day. After the final day, the transponder positions remained nearly unchanged for six months.
SU‐GG‐J‐94: Can We Predict the Severity of Artifacts on 4DCT from the Respiratory Irregularity during 4DCT Image Acquisition?
Authors: Wang Z , George R , Lu W , Hou J , D'Souza W
Published: August 2010

Purpose: To evaluate the correlation between the severity of artifacts on 4DCT and patient's respiratory irregularity. Method and Materials: The 4DCT from 10 different patients were studied. These patients went through two consecutive procedures in a single session: 4DCT with free breathing (FB) followed by another 4DCT with audio and visual (AV) instructions. Phillips big bore Multislice CT scanner with 4D CT capabilities were used to acquire the 4DCT scans. Real Time Position Management system by Varian was used to capture patient's breathing trace during 4DCT acquisition. Describe your methods to quantify 4DCT artifact and respiratory irregularity. Results: Our results of all ten patients indicate that there is an obvious correlation between severity of artifact and breathing irregularity. For each individual patient, the severity of the artifact increases with the breathing irregularity for 4D CT with free breathing. However, it is difficult to use a universal function to describe the correlation among all patients. The most possible cause can lay on the unique dependence of internal organ motion on external motion of each patient. Our results also indicate that the AV coaching helps to reduce the breathing irregularity during the CT acquisition and therefore results in mitigated artifact in 4DCT. Conclusion: We conclude that it is feasible to predict the severity of the artifacts on 4DCT by quantification of breathing irregularity. We also found coaching is helpful to reduce the breathing irregularity. Conflict of Interest: Research sponsored by Philips.
SU‐GG‐J‐97: An MRI Study on the Use of External and Internal Surrogates for Lung Tumor Tracking
Authors: Cerviño L , Du J , Jiang S
Published: August 2010

Purpose: To compare the performance of external and internal surrogates for lung tumor tracking. Method and Materials: 5 healthy volunteers underwent a session of a single sagittal‐slice cine‐MRI with image acquisition frequency of 4Hz. A sagittal slice with a visible vascular structure was selected for imaging. The slice contained the target (vascular structure) and the surrogates (diaphragm, abdomen surface, and chest wall). Volunteers were asked to breathe under 4 different breathing conditions during the MRI session, each one of 5‐minute duration: 1) free breathing; 2) normal inspiration and elongated expiration; 3) irregular breathing; 4) regular inhale through the nose and impeded exhale through the mouth. Target's location was manually identified in all the frames, and the motions of diaphragm, abdomen, and chest wall were characterized using principal component analysis of a region of interest containing them. The correlation between the motion of the different surrogates (diaphragm, chest wall, abdomen) and the target motion was calculated and compared. Artificial neural network was used to estimate the error of using the different surrogates to obtain target motion. Performance of each surrogate has been evaluated in terms of the error at 95% confidence level e95. Results: Diaphragm is the surrogate that correlates best with target motion, even during irregular breathing patterns. The correlation of the abdomen surface with target motion is generally superior to that one obtained with the chest wall, except during some intervals in irregular breathing patterns. Diaphragm led to e95 of up to 1.4 pixels during regular breathing, and as large as 11 pixels during irregular breathing. Using abdomen and chest wall together improves individual tracking results, but does not achieve the performance of the diaphragm. Conclusion: Internal surrogates perform better than external surrogates. A combination of internal and external surrogates will be further investigated.
SU‐GG‐J‐98: Study of Patient Setup Error and Inter‐Fractional Organ Motion for Prostatectomy Patients Using CT‐on‐Rails
Authors: Zhou S , Li S , Chen S , Deng H , Zhang M , Schubert L , Du H , Wahl A , Enke C
Published: August 2010

Purpose: A retrospective study was conducted on daily translation corrections of patient positioning based on CT‐On‐Rails technology to improve our external beam radiotherapy QA program for prostatectomy patients. Methods and Materials: 479 daily translational corrections obtained from 13 consecutive prostatectomy patients were considered. Before each treatment, each patient immobilized in vacuum bag was positioned based on skin markers. After which a 3D daily CT image of the patient was acquired using CT‐On‐Rails. Translational correction in lateral, longitudinal, and vertical directions were obtained by matching soft tissues surrounding patient's prostate bed region in the daily and treatment planning CT images. These corrections reflect the combined positioning error from manual setup and inter‐fractional organ motion/deformation. Basic statistical analysis of these corrections was performed. ANOVA analysis using the translational corrections as response variables and week days or patient study ID as explanatory variables were performed to search for possible dependence of positioning error on individual patient and weekday. Results: Average patient positioning errors in lateral, longitudinal, and vertical directions were small (−0.4mm, 1.4mm, and −0.1mm respectively with standard deviations of 5.4mm, 5.0mm, and 7.3mm). The two statistically significant (p<0.05) but weak correlations were observed between corrections in longitudinal and lateral directions (r = 0.10), and between longitudinal and vertical directions (r=0.18). ANOVA analysis showed statistically significant (p<0.05) difference in individual patient positioning error in all three directions. No such finding was observed for weekdays. Conclusions: For prostatectomy patients, without CT‐On‐Rails, the patient positioning error was patient dependent and the biggest uncertainty was in patient anterior/posterior direction. The performance of our therapists was consistent without significant residual patient setup error. CT‐on‐rails based imaging guidance before individual prostatectomy patient daily radiotherapy was effective in patient positioning error reduction.
SU‐GG‐T‐158: A Coverage Probability Based Method to Estimate Patient‐Specific Planning Organ‐at‐Risk Volumes for the Small Bowel to Account for Organ Motion during Radiotherapy
Authors: Hysing L , Söhn M , Muren L , Alber M
Published: August 2010

Purpose: The small bowel (SB) is a critical organ‐at‐risk (OR) in radiotherapy (RT) of several major pelvic and abdominal tumour sites. Due to its large size, its complex shape and the considerable inter‐patient variation in its mobility, there are at present no method to derive planning OR volumes (PRVs) that can account for individual patterns in SB motion. The aim of the present work was therefore to develop a patient‐specific planning concept for the SB which both has a high sensitivity in predicting SB voxels and is specific, meaning that it does not contain many voxels which will never be visited by the SB. Method and materials: Our method considers the variability in SB wall (SBW) position between a number of initial/pre‐therapy CT‐scans and uses this variability to estimate a ‘soft margin’ around the SBW instances, such that patients with a mobile SBW would get a broader estimated ‘soft margin’ than patiens with a less mobile SBW. The ‘soft margin’ is used to estimate a coverage probability (CP) mat or, the SBW from where patient‐specific PRVs, as generated by applying various CP thresholds. The patient‐specific PRVs, as generated from 2–5 CT‐scans, were evaluated by their sensitivity and specificity with respect to the PRVs generated from all 10–11 available CT‐scans in three patients. Results: The sensitivity and specificity of the patient‐specific PRVs depended on the number of included CT‐scans and the applied CP‐threshold. With three included CT‐scans and a threshold of 0.03, an average sensitivity of 94–96% and specificity of 86–97% was obtained in the three patients. Conclusion: Based on a few CTs, our planning concept generates patient‐specific SB PRVs which are highly sensitive and specific in detecting future SBW voxels.
SU‐GG‐T‐276: Dosimetric Characterization of a Set of Surface Applicators from Xoft Inc
Authors: Kennedy R , Pike T , Davis S , DeWerd L
Published: August 2010

Purpose: A set of conical surface applicators with diameters ranging from 10 mm to 50 mm have been developed by Xoft Inc. These applicators are designed to be used with the Axxent® eBx System, allowing for conformal dose delivery for the treatment of surface lesions. Current methods of output verification for these applicators are based on the AAPM's TG‐61 protocol. Certain components of TG‐61 may need modification for applicators like those under investigation. The goal of this work is an accurate experimental determination of the output of each applicator with comparison to Monte Carlo determined values. Methods and Materials: Air‐kerma rate measurements along the central axis of each applicator were performed using an Attix free‐air chamber, and at the applicator window with two parallel plate chamber models. Each applicator was simulated using MCNP5 and a collision kerma tally was used to determine the air‐kerma rate along the central axis and at the applicator window. Results: Initial measurements of the air‐kerma rate at the applicator window with the parallel plate chambers agreed within the expected uncertainty. Extrapolation of the air‐kerma rate measured with the FAC back to the window surface agreed with the parallel plate chambers within 15% for all but the 50 mm diameter applicator. Monte Carlo calculated values agree with the parallel plate chambers for all applicators except the 50 mm diameter as well. Conclusions: Current forms of output verification for these applicators are based on TG‐61, which was designed for low‐energy external beams, not miniature X‐ray sources in an applicator. Initial work has shown that output verification may be performed using calibrated ionization chambers, however some aspects of TG‐61 need modification for use with applicators like those under investigation here.

Conflict of Interest: Xoft Inc. provided sources and applicators.
SU‐GG‐T‐280: Validation of Intrabeam® Bare Probe 50 KVp X‐Ray Source Delivered Dose and Vendor Calculated Dose, Using Reference Dosimetry in a Water‐Equivalent Phantom
Authors: Dorgu A , Pavord D , Sherouse G , Gong J , Kriminski S
Published: August 2010

Purpose: The ZEISS Intrabeam® is a 50 kVp x‐ray source intended for interstitial brachytherapy. The purpose of this work is to make independent measurement of absolute dose rates in water‐equivalent phantom for validation and commissioning of the dose calculation software provide by the manufacturer. Method and Materials: Measurements were made by placing an ionization chamber into a Plastic Water phantom (formulation DT) parallel to the applicator shaft and at a fixed distance. Our prior work had established very good water‐equivalence of the DT resin at the energy of the Intrabeam. Two different chambers were used: PTW 23342 parallel plate ion chamber (PPIC) and A1SL cylindrical ion chamber. The PPIC was positioned at 1 or 2 cm from the nominal target to the chamber front surface. The A1SL IC was positioned at 1.5, 2.5 and 3.5cm from probe isocenter to chamber center. Measurements were made for 2 minute exposures. Readings from the IC's where converted to dose in water using the formula Dw =M×Nk × KQ × (μen, water÷ρ)/ (μen, air÷ρ). The average spectrum of the beam was assumed to be 20keV. The PPIC was calibrated with at 30kVp beam while the AlSL was calibrated at 50kVp. At this energy range K = Kc thus dose in air is assumed to be dose in water. Results: The computed dose and vendor‐supplied depth dose data where converted to dose rate and compared at our measurement depths. The individual point doses varied from −0.1 ± 3% to 8.2 ± 5% from our measurements. The A1SL cylindrical chamber measurement point shifted 2.5mm upstream from IC center. Conclusion: Dosimetry with the PPIC and A1SL IC agreed with vender calculated data. Our results validate the dose calculation software provided by the manufacturer, confirming suitability for clinical use.
SU‐GG‐T‐286: Cyberknife Quality Assurance Using a Diode Array
Authors: Cernica G , Goss M , Rodgers J
Published: August 2010

Purpose: To investigate whether the use of a linear diode array is feasible in the measurement of flatness and symmetry of a Cyberknife unit for quality assurance purposes. Method and Materials: A Mapcheck (SunNuclear) device with a 7 cm solid water phantom (The Phantom Company) was placed at 80 cm SSD from a G4 Cyberknife unit (Accuray). Profile measurements were obtained for field sizes varying from 40 mm to 60 mm. The SSD was then increased to 150 cm, and to 240 cm, and the process repeated. A spreadsheet was written to calculate the symmetry and centricity of the profile using the Cyberknife. The same profiles were obtained using Gafchromic EBT film (ISPCorp), and the field flatness and symmetry were calculated using FilmQA (3Cognition). Results: EBT Film and Mapcheck results agree within 1% for symmetry. An increased SSD to the Mapcheck (and hence an increase in measurement points) did not demonstrate a significant improvement in the accuracy of the symmetry measurement, nor in an increased accuracy in the measurement of the centricity of the laser. Conclusion: The use of a linear diode array device for quality assurance of a Cyberknife unit has been shown to provide an accurate and time‐efficient way to measure both the symmetry of the Cyberknife profile and the centricity of the laser. We hence have adopted the use of the Mapcheck device for our monthly quality assurance of our Cyberknife unit.
SU‐GG‐T‐287: A Practical Method for Verifying Constancy of Output Factor for Small Fields Shaped by Variable Beam Limiting Devices
Authors: Nikolic M
Published: August 2010

Purpose: To develop a reliable and clinically efficient method for checking the constancy of the output of small field sizes shaped by variable beam collimators. This is an important quality assurance measure because the output is strongly sensitive to even small changes in field size. Such fields typically require the use of diodes for beam measurement. A water phantom is required to center the diode for the output measurements, but a water phantom is impractical for frequent QA checks. We investigate instead a method based on the concept of integrated dose over the whole irradiated field using a large area parallel plate ionization chamber. Method and Materials: A PTW Bragg Peak parallel plate chamber (8 cm) and PTW UNIDOS electrometer were used to measure integrated dose delivered by the CyberKnife® Robotic Radiosurgery System (CK, Accuray Incorporated, Sunnyvale, CA) in a fixed geometry. CK field size was defined using an Iris™ Variable Aperture Collimator (Accuray Incorporated). Field size was determined from radiochromic film exposed simultaneously with the chamber measurement. Results: A quadratic relationship was established between the radiation field size and the temperature/pressure corrected reading from a reproducibly placed parallel plate chamber. For the 5‐mm field size, the smallest available on the CK, this measurement technique detected changes with an uncertainty of 0.1 mm. For this small field, a 0.1‐mm field size change corresponds to relative a change in output of 2%. Conclusion: For a given Linac and its output, a change in an area‐integrated dose reading is a clinically practical method sensitive to the field size changes. The Bragg Peak parallel plate chamber is practical, and large enough to accommodate the range of the field sizes (5 mm to 60 mm) offered by the CyberKnife.

Conflict of Interest: The author is employed by Accuray Incorporated, Sunnyvale, CA.
SU‐GG‐T‐294: Quality Assurance for Small SRS Photon Field Using LUCY Phantom on BrainLab Iplan
Authors: Zhang X , Velasco C , Yan Y , Corry P , Ratanatharathorn V , Moros E
Published: August 2010

Purpose: To use a high precision LUCY phantom designed specifically for stereotactic radiosurgery to perform comprehensive QA and determine which dosimeter to use for different cone sizes with the BrainLAB iPlan TPS. Methods and Materials: An image set was acquired in a Philips Big Bore CT simulator for the whole phantom and transferred to the BrainLAB iPlan TPS for contouring and planning. An SRS cone treatment plan was generated with the LUCY phantom as a patient. A treatment plan for the “patient” was generated using the same clinical approach as used for actual SRS patients. The LUCY patient plan was loaded and mapped onto the LUCY QA phantom with the iPlan Phantom Mapping software to create a QA plan. A PinPoint ionization chamber (0.016cm3) and Gafchromic EDR2 film were employed for dose measurement. Film calibration was performed running a “step‐wedge” calibration procedure. Films were stored overnight to allow for post exposure changes. An optical densitometer was used to read OD. Results and Discussion: The range of percentage differences between calculated and measured doses for the ion chamber was 3.83% to 0.2% for cone sizes of 30mm down to 10mm. For the two smallest cones, 7.5 and 5.0 mm, the percentage difference was beyond clinical tolerance (>5%). All film dosimetry results were within clinical tolerance. Discussion and Conclusion: The DQA for cone size of 30mm down to 10mm were verified with LUCY phantom using both the PinPoint ion chamber and EBT2 film. For cone sizes smaller than 10mm, the PinPoint ion chamber did not measure dose accurately because the ion chamber dimensions were of the same magnitude as the field size. Gafchromic EBT2 film was able to measure the dose for all cone sizes.
SU‐GG‐T‐297: DMLC Leaf Position Tests for Dynamic Arc Using Array Detector Devices
Authors: Ho J , Koltermann P
Published: August 2010

Purpose: For dynamic arc and intensity modulated arc therapies, new challenges have arisen in treatment commissioning and routine physics QA due to additional variation in gantry rotation. A specific dMLC test for leaf position accuracy in dynamic arc deliveries using commercial available 2D array devices was tried. Materials and Methods: The method of the dMLC dynamic test was developed based on a Picket Fence type test which was originally designed for static gantry application. The new tests were performed either at a series of fixed gantry angles in the range of 360 degree or dynamically with a rotating gantry and a dynamical MLC delivery. A PTW Seven29 ion chamber array and a Sun Nuclear MapCheck diode array were employed as detection devices for the measurements. Results: The results measured from both devices showed similar intensity distributions in dynamic Picked Fence patterns. The results were also consistent with our previous film dosimetry studies, which implicates that the leaf position accuracy could be degraded due to gantry dynamical rotation. The difference from the results of two different device measurements were compared with each other, and the pro and con for the measurements using these two devices were discussed. Conclusion: Our tests could detect MLC leaf displacements at specific gantry angles and due to gantry rotation. The test could be implemented for a routine MLC QA for those dynamic arc therapies such as dynamic conformal arc and volumetric modulated arc therapies. While both PTW and Sun Nuclear array detectors could be easily used for a routine arc QA procedure, the PTW Seven29 ion chamber array provides a better resolution and much lesser dependence on the mechanical setup accuracy. Conflict of Interest: Peer Koltermann is an employee of PTW
SU‐GG‐T‐301: Replacing Film with Computed Radiography for Routine Linear Accelerator QA
Authors: Zhang Q , Chan R
Published: August 2010

Purpose: To evaluate the feasibility of using diagnostic computed radiography (CR) plates to replace radiographic films for linear accelerator QA. Method and Materials: Type C Fujifilm CR plate (size 25.2×30.3cm) was tested with image reader FUJIFILM FCR PROFECT CS. Spatial resolution of 10 pixels/mm was used for image reading. The image plate was used without the cassette for linear accelerator QA. Plate or film was setup on the treatment table at isocenter (100cm SSD). A 3mm thick Plexiglas template with stainless steel beads markers was placed over the plate and aligned to the light filed. Exposure of 10 mus or 40 mus was delivered to a 10×10cm and 20×20cm field respectively using a 6 MV photon. The spatial accuracy and spatial uniformity of the CR image were evaluated and compared with Kodak X‐Omat V film. The distances measured in various directions on the CR image and Kodak X‐Omat V film were compared with the actual distances on the template to evaluate the spatial accuracy and image distortion under different conditions. Results: The distance measured in three different directions had error within 1%. Good spatial accuracy and good spatial uniformity were observed. Results were comparable for both FUJI CR plate and Kodak film. Conclusion: Uses of CR plate for routine QA give a comparable result to that of film. Similar results were also observed when used for light field‐radiation field coincidence test, star shot tests and MLC picker fence test. Hence the FUJIFILM CR plate is suitable for the linear accelerator routine QA procedure. With the facing out of films in most hospital, CR plates commonly used in diagnostic radiology made a good and relatively inexpensive substitute for use in routine linear accelerator QA procedure.
SU‐GG‐T‐304: Spreadsheet for Determination of Irradiance Delivered by Cylindrical Diffusers for Intracavitary PDT Treatments
Authors: Ferreira M , Austerlitz C , Sheng C , Benhabib S , Allison R , Sibata C
Published: August 2010

Purpose: This study is aimed to develop a spreadsheet which allows fast determination of laser power and irradiation time for effective dose delivered in PDT intracavitary treatment. Calculations were based on treatment cavity diameter, cylindrical diffuser length, medical radiant exposure prescription (H), and adopted clinical irradiance range (E). Method and Materials: Irradiance profiles delivered by the diffusers were measured with a spherical sensor (2.0 mm diameter). A 630 nm PDT diode laser coupled with sets of different diffusers of 10, 20, 25, and 50 mm length was used as light source. Irradiance was measured from 2–25 mm source to detector distance (SDD). The reference point of measurement was the geometric center of the diffuser and the measurements normalized at its surface (1.59 mm diameter). Curves were fitted with a double exponential decay (0.9349≤r2≤0.9951). Simulations were performed to model experimental results and compared with experimental and clinical methods (calculated by the total irradiance divided by the cylinder area bounded by SDD). Results: Compared with the irradiance calculated clinically using the area bounded by a cylinder, the irradiance measured with spherical sensor differs by as much as −32% at 1.2 cm SDD for a 20 mm length diffuser. For this case, the irradiance calculated clinically is 99.52 mW/cm2 and the true irradiance is 67.78 mW/cm2. To obtain the prescribed radiant exposure, treatment time calculated by the clinical method is wrong by a factor of 1.46. Conclusion: Calculations using the spreadsheet developed in this work allows fast calculations of irradiance, with correct exposure time using the cylindrical diffusers.
SU‐GG‐T‐306: Can Single Dataset in Treatment Planning System Represent Several Beam‐Matched Accelerators?
Authors: Gershkevitsh E , Peraticos A , Raad D Dimitriadis , Aritkan A , Efthymiou T , Stylianou‐Markidou E , Giannos A , Constantinou C
Published: August 2010

Purpose: Beam matched accelerators can provide the flexibility within the radiotherapy department. Furthermore, the accelerators could be represented by single beam data set in the treatment planning system (TPS). The aim of this study was to evaluate the match between dosimetrical characteristics of Elekta accelerators. Method and Materials: The data from three accelerators was used in analysis. Three photon and six electron beams were matched according to the vendor specification. The first accelerator data served as reference. Subset of commissioning measurements for second and third accelerators was performed. The comparison was done in absolute terms, i.e. that PDDs and profiles were used with corresponding accelerator output factors. Results: The average difference between the sets of open field PDDs was −0.7%, −0.1% and −0.1% for 6, 10 and 15MV, respectively. Difference up to −2.2% was observed for 60° motorized wedge PDDs. Aligned profiles showed good agreement, with differences generally less than 1% at dmax and 10 cm for field sizes up to 30×30 cm2. For 40×40 cm2 diagonal profiles at dmax the maximum differences of 2.4% was observed. The difference of up to 4.5% between output factors for small fields have been observed which may result in larger discrepancies in IMRT fields. Conclusion: Beam match for photon beams showed a satisfactory agreement for open fields and support the use of a single data set for TPS. However, vendor's beam matching criteria is not strict enough and better beam matching could be achieved if instead of single value the subset of dosimetrical data is used together with corresponding output factors for both open and wedged fields. Although, scanned data for electron beams showed good agreement larger differences (up to 5%) in electron cone output factors does not support the use of a single dataset in TPS.
SU‐GG‐T‐307: Multi‐Institution Comparison of Varian Linac Electron Commissioning Data: Application to Eclipse Generalized Gaussian Pencil Beam Algorithm Commissioning
Authors: Rasmussen B , Chu K
Published: August 2010

Purpose: To point out clinically relevant issues in the commissioning process of the Eclipse Generalized Gaussian Pencil Beam (GGPB) algorithm and to post a multi‐institution comparison of Varian linac electron commissioning data. Research was performed to quantify the accuracy of commissioning a treatment planning system using either Varian “golden beam data” or common data for several linacs. Using such data may be a useful option for clinics installing or replacing a linac, with the need to make the transition with minimal down time. This work seeks to address whether or not such techniques are clinically acceptable. Method and Materials: Graphical techniques were used to compare the electron commissioning data from multiple independent Varian linacs. The performance of the Varian “golden beam data” for beam modeling using the Eclipse GGPB algorithm was examined by comparing calculations to measured dose distributions as well to calculations made with a fully commissioned pencil beam algorithm in CMS Xio. Results: Depth dose curves from the various independent linacs were typically found to be within a 3 mm distance to agreement, with the Varian “golden beam data” falling within an approximate 5 mm distance to agreement. Comparisons between CMS Xio and Eclipse showed that Eclipse depth dose profiles are less smooth and tend to contain calculation algorithm artifacts. Conclusion: Although good agreement was found between the commissioning data of the clinics in this study, special care should be taken when using data from another linac to commission a treatment planning system. With careful adjustment of the modeling parameters in the treatment planning systems it may be possible to achieve calculation accuracy that is within a clinically acceptable tolerance. However, if optimal accuracy is desired, a full set of commissioning data should be collected for the specific linac being modeled.