bluuebunny/crossref_metadata_2025 huggingface dataset:
Linearization of dose–response curve of the radiochromic film dosimetry system
Authors: Devic Slobodan , Tomic Nada , Aldelaijan Saad , DeBlois Francois , Seuntjens Jan , Chan Maria F. , Lewis Dave
Published: July 2012

Purpose:

Despite numerous advantages of radiochromic film dosimeter (high spatial resolution, near tissue equivalence, low energy dependence) to measure a relative dose distribution with film, one needs to first measure an absolute dose (following previously established reference dosimetry protocol) and then convert measured absolute dose values into relative doses. In this work, we present result of our efforts to obtain a functional form that would linearize the inherently nonlinear dose–response curve of the radiochromic film dosimetry system.

Methods:

Functional form [ζ = (−1)·netOD(2/3)/ln(netOD)] was derived from calibration curves of various previously established radiochromic film dosimetry systems. In order to test the invariance of the proposed functional form with respect to the film model used we tested it with three different GAFCHROMIC™ film models (EBT, EBT2, and EBT3) irradiated to various doses and scanned on a same scanner. For one of the film models (EBT2), we tested the invariance of the functional form to the scanner model used by scanning irradiated film pieces with three different flatbed scanner models (Epson V700, 1680, and 10000XL). To test our hypothesis that the proposed functional argument linearizes the response of the radiochromic film dosimetry system, verification tests have been performed in clinical applications: percent depth dose measurements, IMRT quality assurance (QA), and brachytherapy QA.

Results:

Obtained R2 values indicate that the choice of the functional form of the new argument appropriately linearizes the dose response of the radiochromic film dosimetry system we used. The linear behavior was insensitive to both film model and flatbed scanner model used. Measured PDD values using the green channel response of the GAFCHROMIC™ EBT3 film model are well within ±2% window of the local relative dose value when compared to the tabulated Cobalt‐60 data. It was also found that criteria of 3%/3 mm for an IMRT QA plan and 3%/2 mm for a brachytherapy QA plan are passing 95% gamma function points.

Conclusions:

In this paper, we demonstrate the use of functional argument to linearize the inherently nonlinear response of a radiochromic film based reference dosimetry system. In this way, relative dosimetry can be conveniently performed using radiochromic film dosimetry system without the need of establishing calibration curve.
Integrated multicriterial optimization of beam angles and intensity profiles for coplanar and noncoplanar head and neck IMRT and implications for VMAT
Authors: Voet Peter W. J. , Breedveld Sebastiaan , Dirkx Maarten L. P. , Levendag Peter C. , Heijmen Ben J. M.
Published: July 2012

Purpose:

To quantify improved salivary gland sparing for head and neck cancer patients using intensity‐modulated radiotherapy (IMRT) plans based on integrated computerized optimization of beam orientations and intensity profiles. To assess if optimized nonzero couch angles also improve VMAT plans.

Methods:

Our in‐house developed algorithm iCycle was used for automated generation of multicriterial optimized plans with optimized beam orientations and intensity profiles, and plans with optimized profiles for preselected beam arrangements. For 20 patients, five IMRT plans, based on one “wish‐list,” were compared: (i) and (ii) seven‐ and nine‐beam equiangular coplanar plans (iCycle7equi, iCycle9equi), (iii) and (iv) nine‐beam plans with optimized coplanar and noncoplanar beam orientations (iCyclecopl, iCyclenoncopl), and (v) a nine‐beam coplanar plan with optimized gantry angles and one optimized couch rotation (iCyclecouch). VMAT plans without and with this optimized couch rotation were evaluated.

Results:

iCyclenoncopl resulted in the best salivary gland sparing, while iCyclecouch yielded similar results for 18 patients. For iCycle7equi, submandibular gland NTCP values were on average 5% higher. iCycle9equi performed better than iCycle7equi. iCyclecopl showed further improvement. Application of the optimized couch angle from iCyclecouch also improved NTCP values in VMAT plans.

Conclusions:

iCycle allows objective comparison of competing planning strategies. Integrated optimization of beam profiles and angles can significantly improve normal tissue sparing, yielding optimal results for iCyclenoncopl.
Consistency of breast density measured from the same women in four different MR scanners
Authors: Chen Jeon‐Hor , Chan Siwa , Liu Yi‐Jui , Yeh Dah‐Cherng , Chang Chih‐Kai , Chen Li‐Kuang , Pan Wei‐Fan , Kuo Chih‐Chen , Lin Muqing , Chang Daniel H. E. , Fwu Peter T. , Su Min‐Ying
Published: July 2012

Purpose:

To compare the breast volume (BV), fibroglandular tissue volume (FV), and percent density (PD) measured from breast MRI of the same women using four different MR scanners.

Methods:

The study was performed in 34 healthy Asian volunteers using two 1.5T (GE and Siemens) and two 3T (GE and Philips) MR scanners. The BV, FV, and PD were measured on nonfat‐suppressed T1‐weighted images using a comprehensive computer algorithm‐based segmentation method. The scanner‐to‐scanner measurement difference, and the coefficient of variation (CV) among the four scanners were calculated. The measurement variation between two density morphological patterns presenting as the central type and the intermingled type was separately analyzed and compared.

Results:

All four scanners provided satisfactory image quality allowing for successful completion of the segmentation processes. The measured parameters between each pair of MR scanners were highly correlated, with R2 ≥ 0.95 for BV, R2 ≥ 0.99 for FV, and R2 ≥ 0.97 for PD in all comparisons. The mean percent differences between each pair of scanners were 5.9%–7.8% for BV, 5.3%–6.5% for FV, 4.3%–7.3% for PD; with the overall CV of 5.8% for BV, 4.8% for FV, and 4.9% for PD. The variation of FV was smaller in the central type than in the intermingled type (p = 0.04).

Conclusions:

The results showed that the variation of FV and PD measured from four different MR scanners is around 5%, suggesting the parameters measured using different scanners can be used for a combined analysis in a multicenter study.
Poster — Thur Eve — 21: ROC analysis in patient specific quality assurance
Authors: Carlone M , Cruje C , McCabe R , Nielsen M , Macpherson M
Published: July 2012

INTRODUCTION: Many institutions rely on a patient specific measurement for IMRT/VMAT patient QA. In diagnostic imaging, radiologists use Receiver Operator Curves (ROC) to help quantify the value of a diagnostic imaging test. The purpose of this work is to investigate the value or ROC methodology for patient specific IMRT QA. METHODS AND MATERIALS: Beam fluences for 34 prostate IMRT patients were analyzed using gamma analysis. For half of these, measurements were done using the planned beam fluences. For the rest, perturbations to the MLC leaf positions were introduced. Gamma analysis was then used to measure fluence differences. Assuming that the unperturbed fluencies were positive measurements, distributions of true positive and false negatives were calculated. RESULTS: For poorly performing beam delivery systems the choice of γ‐DTA criterion has little effect on test sensitivity and specificity. The AUC is increased by about 10% for high performance beam delivery systems. For a 3%/3mm γ‐DTA condition, ideal cut off values are reasonably independent of MLC performance. At a tighter γ‐DTA condition of 2%/2mm, then the optimal sensitivity and specificity of the test is more dependent on MLC performance. DISCUSSION: For a pass‐fail test such as the γ‐DTA map is, it is important to choose an optimal cut off value to maximize the sensitivity and specificity of the test. ROC methodology allows users to follow a prescriptive method to obtain ideal cut‐off values for gamma analysis, and to assess improvements in sensitivity and specificity for higher performing beam delivery system.
Poster — Thur Eve — 22: Bone heterogeneity in kV x‐ray radiotherapy
Authors: Chow J , Grigorov G
Published: July 2012

This study evaluated the dosimetric impact of bone heterogeneity on the surface dose and dose prescription, when dose is assumed to be prescribed on a homogeneous medium in kV x‐ray radiotherapy. A heterogeneous phantom containing a thin water layer (thickness = 1–5 mm) over a bone (thickness =1 cm) was used to mimic treatment sites of forehead, knee and chest wall. The phantom was irradiated by a 220 kVp photon beam with field size of 5 cm diameter. Percentage depth dose, surface dose and photon energy spectrum with different thicknesses of water were determined using Monte Carlo simulations (the EGSnrc code) with experimental verifications using parallel‐plate ionization chamber and radiochormatic film. Our results (treatment cone of 5 cm diameter) showed that the surface dose increased in a range of 2.5–3.7% when the water layer above the bone was increased from 1 to 5 mm. However, the surface dose did not increase linearly with the increase of water thickness, and the maximum increase of surface dose was found at a water thickness of 3 mm. Results of the percentage depth dose showed that the maximum bone dose was about 210% higher than the surface dose in the heterogeneous phantom. It is concluded that in treatment sites having a thin layer of tissue over a bone in kV x‐ray radiotherapy, if increased surface dose due to bone heterogeneity is not considered, this will result in an additional dosimetric uncertainty narrowing down the total error margin (5%).
Poster — Thur Eve — 23: Statistical analysis and verification of the percentage depth dose calculation based on the tissue maximum ratio in external beam radiotherapy
Authors: Isa M , Iqbal K , Afzal M , Buzdar S , Chow J
Published: July 2012

The aim of this study is to perform a statistical analysis to verify the calculated percentage depth dose (PDD) based on the tissue maximum ratio (TMR) with the PDD measurements taken in water. 6 and 15 MV photon beams produced by a Varian linear 2100 C/D accelerator were used. PDDs and TMRs were measured at various depths and field sizes (5 × 5, 10 × 10, 15 × 15, 20 × 20 and 30 × 30 cm2) using a PTW 31006 ionization chamber and a scanning water tank. By comparing the calculated and measured PDD results, it was seen that for larger field sizes the deviation between the calculated and measured PDD was smaller. Deviations between the calculated and measured results were found to be higher in the build‐up regions of the 6 and 15 MV photon beams. For the statistical analyses, t‐tests were performed using the measured and calculated PDDs for each field size but showed insignificant deviations for the 6 and 15 MV photon beams. The mean t‐test values are 0.952 and 0.970 for the 6 and 15 MV photon beams, respectively. The difference between the calculated and measured PDD is within the acceptable range according to the ICRU reports (ICRU Report No. 24, 1976). We conclude that accurate calculation of the PDD using the measured TMR data is possible, which is useful as the PDD cannot be measured directly.
Poster — Thur Eve — 26: Interfraction reproducibility of heart position during breast irradiation using Active Breathing Control
Authors: Comsa D , Zhang B , Mosely D , Yeung I
Published: July 2012

The moderate deep‐inspiration breath hold (mDIBH) technique using the Active Breathing Coordinator (ABC) from Elekta is used in our clinic to lower the heart dose during left breast irradiations. The purpose of this work was to investigate the interfraction reproducibility of the heart to chest distance during these treatments and to evaluate the dosimetric effect of any changes in the heart position. Daily CBCT images were available for 5 patients who had been treated with ABC tangents and a cavity boost. On these images, one‐dimensional measurements of the distance between the heart and the chest wall were taken at two anatomical locations corresponding roughly with the location where the radiation field most likely intercepts the heart. The average change in this distance was interpreted as a shift of the heart position. To assess the effect of this shift on the delivered heart dose, the heart contours in the clinical plans of the corresponding patients were shifted towards the treatment field using standard Pinnacle tools. Although the ABC device allows good reproducibility of the volume of air held, this does not warrant reproducibility of heart position for all patients during treatment. The largest average heart shift extracted from CBCT images in this study was 6.2mm. The heart dose reconstructed using this shift for the corresponding patient also showed the largest effect. However, even in the presence of a systematic heart shift of this magnitude, the ABC plan still showed superior heart dose reduction compared to the free‐breathing plan.
Poster — Thur Eve — 30: Comparison of treatment planning and delivery performance of VMAT versus IMRT
Authors: Bergman A , Vollans E , Duzenli C
Published: July 2012

The purpose of this study was to determine whether VMAT (Varian RapidArc ™) treatment planning and delivery performance is in compliance with accepted quality assurance tolerances developed for sliding window IMRT. We present an analysis of data for over 1300 patients treated with VMAT and IMRT over a period of three years. Data was acquired on 6 dosimetrically matched linacs for sites including head and neck, brain, gynaecological, and a variety of other cancer cases treated with 6 MV. We have demonstrated that it is possible to dosimetrically match multiple Varian iX linacs with the millennium series MLC using a sliding gap and intercept test. QA is performed by Monte Carlo simulation and ion chamber measurement comparisons with Varian Eclipse TPS as well as linac log file analysis of MLC positions, gantry angles and monitor units on each patient. Point dose and 3D gamma analysis indicate that agreement between Eclipse and measurement or Monte Carlo calculation is site specific, with the dosimetric leaf gap parameter in Eclipse optimized for the most frequently treated site Point dose agreement within 2% and gamma pass rate of > 95% (3%/ 3 mm) is achievable for all sites for both IMRT and VMAT. Linac log file analysis indicates that planned MLC positions are achieved within 2 mm >99.7% of the time for both sliding window IMRT and VMAT. Planned gantry angles are achieved within 0.6 mm 99.8% of the time and planned MU's within 0.1 mm are achieved 99.8% of the time for VMAT.
Poster — Thur Eve — 33: A comprehensive analysis of the effect of respiratory motion on the delivery of IMRT to advanced stage non‐small cell lung cancer
Authors: Hopkins G , Goosheh S , Gaede S
Published: July 2012

The goal of this work was to quantify the interplay effect of various IMRT delivery techniques in the treatment of Stage III non‐small cell lung cancer. Five patients with significant tumour motion were retrospectively planned on the average 4D‐CT dataset with eight different IMRT techniques: three Tomotherapy techniques with different beam‐widths, two step‐and‐shoot (SS‐IMRT) with different complexity, one sliding‐window (SW‐IMRT), and two VMAT techniques (RapidArc and SmartArc). Each plan was calculated on a delivery verification phantom that was mounted on a programmable respiratory motion platform and delivered under the following motion conditions: 1) Static; 2) sinusoidal with 4 different amplitudes; 3) Real Patient Breathing. A standard 3%/3mm gamma analysis compared the sum of all 30 fractions to their corresponding 60Gy/30fx plan. One‐way ANOVA was conducted for respiratory motion amplitude and IMRT modality, separately. There were no significant differences amongst the modalities at any amplitude level. However, for individual modalities, there were significant differences amongst different amplitudes except for Tomo‐2.5cm (p=0.260). Post‐hoc Tukey tests determined that detectable significant differences amongst any motion level, including real‐patient breathing, were observed when compared to the 20mm amplitude for all modalities except Tomo‐2.5cm and SmartArc. SW‐IMRT showed significant differences at 15mm when compared to both static (p=0.033) and 5mm (p=0.008). All methods except for RapidArc averaged out to clinically acceptable gamma pass rates up to 15mm. In conclusion, for motion levels above 15mm, the interplay effect can be clinically unacceptable. However, the interplay effect at these motion levels does not appear to be modality dependent.
Poster — Thur Eve — 40: Dynamic arc sliding window tests for checking MLC gap consistency in rapid arc delivery
Authors: Mei X , Konieczny J , Leszczynsky K
Published: July 2012

MLC gap control is critical for dosimetric accuracy in rotational IMRT (RapidArc, VMAT) treatments. Systematic MLC gap change of 1 mm may cause 3–4% change of EUD to PTV for a typical H&N RapidArc plan. Therefore it is important to monitor MLC gap through QC procedures. For this purpose, we have created dynamic arc sliding window (SW) plans with fixed width MLC slits sliding across a jaw defined field. Plans with MLC slit widths of 5, 10, 15, and 20 mm, respectively, and the same length of 20 cm (in Y direction) were created with 6MV photons in a single arc of gantry angles from 182° to 178°. Dose delivered from these SW plans was measured using an ion chamber in a cylindrical phantom placed at isocentre, and values for dosimetric leaf gap (DLG) were derived based on relative dose measurements. DLG measured in dynamic arc SW tests agrees with that measured in fixed gantry angle SW fields to within 0.02 mm. We also extracted the MLC leaf gaps during MLC travels in these dynamic arc SW deliveries from MLC positions recorded in dynalog files, and compared to the MLC gaps in fixed gantry SW fields. We found that MLC leaf gaps were maintained excellently constant whether in dynamic arc or fixed gantry angle SW delivery, with typical standard deviation of MLC gaps of only ∼0.01mm for all involved leaf pairs. We believe these dynamic arc SW tests are very useful for checking MLC leaf constancy for RapidArc delivery.
Poster — Thur Eve — 42: Dynamic delivery quality assurance on Elekta linacs
Authors: Munger P , Nicol S
Published: July 2012

Introduction Recently, Elekta linacs have gained the capability to deliver dynamic fields, such as sliding window IMRT or VMAT fields. Because of the potential complexity of such delivery modes, linac QA and patient‐specific QA are of prime importance. The aim of this paper is to explain the fundamental concepts of this new mode of operation on Elekta linacs as well as to introduce the linear‐α slit, a novel dynamic QA sequence which allows the performance of the delivery system to be objectively aassessed against dosimetric measurements. In the Elekta dynamic mode, dose rate can only take a limited number of discontinuous values. We have used the uniform slit, a simple slit moving at a constant speed across the field to verify how the linac selects leaf speed and dose rate for a given MU setting. Based on this, we expose the principles behind the linear‐α slit, which exhibits two main characteristics: 1) it exercises the linac over a range of dose rates and 2) it produces a dose distribution which is theoretically equivalent to the uniform slit. Discrepancies between measurements of the linear‐α slit and the uniform slit directly reflects problems with the delivery. The linear‐α slit has been recently introduced in our routine monthly linac QA. We hope it will nicely complement patient‐specific QA, and exising linac‐QA.
Poster — Thur Eve — 44: Digital tomosynthesis image quality in a Co‐60 treatment beam
Authors: Marsh M , Schreiner LJ , Kerr A
Published: July 2012

Image guidance capability is an important feature of modern radiotherapy machines. Cobalt‐60 units will require some form of image guidance if they are to be brought up to modern standards. Imaging in the treatment beam is an appealing option, for reasons of simplicity and cost, but the dose needed to produce cone beam CT images in a Co‐60 treatment beam is too high for this modality to be clinically useful. Digital tomosynthesis (DT) offers a quasi‐3D image, of sufficient quality to identify bony anatomy or fiducial markers, while delivering a much lower dose than CBCT. A series of experiments were conducted on a prototype Co‐60 cone beam imaging system to quantify the resolution, selectivity, geometric accuracy and contrast sensitivity of Co‐60 DT. Although the resolution was severely limited by the penumbra cast by the ∼2cm diameter source, it was possible to identify high contrast objects on the order of 1 mm in width, and bony anatomy in anthropomorphic phantoms was clearly recognizable. Low contrast sensitivity down to electron density differences of 3% was obtained, for uniform features of similar thickness. The conventional shift‐and‐add algorithm was compared to the FDK filtered backprojection algorithm using several different spatial filters. The Co‐60 DT images were obtained with a total dose of 5 to 15 cGy. We conclude that, should Co‐60 radiotherapy units be upgraded with image guidance capabilities, filtered backprojection DT in the treatment beam is a versatile and promising modality that would be well suited to the task of patient positioning.
Poster — Thur Eve — 50: Planning and delivery accuracy of stereotactic radiosurgery with Tomotherapy as compared to linear‐accelerator and robotic based radiosurgery
Authors: Thakur V , Soisson E , Ruo R , Doucet R , Parker W , Seuntjens J
Published: July 2012

This study includes planning and delivery comparison of three stereotactic radiosurgery techniques : Helical Tomotherapy (HT), circular collimator‐based Linear‐accelerator and robotic‐radiosurgery. Plans were generated for two spherical targets of diameter 6 mm and 10 mm contoured at the center of a Lucite phantom, using similar planning constrains. Planning comparison showed that average conformality (0–1best) for Linear‐accelerator, robotic‐radiosurgery and HT was 1.43, 1.24, and 1.77 and gradient index (less is better) was 2.72, 4.50 and 13.56 respectively. For delivery comparison, plans were delivered to radiochromic film and measured dose was compared with the planned dose. For Linear‐accelerator and robotic‐radiosurgery more than 99% pixels‐passing a gamma criteria of 3% dose difference and 1 mm distance to agreement where as for HT this value was as low as 40% for off‐axis targets. Further investigation of the delivery accuracy as a function of the location of the target with in the bore was initiated using small volume A1SL (0.057 cm3) and MicroLion liquid ion chamber (0.0017 cm3). Point dose measurements for targets located at the center and 10 cm away from the center of the bore showed that delivered dose varied by more than 15% for targets placed away from the center of the bore as opposed to at the center. In conclusion, Linear‐accelerator and the robotic‐radiosurgery techniques showed preferable gradient and conformality. For HT, point dose measurements were significantly lower than predicted by the TPS when the target was positioned away from the isocenter, while they were found to be higher at isocenter.
Poster — Thur Eve — 51: Three‐dimensional in‐vivo EPID dosimetry of IMRT and VMAT treatments
Authors: Van Uytven E , Van Beek T , Chytyk K , McCurdy BMC
Published: July 2012

As radiation treatment delivery becomes more complex, including dynamic IMRT and VMAT, the argument for routine patient dose verification becomes more compelling. This work demonstrates a technique that utilizes our pre‐existing portal dose image prediction algorithm to compute 3D patient dose from recorded on‐treatment portal images. This approach can be applied on CT simulation data or daily cone‐beam CT data sets. Here we demonstrate the robustness of our dose reconstruction technique with phantom and patient examples, with delivery schemes including IMRT and VMAT. For an example prostate treatment site, 3D dose distributions reconstructed in the patient model are computed for each fraction, and DVHs presented. Results indicate that the patient dose reconstruction algorithm compares well with treatment planning system computed doses for controlled test situations. For patient examples the 3D chi comparison values (similar to the gamma comparison) ranged from 94.5% to 100% agreement for voxels > 10% maximum dose for all treatments and phantom cases. We show an example where the DVH for fraction nine of a prostate treatment fails acceptability criteria, due to a previously unnoticed positioning error. Future work involves building our patient dose reconstruction into a QA package, subsequently integrating it into a clinical workflow. We are also investigating the use of this tool as a backbone for an in‐house adaptive radiotherapy implementation. This work is supported by Varian Medical Systems.
Poster — Thur Eve — 61: A dosimetric comparison between single‐arc and double‐arc prostate VMAT plans
Authors: Chow J , Jiang R
Published: July 2012

This study compared the dosimetry between prostate plans using the single‐arc and double‐arc technique in volumetric modulated arc therapy (VMAT). Single‐arc and double‐arc prostate VMAT plans were created in five patients with prostate volumes ranging from 39.0–68.1 cm3. 6 MV photon beams generated by a Varian 21EX linac with 120‐leaf multileaf collimator, and prescription dose of 78 Gy with 39 fractions were used. Dose‐volume histograms of the target and critical organs, dose criteria such as the planning target volume (PTV) D99%, rectum D30% and rectum D50%, and dose profiles along the anterior‐posterior and lateral directions crossing the isocenter were determined using the Eclipse treatment planning system with RapidArc optimization. It is found that the double‐arc technique provided a better PTV coverage and rectal sparing compared to the single‐arc for all patients. For dose profiles crossing the isocenter, it is seen that the dose in the anterior‐posterior direction was shifted to the lateral, when the single‐arc technique was replaced by the double‐arc. This dosimetric change made the dose in the rectal position shift laterally to the femoral head, resulting in a lower rectal dose. The calculated monitor units (MUs) were 471–600 for the single‐arc plans and 587–730 for the double‐arc. Although the double‐arc plans on average required more MUs of about 20%, and almost twice the treatment time compared to the single‐arc, it is worthwhile to be considered particularly in some cases, where the dose criteria are difficult to achieve using the single‐arc technique.
Poster — Thur Eve — 65: Optimization of an automatic image contouring system for radiation therapy
Authors: Hamilton T , Nedialkov N , Wierzbicki M
Published: July 2012

Intensity modulated radiation therapy (IMRT) is an advanced technique used to concentrate the prescribed dose in the tumour while minimizing exposure to healthy tissues. Success in IMRT is greatly dependent upon the localization of the target volume and normal tissue, thus accurate contouring is crucial. In this paper, we describe an automated atlas‐based image contouring system and our approach for improving the system by performing a full‐scale optimization of registration parameters using high‐performance computing. To achieve this, we use manually pre‐contoured CT images of ten head and neck patients. For any parameter set, each patient data is registered with the remaining patients. Accuracy of the resulting contours is determined automatically by comparing their overlap with manually defined targets using Dice's similarity coefficient (DSC). This allows us to compare all permutations of the image registration parameter sets and input data to investigate their impact on final contour accuracy. Investigating the parameter space required 27,000 image registrations and 216,000 DSC computations. To perform these registrations we introduced a large cluster of high‐performance computers and developed a parallel testing harness. The metrics collected from the tests show a wide range of performance, indicating that parameter selection is crucial in our contouring system. By selecting an optimized parameter set, we increased the mean overlap of the automatically contoured regions of interest by 50% and reduced registration time by 50% compared to the original parameters. Our findings illustrate that full‐scale optimization is an effective method for improving the performance of the automated image contouring system.
Poster — Thur Eve — 66: A planning comparison between RapidArc and intensity modulated radiotherapy for head and neck cancer
Authors: Jiang R , Fleck A , Schaly B , Charland P , Zhan L , Alliet K , Rachakonda R , Osei E
Published: July 2012

Volumetric modulated arc therapy (VMAT) has recently been used to improve the dose distribution and efficiency of treatment delivery over the standard intensity‐modulated radiotherapy (IMRT) technique. This study compares the dosimetry between RapidArc plan and standard IMRT plan for head and neck cancer. Three head and neck patients treated clinically with sliding window intensity‐modulated radiotherapy (IMRT) technique at Grand River Regional Cancer Center were selected randomly and re‐planned using RapidArc technique with 6 MV photon beams generated by a Varian 21EX linac with 120‐leaf multileaf collimator. Three dose prescriptions were used to deliver 70 Gy, 63 Gy and 58.1 Gy to the regions of the primary tumors, intermediate‐risk nodes and low‐risk nodal level, respectively, in 35 fractions. Dosimetric comparison based on the dose‐volume histogram, target coverage, organ at risk (OAR) dose sparing were studied between the RapidArc plan and IMRT plan. RapidArc technique from Varian Medical Systems showed superior target coverage, better OAR sparing, fewer monitor units per fraction with less treatment time over IMRT technique for head and neck cancers. The average homogeneity index, defined as the difference between the percentage dose covering 5% and 95% of the PTV, is 9.5 for RapidArc plan and 10.5 for IMRT plan. All RapidArc plans met the dose objectives for the primary OAR: spinal cord, brainstem, brain etc. Both parotid mean dose and D50% are lower for RapidArc plan than those of the IMRT plan. The technique is currently being used clinically at our cancer center.
Poster — Thur Eve — 72: Conversion of helical tomotherapy plans into clinically favourable step‐and‐shoot IMRT plans deliverable on a c‐arm linac
Authors: Studinski RCN , Alexander A , La Russa D
Published: July 2012

The treatment planning software SharePlan is designed to convert dose distributions generated by the TomoTherapy planning station into step‐and‐shoot IMRT plans deliverable on a c‐arm linear accelerator. Five anal canal patients who were planned for TomoTherapy treatments were exported into a SharePlan system and plans were generated for delivery on an Elekta Synergy unit. A total of 80 plans were generated for those five patients, with either seven, nine, eleven or twenty‐one gantry angles and different priorities between focusing on matching either the target doses or healthy tissue sparing of the TomoTherapy plan. The plans generated by SharePlan, while often not matching target coverage at prescription, matched well the TomoTherapy coverage at 95% and 105% of the prescription dose. Organ at risk dose, when heavily emphazied in the SharePlan calculations matched or bettered the TomoTherapy dose due to the placement of the beams and the sharper sup‐inf fall off of the dose distribution on a linac. For one of the patients, it was possible to produce a better DVH with SharePlan than the original TomoTherapy plan for those reasons. The TomoTherapy plans boasted significantly shorter delivery times than the plans generated with SharePlan.
Poster — Thur Eve — 76: A quality control to achieve planning consistency in arc radiotherapy of the prostate
Authors: Zeng G , Murphy J , Annis S‐L , Wu X , Wang Y , McGowan T , Macpherson M
Published: July 2012

Purpose: To report a quality control program in prostate radiation therapy at our center that includes semi‐automated planning process to generate high quality plans and in‐house software to track plan quality in the subsequent clinical application. Material and methods: Arc planning in Eclipse v10.0 was preformed for both intact prostate and post‐prostatectomy treatments. The planning focuses on DVH requirements and dose distributions being able to tolerate daily setup variations. A modified structure set is used to standardize the optimization, including short rectum and bladder in the fields to effectively tighten dose to target and a rectum expansion with 1cm cropped from PTV to block dose and shape posterior isodose lines. Structure, plan and optimization templates are used to streamline plan generation. DVH files are exported from Eclipse to a quality tracking software with GUI written in Matlab that can report the dose‐volume data either for an individual patient or over a patient population. Results: For 100 intact prostate patients treated with 78Gy, rectal D50, D25, D15 and D5 are 30.1±6.2Gy, 50.6±7.9Gy, 65.9±6.0Gy and 76.6±1.4Gy respectively, well below the limits 50Gy, 65Gy, 75Gy and 78Gy respectively. For prostate bed with prescription of 66Gy, rectal D50 is 35.9±6.9Gy. In both sites, PTV is covered by 95% prescription and the hotspots are less than 5%. Conclusion: The semi‐automated planning method can efficiently create high quality plans while the tracking software can monitor the feedback from clinical application. It is a comprehensive and robust quality control program in radiation therapy.
Sci—Fri AM: Imaging — 03: Temperature dependence of a SiPM detector for an MR compatible PET system
Authors: Goertzen AL , Zhang X , Liu C‐Y , Kozlowski P , Retière F , Ryner L , Sossi V , Stortz G , Thompson CJ
Published: July 2012

Silicon photomultiplier (SiPM) detectors are rapidly becoming the detector of choice for research and development of new detectors for positron emission tomography (PET) due to their combination of high gain, fast timing, compact form factor and ability to function in a magnetic field. We are investigating using SiPM based detectors in a compact PET system designed to be inserted into a 7T animal MRI system and enable simultaneous PET/MRI imaging. In order to understand the level of thermal stability required for this PET system, we examined the stability of a prototype SiPM detector vs. temperature. A detector was constructed using a SensL SPMArray4 SiPM array coupled to a LYSO scintillator crystal array. The temperature of the detector was varied between 23 and 60°C in 5°C steps. At each temperature setting data were collected to characterize the detector flood histogram, photopeak amplitude and energy resolution at 511 keV, timing resolution and signal arrival time. While the flood image showed no noticeable changes with temperature, the 511 keV photopeak amplitude showed a linear decrease of 1.5%/°C and the energy resolution degraded by 0.08%/°C. The timing resolution degraded by 1.5 ns, from 3.5 ns to 5 ns when the temperature changed from 23 to 60°C. Over this temperature range there was a shift in the signal arrival time of approximately 3 ns. These results demonstrate that the detector can be operated over a wide range of temperature, giving a large degree of flexibility in choosing an operating temperature set‐point for our PET system.
Sci—Fri AM: Imaging — 04: SPECT‐based functional lung imaging in the prediction of radiation pneumonitis: A retrospective clinical and dosimetric correlation
Authors: Hoover D , Reid RH , Rodrigues G , Wong E , Stitt L , Yaremko BP
Published: July 2012

Purpose: To investigate whether functionally‐weighted dose‐volume histogram (DVH) parameters are more predictive of radiation‐induced pneumonitis (RP) than standard parameters such as V20 and mean lung dose (MLD). Materials and Methods: A retrospective chart review identified 26 patients who received curative‐intent radiation therapy for primary carcinoma of the lung. Prior to treatment, all patients received single photon emission computed tomography (SPECT) to assess both lung ventilation and lung perfusion. Patients were assessed for clinical RP using standard criteria and were separated into a non‐RP group (RP grade < 2) and an RP‐group (RP grade ≥ 2). Standard DVH parameters (V10, V20, V30, MLD) and their function‐weighted counterparts (for perfusion: pF10, pF20, pF30, pMLD; for ventilation: vF10, vF20, vF30, vMLD) were evaluated for each group. Receiver operating characteristics (ROC) curves were created and the area under the curve (AUC) computed. Results: 7 of 26 patients had grade ≥ 2 pneumonitis. Both pF20 (p=0.022) and vF20 (p=0.036) were significantly different between the 2 groups; V20 was not (p=0.06). Both pF30 (p=0.008) and vF30 (p=0.025) were significantly different between groups while V30 failed to reach significance (p=0.072). Standard MLD (p=0.011), pMLD (p=0.001), and vMLD (p=0.011) were all significantly different. The ROC curves indicated that both the perfusion‐weighted parameters and the ventilation‐weighted parameters outperformed the standard DVH parameters as predictors of RP grade ≥2. Conclusions: SPECT‐based, function‐weighted DVH parameters appear to be useful as predictors of RP.
Sci—Fri AM: Imaging — 08: Comparison of single and dual layer detector blocks for pre‐clinical MRI‐PET
Authors: Thompson C , Stortz G , Goertzen A , Berg E , Retière F , Kozlowski P , Ryner L , Sossi V
Published: July 2012

Our group is developing a small animal PET scanner which would fit in a 7 Tesla MRI scanner to provide simultaneous PET and MR acquisitions and images. There is very little room for the PET detectors and we must use nonmagnetic materials. This presentation describes preliminary work with two PET detectors consisting of LYSO blocks each with two crystal layers: 49 1.67*1.67*6.0 mm on the bottom layer and 36 4.0 mm deep crystals on the top layer. These are mounted on SensL 4*4 arrays of silicon photo‐multipliers whose outputs are multiplexed to provide three signals: Energy, X, Y. These detectors were mounted on translation stages and scanned past a 0.25 mm diameter 370 MBq Na‐18 source embedded in tissue equivalent plastic. The results were compared with similar single layer crystal blocks with 10 mm deep crystals to assess the advantage of dual layer crystals to reduce radial blurring in a PET scanner with a diameter of only 64 mm. The ability to identify correctly each crystal is reported as its resolvability index, (RI), defined as the (crystal response FWHM)/(crystal's separation) in the crystal identification matrix. For the dual layer block RI =0.44, and for dual layer block RI=0.22. The coincidence response resolution for the single layer block varied from 1.23±0.05 mm at the centre of the scanner to 3.09±0.10 mm at 15.8 mm radius, while the dual layer block varied from 1.31±0.06 to 1.96±0.51 mm over the same range, confirming the Monte‐Carlo simulations showing reduced radial blurring.
Sci—Fri AM: Imaging — 09: Serial estimation of cross‐talk for correction in dual‐isotope imaging with dynamic tracers
Authors: Wells RG , Lockwood J , Wei L , Duan D , Fernando P , Bensimon C , Ruddy TD
Published: July 2012

The recent radioisotope shortage has led to interest in non‐Tc99m‐based tracers. We have developed a novel I‐123‐labelled myocardial perfusion imaging tracer. We compare the I123‐tracer to the clinical standard of Tc99m tetrofosmin in vivo in a rat model using a small‐animal SPECT/CT camera. SPECT distinguishes different isotopes based on the different energies of the emitted gamma rays and thus allows simultaneous comparison of two tracer distributions in the same animal. Dual‐isotope imaging is complicated by cross‐talk between the energy windows of the isotopes. Standard energy‐window‐based correction methods are difficult to employ because of the proximity in energy of Tc99m (140keV) and I123 (159keV). Imaging the second tracer's energy window prior to its injection provides an estimate of the cross‐talk. However, this estimate is only accurate if the tracer distribution is static. We use serial imaging prior to the introduction of the second tracer to estimate the dynamics of the first tracer and interpolate the cross‐talk images to provide a more accurate correction. We used rat models of myocardial disease (n=3). I123 tracer was injected and imaged for one hour at 20min intervals. The Tc99m tetrofosmin was then injected and 30min later, a dual‐isotope image was obtained. The impact of this approach is assessed by comparing the differences in the Tc99m‐tetrofosmin image using this method with correction by simple correction for physical decay. The interpolative approach improves the accuracy of the correction by 2%–5% and thereby enhances the comparison of the two tracers.
Sci—Fri PM: Delivery — 02: CT image guidance strategies for dose‐adaptive IMRT of the prostate
Authors: Battista J , Johnson C , Kempe J , Turnbull D , VanDyk J , Bauman G
Published: July 2012

On‐line CT imaging in the radiotherapy room has become the norm for targeted intensity‐modulated radiotherapy (IMRT), enabling precise adjustments of the daily patient setup based on soft tissue visualization. Corrections for plasticity of the anatomy and dose deformation are within technological reach but will require more on‐line resources. We have developed a computer model that allows exploration of “what if” scenarios for assessing the benefits of Image Guidance strategies in terms of the multi‐fraction dose distribution and DVH metrics (Target D95 and rectum V70). In this work we report on changes in anatomy and resultant dose distribution as observed in 35 daily megavoltage CT (MVCT) scans of the pelvis during prostate therapy for 13 patients. Our goal is to assess the effectiveness and efficiency of various adaptive strategies involving imaging schedule with and without dose re‐planning of 5‐field IMRT with 18 MV x‐rays. Our research questions are: To what extent do radiation dose distributions delivered to individual patients (in vivo) diverge from the planned dose distributions (in silico)? Is there a robust schedule of CT image guidance, with or without dose re‐planning that will mitigate discrepancies? For prostate IMRT, we conclude that image guidance schedule can be relaxed when generous GTV margins (10/7mm) are used. Tighter margins (isotropic 5 mm) reduce the dose to the rectum as expected. However, daily re‐planning may be required to maintain adequate target coverage as planned when tighter margins are used.
Sci—Fri PM: Delivery — 07: Cyberknife relative output factor measurements using fiber‐coupled luminescence, MOSFETS and RADPOS dosimetry system
Authors: Ploquin N , Kertzscher G , Vandervoort E , Cygler JE , Andersen CE , Francescon P
Published: July 2012

Novel dosimetry systems based on Al2O3:C radioluminescence (RL) and a 4D dosimetry system (RADPOS) from Best Medical Canada were used to measure the relative output factor (ROF) on Cyberknife. Measurements were performed in a solid water phantom at the depth of 1.5 cm and SSD = 78.5 cm for cones from 5 to 60 mm. ROFs were also measured using a mobileMOSFET system (Best Medical Canada) and EBT1 and EBT2 GAFCHROMIC® (ISP, Ashland) radiochromic films. For cone sizes 12.5–60 mm all detector results were in agreement within the measurement uncertainty. The microMOSFET/RADPOS measurements (published corrections applied) yielded ROFs of 0.650 ± 1.9%, 0.811 ± 0.9% and 0.843 ± 1.7% for the 5, 7.5 and 10 mm cones, respectively, and were in excellent agreement with radiochromic film values (averaged for EBT1 and EBT2) of 0.645 ± 1.4%, 0.806 ± 1.1% and 0.859 ± 1.1%. Monte‐Carlo calculated correction factors were applied to the RL readings to correct for excessive scatter due to the relatively high effective atomic number of Al2O3 (Z=10.2) compared to water for the 5, 7.5 and 10 mm cones. When these corrections are applied to our RL detector measurements, we obtain ROFs of 0.656 ± 0.3% and 0.815 ± 0.3% and 0.865 ± 0.3% for 5, 7.5 and 10 mm cones. Our study shows that the microMOSFET/RADPOS and optical fiber‐coupled RL dosimetry system are well suited for Cyberknife cone output factors measurements over the entire range of field sizes, provided that appropriate correction factors are applied for the smallest cone sizes (5, 7.5 and 10 mm).
Sci—Fri PM: Delivery — 08: Characterizing the spatially varying fluence and spectra of a kV imaging source for dose calculations
Authors: Poirier Y , Kouznetsov A , Tambasco M
Published: July 2012

Kilovoltage (kV) daily image‐guided radiotherapy (IGRT) procedures accumulate radiation dose within the patient that is currently not routinely incorporated in the treatment plan. As part of the process of developing a patient‐specific kV dose computation tool, the kV x‐ray source must be characterized. We propose a simple, clinically feasible experimental characterization method using in‐air dose measurements along the transverse axis. We determine half‐value layer (HVL) along the transverse axis, from which we derive the HVL‐specific mass‐absorption coefficient, which is used to determine beam fluence. These values are interpolated over the entire field. The spectrum at each interpolation point in the field is found from HVL and accelerating potential (kVp) using third‐party software Spektr. We use this method to characterize the spatially varying fluence and spectra of a Varian® On‐Board Imaging® source for energies 80, 100 and 125 kVp. This characterization is used to compute dose within a heterogeneous phantom, using our previously validated in‐house dose computation software, which we compare with relative dose measurements. We show that for a 10×10 cm2 field size using no added filtration, the agreement for all three energies is within 2% for the central depth‐dose profile and within 2.6% for the transverse profiles. This clinically feasible experimental characterization method for kV imaging sources represents a crucial step in the development of a patient‐specific dose computation tool.
Sci—Fri PM: Delivery — 09: Response of a tumor xenograft model to radiation therapy using magnetic resonance spectroscopy
Authors: Tessier AG , Yahya A , Larocque M , Fallone BG , Syme A
Published: July 2012

9.4T 1H magnetic resonance spectroscopy (MRS) was utilized to track the response of mouse xenograft glioblastoma multiform (GBM) brain tumors to single fraction radiation therapy. Six metabolites were analyzed with LCModel: alanine (Ala), myo‐inositol (Ins), taurine (Tau), creatine and phosphocreatine (Cr + PCr), glutamine and glutamate (Glu + Gln), and total choline (glycerophosphocholine + phosphocholine) (GPC + PCh). 11 mice received 800 cGy of 200 kVp x‐rays, 5 were untreated controls. PRESS spectra (27 μL volumes) were acquired at multiple time points for treated and control animals. In treated animals, all metabolite : water ratios decreased 3 days post‐treatment, with further decreases at day 7, and then increases at day 14 relative to the 7 day mark. Concentrations on day 7 relative to pre‐treatment were as follows: 0.42 (Ala), 0.43 (Ins), 0.68 (Tau), 0.52 (GPC+PCh), 0.49 (Cr + PCr) and 0.78 (Glu + Gln). Metabolite ratios did not correlate with tumor volume in control animals, suggesting a real therapeutic response was observed. Our 1H MRS data suggests that perturbations in the metabolic signature of GBM cancers occur in response to irradiation. Such changes in the metabolite : water concentration ratios could potentially be exploited for the improvement of radiotherapy.