Delivered Dose Distribution Research Articles

SU‐E‐J‐15: A Patient‐Centered Scheme to Mitigate Impacts of Treatment Setup Error

Purpose:Current Intensity Modulated Radiation Therapy (IMRT) is plan‐centered. At each treatment fraction, we position the patient to match the setup in treatment plan. Inaccurate setup can compromise delivered dose distribution, and hence leading to suboptimal treatments. Moreover, current setup approach via couch shift under image guidance can correct translational errors, while rotational and deformation errors are hard to address. To overcome these problems, we propose in this abstract a patient‐centered scheme to mitigate impacts of treatment setup errors.Methods:In the patient‐centered scheme, we first position the patient on the couch approximately matching the planned‐setup. Our Supercomputing Online Replanning Environment (SCORE) is then employed to design an optimal treatment plan based on the daily patient geometry. It hence mitigates the impacts of treatment setup error and reduces the requirements on setup accuracy. We have conducted simulations studies in 10 head‐and‐neck (HN) patients to investigate the feasibility of this scheme. Rotational and deformation setup errors were simulated. Specifically, 1, 3, 5, 7 degrees of rotations were put on pitch, roll, and yaw directions; deformation errors were simulated by splitting neck movements into four basic types: rotation, lateral bending, flexion and extension. Setup variation ranges are based on observed numbers in previous studies. Dosimetric impacts of our scheme were evaluated on PTVs and OARs in comparison with original plan dose with original geometry and original plan recalculated dose with new setup geometries.Results:With conventional plan‐centered approach, setup error could lead to significant PTV D99 decrease (−0.25∼+32.42%) and contralateral‐parotid Dmean increase (−35.09∼+42.90%). The patientcentered approach is effective in mitigating such impacts to 0∼+0.20% and −0.03∼+5.01%, respectively. Computation time is <128 s.Conclusion:Patient‐centered scheme is proposed to mitigate setup error impacts using replanning. Its superiority in terms of dosimetric impacts and feasibility has been shown through simulation studies on HN cases.

SU‐E‐CAMPUS‐T‐01: Analysis of the Precision of Patient Set‐Up, and Fidelity of the Delivered Dose Distribution in Proton Therapy of Ocular Tumors

Purpose:To quantify daily set‐up variations in fractionated proton therapy of ocular melanomas, and to assess the effect on the fidelity of delivered distribution to the plan.Methods:In a typical five‐fraction course, daily set‐up is achieved by matching the position of fiducial markers in orthogonal radiographs to the images generated by treatment planning program. A patient maintains the required gaze direction voluntarily, without the aid of fixation devices. Confirmation radiographs are acquired to assess intrafractional changes. For this study, daily radiographs were analyzed to determine the daily iso‐center position and apparent gaze direction, which were then transferred to the planning system to calculate the dose delivered in individual fractions, and accumulated dose for the entire course. Dose‐volume metrics were compared between the planned and accumulated distributions for the tumor and organs at risk, for representative cases that varied by location within the ocular globe.Results:The analysis of the first set of cases (3 posterior, 3 transequatorial and 4 anterior tumors) revealed varying dose deviation patterns, depending on the tumor location. For anterior and posterior tumors, the largest dose increases were observed in the lens and ciliary body, while for the equatorial tumors, macula, optic nerve and disk, were most often affected. The iso‐center position error was below 1.3 mm (95%‐confidence interval), and the standard deviation of daily polar and azimuthal gaze set‐up were 1.5 and 3 degrees, respectively.Conclusion:We quantified interfractional and intrafractional set‐up variation, and estimated their effect on the delivered dose for representative cases. Current safety margins are sufficient to maintain the target coverage, however, the dose delivered to critical structures often deviates from the plan. The ongoing analysis will further explore the patterns of dose deviation, and may help to identify particular treatment scenarios which are at a higher risk for such deviations.

TH‐E‐BRE‐05: Analysis of Dosimetric Characteristics in Two Leaf Motion Calculator Algorithms for Sliding Window IMRT

Purpose:The Smart leaf motion calculator (SLMC) in Eclipse treatment planning system is an advanced fluence delivery modeling algorithm as it takes into account fine MLC features including inter‐leaf leakage, rounded leaf tips, non‐uniform leaf thickness, and the spindle cavity etc. In this study, SLMC and traditional Varian LMC (VLMC) algorithms were investigated, for the first time, in dosimetric characteristics and delivery accuracy of sliding window (SW) IMRT.Methods:The SW IMRT plans of 51 cancer cases were included to evaluate dosimetric characteristics and dose delivery accuracy from leaf motion calculated by SLMC and VLMC, respectively. All plans were delivered using a Varian TrueBeam Linac. The DVH and MUs of the plans were analyzed. Three patient specific QA tools – independent dose calculation software IMSure, Delta4 phantom, and EPID portal dosimetry were also used to measure the delivered dose distribution.Results:Significant differences in the MUs were observed between the two LMCs (p≤0.001).Gamma analysis shows an excellent agreement between the planned dose distribution calculated by both LMC algorithms and delivered dose distribution measured by three QA tools in all plans at 3%/3 mm, leading to a mean pass rate exceeding 97%. The mean fraction of pixels with gamma < 1 of SLMC is slightly lower than that of VLMC in the IMSure and Delta4 results, but higher in portal dosimetry (the highest spatial resolution), especially in complex cases such as nasopharynx.Conclusion:The study suggests that the two LMCs generates the similar target coverage and sparing patterns of critical structures. However, SLMC is modestly more accurate than VLMC in modeling advanced MLC features, which may lead to a more accurate dose delivery in SW IMRT. Current clinical QA tools might not be specific enough to differentiate the dosimetric discrepancies at the millimeter level calculated by these two LMC algorithms.NIH/NIGMS grant U54 GM104944, Lincy Endowed Assistant Professorship

WE-F-16A-03: 3D Printer Application in Proton Therapy: A Novel Method to Deliver Passive-Scattering Proton Beams with a Fixed Range and Modulation for SRS and SRT

Purpose: To present a novel technique to deliver passive-scattering proton beam with fixed range and modulation using a 3D printed patient-specific bolus for proton stereotactic radiosurgery and radiotherapy. Methods: A CIRS head phantom was used to simulate a patient with a small brain lesion. A custom bolus was created in the Eclipse Treatment Planning System (TPS) to compensate for the different water equivalent depths from the patient surface to the target from multiple beam directions. To simulate arc therapy, a plan was created on the initial CT using three passive-scattering proton beams with a fixed range and modulations irradiating from different angles. The DICOM-RT structure file of the bolus was exported from the TPS and converted to STL format for 3D printing. The phantom was rescanned with the printed custom bolus and head cup to verify the dose distribution comparing to the initial plan. EBT3 films were placed in the sagital plane of the target to verify the delivered dose distribution. The relative stopping power of the printing material(ABSplus-P430) was measured using the Zebra multi-plate ion chamber. Results: The relative stopping power of the 3D printing material, ABSplus-P430 was 1.05 which is almost water equivalent. The dose difference between verificationmore » CT and Initial CT is almost negligible. Film measurement also confirmed the accuracy for this new proton delivery technique. Conclusion: Our method using 3D printed range modifiers simplify the treatment delivery of multiple passive-scattering beams in treatment of small lesion in brain. This technique makes delivery of multiple beam more efficient and can be extended to allow arc therapy with proton beams. The ability to create and construct complex patient specific bolus structures provides a new dimension in creating optimized quality treatment plans not only for proton therapy but also for electron and photon therapy.« less

Image-guided IMRT for localized prostate cancer with daily repositioning: Inferring the difference between planned dose and delivered dose distribution

IntroductionTo investigate the dosimetric impact of daily on-line repositioning during a full course of IMRT for prostate cancer. Materials and methodsTwenty patients were treated with image-guided IMRT. Each pre-treatment plan (Plan A) was compared with a post-treatment plan sum (Plan B) based on couch shifts measured. The delivered dose to the prostate without a daily repositioning was inferred by considering each daily couch shift during the whole course of image-guided IMRT (i.e. plan B). Dose metrics were compared for prostate CTV (P-CTV) and PTV (P-PTV) and for organs at risk. Ten patients were treated with a 5 mm margin and 10 patients with a 10 mm margin. ResultsFor plan A vs plan B: the average D95, D98, D50, D mean and EUD were: 76.4 Gy vs 73.9 Gy (p = 0.0007), 75.4 Gy vs 72.3 Gy (p = 0.001), 78.9 Gy vs 78.4 Gy (p = 0.014), 78.7 Gy vs 77.8 Gy (p = 0.003) and 78.1 Gy vs 75.9 Gy (p = 0.002), respectively for P-CTV, and 73.2 Gy vs 69.3 Gy (p = 0.0006), 70.7 Gy vs 66.0 Gy (p = 0.0008), 78.3 Gy vs 77.5 Gy (p = 0.001), 77.8 Gy vs 76.4 Gy (p = 0.0002) and 74.4 Gy vs 69.2 Gy (p = 0.003), respectively for P-PTV. Margin comparison showed no differences in dose metrics between the two plans except for D98 of the rectum in plan B which was significantly higher with a 10 mm margin. ConclusionsThe absence of daily image-guided IMRT resulted in a significantly less uniform and less homogeneous dose distribution to the prostate. A reduction in PTV margin showed neither a lower target coverage nor a better spare of OAR with and without daily image-guided IMRT.

A dosimetric comparison of volumetric modulated arc therapy with step-and-shoot intensity modulated radiation therapy for prostate cancer

PurposeTo compare variable dose-rate volumetric modulated arc therapy (VMAT) with 7-field, step-and-shoot intensity modulated radiation therapy (IMRT) in prostate cancer patients treated with a consistent planning target volume (PTV) to a uniform total radiation therapy dose. Methods and materialsWe studied 32 patients who received 8100 cGy in 45 daily fractions to their prostate and proximal 1 cm of the seminal vesicles using variable dose rate VMAT (n = 22) or 7-field, step-and-shoot IMRT (n = 10) for intermediate-risk or high-risk prostate cancer between July 2010 and April 2013. In 90% of patients, VMAT was delivered with 2 arcs. To have an unbiased comparison of plan quality, patients who were treated with VMAT were also planned with IMRT and vice versa. Each patient served as his own control for the comparison. ResultsVMAT reduced median radiation beam-on time from 4.3 to 3.4 minutes (P = .03). There was no statistically significant difference in PTV volumes between the VMAT and step-and-shoot IMRT groups (P = .76). VMAT dose distributions were more homogeneous (P = .003). There was no difference between groups with regard to rectal V60, V65, V70, V75, bladder V65, V70, V75, V80, or femoral heads V33. ConclusionsTwo-arc VMAT resulted in shorter beam-on times and more homogenous dose distributions than 7-field, step-and-shoot IMRT for prostate cancer. With decreased beam-on time, the intrafraction motion during irradiation is reduced, thus improving confidence that the delivered dose distribution agrees with the plan.

γ+ index: A new evaluation parameter for quantitative quality assurance

PurposeThe accuracy of dose delivery and the evaluation of differences between calculated and delivered dose distributions, has been studied by several groups. The aim of this investigation is to extend the gamma index by including radiobiological information and to propose a new index that we will here forth refer to as the gamma plus (γ+). Furthermore, to validate the robustness of this new index in performing a quality control analysis of an IMRT treatment plan using pure radiobiological measures such as the biologically effective uniform dose (D¯¯) and complication-free tumor control probability (P+). Material and methodsA new quality assurance index, the (γ+), is proposed based on the theoretical concept of gamma index presented by Low et al. (1998). In this study, the dose difference, including the radiobiological dose information (biological effective dose, BED) is used instead of just the physical dose difference when performing the γ+ calculation. An in-house software was developed to compare different dose distributions based on the γ+ concept. A test pattern for a two-dimensional dose comparison was built using the in-house software platform. The γ+ index was tested using planar dose distributions (exported from the treatment planning system) and delivered (film) dose distributions acquired in a solid water phantom using a test pattern and a theoretical clinical case. Furthermore, a lung cancer case for a patient treated with IMRT was also selected for the analysis. The respective planar dose distributions from the treatment plan and the film were compared based on the γ+ index and were evaluated using the radiobiological measures of P+ and D¯¯. ResultsThe results for the test pattern analysis indicate that the γ+ index distributions differ from those of the gamma index since the former considers radiobiological parameters that may affect treatment outcome. For the theoretical clinical case, it is observed that the γ+ index varies for different treatment parameters (e.g. dose per fraction). The dose area histogram (DAH) from the plan and film dose distributions are associated with P+ values of 50.8% and 49.0%, for a D¯¯ to the target of 54.0Gy and 53.3Gy, respectively. ConclusionThe γ+ index shows advantageous properties in the quantitative evaluation of dose delivery and quality control of IMRT treatments because it includes information about the expected responses and radiobiological doses of the individual tissues.

EP-1658: Does the delivery technique impact the effect of respiratory motion in stereotactic ablative body radiotherapy?

Purpose/Objective: To examine whether the impact of respiratory motion on the delivered dose distributions for a range of tumour sizes in SABR, differs between 3DCRT,sliding window IMRT and RapidArc. Materials and Methods: Three patient data sets with different lung tumour sizes were selected: T1=6cc, T2=31cc and T3=60cc. Each was planned in Eclipse for SABR using 3DCRT, sliding window IMRT and VMAT, creating 9 treatment plans which were then delivered to a dynamic thorax phantom. The phantom was programmed to move at a typical patient breathing amplitude of 15mm with a period of 5 seconds and Varian linacs were used for the delivery. EBT3 Gafchromic film was used in coronal and sagittal planes for measuring dose distributions. Static phantom measurements were compared with the TPS calculated plans to establish agreement between expected and measured dose distributions without motion, using the software OmniPro I'mRT. Comparisons of static and dynamic phantom measurements followed. Global gamma analysis was used to carry out a relative comparison between the three delivery techniques. Five regions of the gamma index map (Middle, Proximal Left, Proximal Right, Distal Left, Distal Right) were analysed to quantify the differences along the axis of motion. The criteria used for the gamma analysis were 3%/3mm, with a threshold of 20%. Results: The setup and delivery accuracy was confirmed by the agreement between planned and static delivered dose distributions. The average percentage of pixels passing was 100% (T1),100% (T2) and 98.46% (T3). The comparison of films with and without motion gave lower percentages of pixels passing, ranging between 33.68 - 59.94% (T1), 47.86 - 61.77% (T2) and 43.44 - 64.32% (T3). Comparison of the delivery technique, showed passing rates of 33.63 - 52.25% (3DCRT), 43.44 - 64.52% (IMRT) and 46.58- 56.08% (VMAT). Analysis of the five regions for all delivery techniques gave averages of 93.76% (Middle), 58.7% (Proximal) and 12.8% (Distal). For 3DCRT results were 87.08% (Middle), 46.52% (Proximal) and 12.54% (Distal), for IMRT were 96.45%, 69.20%, 14.14% and for VMAT 97.75%, 60.39% and 11.71%, respectively. Conclusions: The results are indicative of the intra-fractional respiratory motion-induced dosimetric inaccuracies caused in three SABR delivery techniques.On average, the impact is greatest in the distal regions, significant in the proximal regions, whereas the middle region is less susceptible to these effects. No noticeable difference was observedbetween coronal and sagittal planes. The results also suggest that the effect of motion is greater in the proximal regions for 3DCRT in relation to the other techniques, particularly with smaller tumour sizes.

A preliminary study: The impact of manufacturing conditions on the dosimetric characteristics of N-vinylpyrrolidone based polymer gels

s / Physica Medica 30 (2014) e45ee74 e71 were also investigated with the Delta4PT phantom by performing the test in oblique gantry angles. Results: Results obtained through the two methods showed very good agreement as far as significant dispositions are concerned. For slight inaccuracies Delta4PT provided accurate numerical results. As far as gravity effects are concerned a very slight diversification was observed for an oblique gantry setup. Discussion: EBT2 films provide a relatively fast, qualitative visual inspection of the significant leaf dispositions. When slight inaccuracies need to be revealed or precise numerical results for each leaf position are needed, Delta4PT provides the desired accuracy. In treatment modalities where a higher accuracy is required in the delivered dose distribution, such as in IMRT, a 3D dosimetric phantom is recommended for the MLC positional inspection.

Feasibility of using the Vero SBRT system for intracranial SRS

The Vero SBRT system was benchmarked in a planning study against the Novalis SRS system for quality of delivered dose distributions to intracranial lesions and assessing the Vero system's capacity for SRS. A total of 27 patients with one brain lesion treated on the Novalis system, with 3 mm leaf width MLC and C‐arm gantry, were replanned for Vero, with a 5 mm leaf width MLC mounted on an O‐ring gantry allowing rotations around both the horizontal and vertical axis. The Novalis dynamic conformal arc (DCA) planning included vertex arcs, using 90° couch rotation. These vertex arcs cannot be reproduced with Vero due to the mechanical limitations of the O‐ring gantry. Alternative class solutions were investigated for the Vero. Additionally, to distinguish between the effect of MLC leaf width and different beam arrangements on dose distributions, the Vero class solutions were also applied for Novalis. In addition, the added value of noncoplanar IMRT was investigated in this study. Quality of the achieved dose distributions was expressed in the conformity index (CI) and gradient index (GI), and compared using a paired Student's t‐test with statistical significance for p‐values ≤0.05. For lesions larger than 5 cm3, no statistical significant difference in conformity was observed between Vero and Novalis, but for smaller lesions, the dose distributions showed a significantly better conformity for the Novalis (ΔCI=13.74%, p=0.0002) mainly due to the smaller MLC leaf width. Using IMRT on Vero reduces this conformity difference to nonsignificant levels. The cutoff for achieving a GI around 3, characterizing a sharp dose falloff outside the target volume was 4 cm3 for Novalis and 7 cm3 for Vero using DCA technique. Using noncoplanar IMRT, this threshold was reduced to 3 cm3 for the Vero system. The smaller MLC and the presence of the vertex fields allow the Novalis system to better conform the dose around the lesion and to obtain steeper dose falloff outside the lesion. Comparable dosimetric characteristics can be achieved with Vero for lesions larger than 3 cm3 and using IMRT.PACS number: 87.55.D

A simple planning technique of craniospinal irradiation in the eclipse treatment planning system.

A new planning method for Craniospinal Irradiation by Eclipse treatment planning system using Field alignment, Field-in-Field technique was developed. Advantage of this planning method was also studied retrospectively for previously treated five patients of medulloblastoma with variable spine length. Plan consists of half beam blocked parallel opposed cranium, and a single posterior cervicospine field was created by sharing the same isocenter, which obviates divergence matching. Further, a single symmetrical field was created to treat remaining Lumbosacral spine. Matching between a inferior diverging edge of cervicospine field and superior diverging edge of a Lumbosacral field was done using the field alignment option. ′Field alignment′ is specific option in the Eclipse Treatment Planning System, which automatically matches the field edge divergence as per field alignment rule. Multiple segments were applied in both the spine field to manage with hot and cold spots created by varying depth of spinal cord. Plan becomes fully computerized using this field alignment option and multiple segments. Plan evaluation and calculated mean modified Homogeneity Index (1.04 and 0.1) ensured that dose to target volume is homogeneous and critical organ doses were within tolerance. Dose variation at the spinal field junction was verified using ionization chamber array (I′MatriXX) for matched, overlapped and gap junction spine fields; the delivered dose distribution confirmed the ideal clinical match, over exposure and under exposure at the junction, respectively. This method is simple to plan, executable in Record and Verify mode and can be adopted for various length of spinal cord with only two isocenter in shorter treatment time.

A retrospective analysis for patient-specific quality assurance of volumetric-modulated arc therapy plans

Volumetric-modulated arc therapy (VMAT) is now widely used clinically, as it is capable of delivering a highly conformal dose distribution in a short time interval. We retrospectively analyzed patient-specific quality assurance (QA) of VMAT and examined the relationships between the planning parameters and the QA results. A total of 118 clinical VMAT cases underwent pretreatment QA. All plans had 3-dimensional diode array measurements, and 69 also had ion chamber measurements. Dose distribution and isocenter point dose were evaluated by comparing the measurements and the treatment planning system (TPS) calculations. In addition, the relationship between QA results and several planning parameters, such as dose level, control points (CPs), monitor units (MUs), average field width, and average leaf travel, were also analyzed. For delivered dose distribution, a gamma analysis passing rate greater than 90% was obtained for all plans and greater than 95% for 100 of 118 plans with the 3%/3-mm criteria. The difference (mean ± standard deviation) between the point doses measured by the ion chamber and those calculated by TPS was 0.9% ± 2.0% for all plans. For all cancer sites, nasopharyngeal carcinoma and gastric cancer have the lowest and highest average passing rates, respectively. From multivariate linear regression analysis, the dose level (p = 0.001) and the average leaf travel (p < 0.001) showed negative correlations with the passing rate, and the average field width (p = 0.003) showed a positive correlation with the passing rate, all indicating a correlation between the passing rate and the plan complexity. No statistically significant correlation was found between MU or CP and the passing rate. Analysis of the results of dosimetric pretreatment measurements as a function of VMAT plan parameters can provide important information to guide the plan parameter setting and optimization in TPS.

Discrepancies between Calculated and Delivered Dose Distributions of Respiratory Gated IMRT Fields according to the Target Motion Ranges for Lung and Liver Cancer Patients

242 This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0013701 and 2013R1A1A2012013). Received 15 October 2014, Revised 12 November 2014, Accepted 19 November 2014 Correspondence: Sangwook Lim (medicalphysics@hotmail.com) Tel: 82-51-990-6393, Fax: 82-2-6280-6247 cc This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Discrepancies between Calculated and Delivered Dose Distributions of Respiratory Gated IMRT Fields according to the Target Motion Ranges for Lung and Liver Cancer Patients

Design and implementation of a film dosimetry audit tool for comparison of planned and delivered dose distributions in high dose rate (HDR) brachytherapy

A novel phantom is presented for ‘full system’ dosimetric audit comparing planned and delivered dose distributions in HDR gynaecological brachytherapy, using clinical treatment applicators. The brachytherapy applicator dosimetry test object consists of a near full-scatter water tank with applicator and film supports constructed of Solid Water, accommodating any typical cervix applicator. Film dosimeters are precisely held in four orthogonal planes bisecting the intrauterine tube, sampling dose distributions in the high risk clinical target volume, points A and B, bladder, rectum and sigmoid. The applicator position is fixed prior to CT scanning and through treatment planning and irradiation. The CT data is acquired with the applicator in a near clinical orientation to include applicator reconstruction in the system test. Gamma analysis is used to compare treatment planning system exported RTDose grid with measured multi-channel film dose maps. Results from two pilot audits are presented, using Ir-192 and Co-60 HDR sources, with a mean gamma passing rate of 98.6% using criteria of 3% local normalization and 3 mm distance to agreement (DTA). The mean DTA between prescribed dose and measured film dose at point A was 1.2 mm. The phantom was funded by IPEM and will be used for a UK national brachytherapy dosimetry audit.

Measurement of characteristic prompt gamma rays emitted from oxygen and carbon in tissue-equivalent samples during proton beam irradiation

The purpose of this work was to characterize how prompt gamma (PG) emission from tissue changes as a function of carbon and oxygen concentration, and to assess the feasibility of determining elemental concentration in tissues irradiated with proton beams. For this study, four tissue-equivalent water–sucrose samples with differing densities and concentrations of carbon, hydrogen, and oxygen were irradiated with a 48 MeV proton pencil beam. The PG spectrum emitted from each sample was measured using a high-purity germanium detector, and the absolute detection efficiency of the detector, average beam current, and delivered dose distribution were also measured. Changes to the total PG emission from 12C (4.44 MeV) and 16O (6.13 MeV) per incident proton and per Gray of absorbed dose were characterized as a function of carbon and oxygen concentration in the sample. The intensity of the 4.44 MeV PG emission per incident proton was found to be nearly constant for all samples regardless of their carbon concentration. However, we found that the 6.13 MeV PG emission increased linearly with the total amount (in grams) of oxygen irradiated in the sample. From the measured PG data, we determined that 1.64 × 107 oxygen PGs were emitted per gram of oxygen irradiated per Gray of absorbed dose delivered with a 48 MeV proton beam. These results indicate that the 6.13 MeV PG emission from 16O is proportional to the concentration of oxygen in tissue irradiated with proton beams, showing that it is possible to determine the concentration of oxygen within tissues irradiated with proton beams by measuring 16O PG emission.

Analysis and evaluation of planned and delivered dose distributions: practical concerns with γ- and χ- Evaluations

One component of clinical treatment validation, for example in the commissioning of new radiotherapy techniques or in patient specific quality assurance, is the evaluation and verification of planned and delivered dose distributions. Gamma and related tests (such as the chi evaluation) have become standard clinical tools for such work. Both functions provide quantitative comparisons between dose distributions, combining dose difference and distance to agreement criteria. However, there are some practical considerations in their utilization that can compromise the integrity of the tests, and these are occasionally overlooked especially when the tests are too readily adopted from commercial software. In this paper we review the evaluation tools and describe some practical concerns. The intent is to provide users with some guidance so that their use of these evaluations will provide valid rapid analysis and visualization of the agreement between planned and delivered dose distributions.

Integration of a real‐time tumor monitoring system into gated proton spot‐scanning beam therapy: An initial phantom study using patient tumor trajectory data

In spot-scanning proton therapy, the interplay effect between tumor motion and beam delivery leads to deterioration of the dose distribution. To mitigate the impact of tumor motion, gating in combination with repainting is one of the most promising methods that have been proposed. This study focused on a synchrotron-based spot-scanning proton therapy system integrated with real-time tumor monitoring. The authors investigated the effectiveness of gating in terms of both the delivered dose distribution and irradiation time by conducting simulations with patients' motion data. The clinically acceptable range of adjustable irradiation control parameters was explored. Also, the relation between the dose error and the characteristics of tumor motion was investigated. A simulation study was performed using a water phantom. A gated proton beam was irradiated to a clinical target volume (CTV) of 5 × 5 × 5 cm(3), in synchronization with lung cancer patients' tumor trajectory data. With varying parameters of gate width, spot spacing, and delivered dose per spot at one time, both dose uniformity and irradiation time were calculated for 397 tumor trajectory data from 78 patients. In addition, the authors placed an energy absorber upstream of the phantom and varied the thickness to examine the effect of changing the size of the Bragg peak and the number of required energy layers. The parameters with which 95% of the tumor trajectory data fulfill our defined criteria were accepted. Next, correlation coefficients were calculated between the maximum dose error and the tumor motion characteristics that were extracted from the tumor trajectory data. With the assumed CTV, the largest percentage of the data fulfilled the criteria when the gate width was ± 2 mm. Larger spot spacing was preferred because it increased the number of paintings. With a prescribed dose of 2 Gy, it was difficult to fulfill the criteria for the target with a very small effective depth (the sum of an assumed energy absorber's thickness and the target depth in the phantom) because of the sharpness of the Bragg peak. However, even shallow targets could be successfully irradiated by employing an adequate number of paintings and by placing an energy absorber of sufficient thickness to make the effective target depth more than 12 cm. The authors also observed that motion in the beam direction was the main cause of dose distortion, followed by motion in the lateral plane perpendicular to the scan direction. The results suggested that by properly adjusting irradiation control parameters, gated proton spot-scanning beam therapy can be robust to target motion. This is an important first step toward establishing treatment plans in real patient geometry.

SU‐E‐T‐223: Dose Distribution Verification in a Moving Target Using Moving Platform and 2D Diode Array

Purpose: The aim of this study was to carry out a dosimetric verification of statically planned sliding window IMRT for respiratory‐like moving targets using 2D diode array. Methods: Ten 5‐field IMRT plans for mobile lung tumors were created. The target volumes were delineated on the free breathing CT scans. Three tumor motion trajectories have been simulated, with the largest movement amplitude in cranio‐caudal direction of 4 mm, 6 mm and 8 mm, and breathing periods of 4 s. The breathing motion was simulated using the motion kernels derived from the literature data. The moving platform (MotionSim XY/4D‐Sun Nuclear) with the diode array MapCHECK 2 (Sun Nuclear) was used.The evaluation was based on the Gamma analysis (3mm, 3%). The scoring parameter was the percentage of the field area (defined by the jaws plus 1 cm margin) resulting with γ &lt; 1 (%score). Results: The average score for static target equals 99.43% (γ=0.330). When the breathing trajectory with maximum amplitude of 4 mm was applied we have found an average score of 97.81% (γ=0.386), which fitted well to the plan acceptance criteria (%score &gt; 95% and average gamma &lt; 0.6). For 6 mm amplitude average score was 94.18% (γ=0.468). However when 8 mm movement of target was applied we have observed significant change in analysed parameters: average score and γ of 83.68% and 0.572, respectively. Conclusion: This study showed that statically planned nonuniform dose distribution could cause the tumor under‐dosage due to interplay effect. It is therefore crucial to verify amplitude of individual patient breathing motion trajectory and take it into account by increased margins.For maximal amplitudes of breathing trajectory below 4 mm the disagreement between planned and delivered dose distribution can be neglected. Whereas respiratory motion with amplitude above 6 mm leads to significant changes in delivered dose distribution.

SU-E-J-221: MVCT-Detector Data Sinograms for In Vivo Quality Assurance of Helical TomoTherapy Treatments of Left Sided Breast Cancer Patients: Effect of Soft-Tissue Deformations

Purpose: Objectives are to compute delivered doses in megavoltage CT (MVCT) datasets using the MVCT‐detector‐data sinograms (MVCT‐DDS), collected during breast‐cancer treatments, and to study the impact of soft‐tissue‐deformations (SD) on dose‐volume‐histograms (DVHs) of the PTVs and organs at risk (OARs). Methods: Tomotherapy Delivery Verification (DV) software was used to reconstruct delivered fluence using pretreatment MVCTs and MVCT‐DDS for 4 patients (20 fractions) treated in our clinic. Delivered fluence was back projected onto the MVCT datasets to compute dose distributions to the patients anatomy at the time of delivery. A commercial deformation algorithm was used to deform planning CT‐datasets to match the pre‐treatment anatomy. The same deformation matrix was applied to plan structures (PS) for true volumes. Both image fusions and deformed structures (DS) were reviewed by a radiation oncologist and a medical physicist. DS were manually edited when necessary. Only fractions with the entire treatment volume contained in the MVCT scans were used. CT‐datasets, planned and reconstructed‐doses (RD), PS and DS were exported to CERR for computing dose‐volume‐indices (DVI). DVI of the planned and reconstructed doses, of both non‐deformed (RD‐ND) and deformed (RD‐D) structures, were averaged and compared. Dose deformations were ignored in this study. Results: PTV D9 5 was 49.1±0.6Gy, 48.0±0.8Gy, 45.8±1.0Gy for the plan, RD‐ND and RD‐D volumes. PTV dose heterogeneity (D5–D9 5) increased by 3.0Gy for deformed targets. Mean dose (MD) of both lungs reduced slightly for both non‐deformed and deformed volumes, however, they were not significant. Heart MD, V2 5 and V3 5 were also non‐significantly reduced. The Sign tests revealed that absolute relative differences between planned and delivered dose distributions were significantly greater than 5% (p<0.05) for D9 5 and D5–D9 5 and heart V2 5 . Conclusion: This study showed that its feasible to reconstruct delivered fraction‐doses in pretreatment MVCT datasets utilizing MVCT‐DDS. Soft‐tissue deformations are important for accurate dose volume evaluations. Research is partly funded by Tomotherapy Inc now called Accuray Inc

TU‐E‐141‐04: Dose Reconstruction for DMLC Tracking and Gating in Adaptive Prostate Radiotherapy

Purpose: To assess the accuracy of dynamic multileaf collimator (DMLC) tracking delivery and the comparison to gating as an intervention strategy for adaptive prostate radiotherapy. Methods: An experimentally benchmarked isocenter shift dose reconstruction method was used to retrospectively assess the dose distributions delivered during previously observed intensity modulated arc therapy (IMAT)(RapidArc) fractions and compared to the dose distributions which would have been delivered had DMLC tracking or gating intervention strategies been implemented. Delivered dose distributions were constructed by incorporating the monitored prostate motions with the patients original treatment plan, to mimic the treatment delivery. These were compared with dose distributions calculated for DMLC tracking delivery created using the same dose reconstruction method with the addition of MLC positions from dynalog files obtained during tracking simulation with the original motion input. The tracking dose distributions were then compared to the gated dose distributions, constructed using altered motion files created to mimic the application of a gating threshold of 3mm for 5 seconds. Results: The delivered dose distributions showed that dosimetric effects of intrafraction prostate motion were substantial for some fractions with a decrease of more than 5% from the planned PTV D95 value. Evaluation of dose distributions for DMLC tracking delivery showed that tracking was effective in improving the PTV D95 to within 2% of planned value for all fractions, while gating improved the PTV D95 to within 4% of the planned values. Conclusion: Intrafraction prostate motion can Result in a decrease in target coverage. The results of our study suggest that the implementation of DMLC tracking would allow for higher accuracy in the delivery of prostate cancer radiotherapy treatments beyond the accuracy achievable through gating intervention. This work is supported by an NHMRC Australia Fellowship