Constant Dose Rate Research Articles

Angular dependence of the output of a kilovoltage X‐ray therapy unit

During the recommissioning of a Philips RT‐250 kilovoltage X‐ray unit, unexpected output variations with tube head rotation (cross‐plane) and tube head tilt (in‐plane) were observed. The measured output showed an increase of up to 7.3% relative to the neutral position (0° in‐plane and 0° cross‐plane) over the possible range of angles of in‐plane rotation for 75 kVp (half‐value layer, HVL=1.84 mm Al). A less pronounced but noticeable output change (with respect to the neutral position) was observed for cross‐plane rotation reaching 2% for the 225 kVp beam (HVL=0.90 mm Cu). This output variation was observed while manually adjusting the current to maintain constancy according to the current meter gauge. In order to address the observed output dependence with head orientation, the dose rate monitor chamber of the kilovoltage unit was calibrated to monitor the beam output in real time. The dose rate was manually adjusted to maintain a constant dose rate (in r/min) as displayed on the r/min gauge. This approach resulted in maintaining beam output for the 75 kVp and the 225 kVp beams within ±2% for the in‐plane angle variation and ±0.5% for the cross‐plane angle variation. A daily output check that includes ion chamber‐based measurements at the neutral position and an in‐plane angle of 45° has been implemented using the constant dose rate approach to monitor the stability of the X‐ray beams. As a result of the output variations with in/cross‐plane rotation, the quality control (QC) procedures that are typically used for clinical setup have been modified to test the stability of the beams under the non‐neutral positioning of the X‐ray tube.PACS number: 87.56.Fc

SU-GG-T-237: Impact of Modeling the Treatment Couch for IMAT Treatments

Purpose: Intensity modulated arc therapy (IMAT) can result in superior plan quality and delivery efficiency. As part of the treatment planning optimization process the impact of the treatment couch is not always considered leading to treatments not being delivered as planned. In this work the dosimetric differences between plans calculated with and without the couch are studied using a phantom and then the clinical impact of delivering IMAT plans without the treatment couch outlined assessed. Method and Materials: Single and dual arc IMAT plans, obtained using Oncentra MasterPlan v3.3sp1 (Nucletron, BV), were planned and recalculated on a CT scan set of a 2D ionization chamber array (IBA MatriXX Evolution) positioned between slabs of 30×30cm solid water. The couch was either not considered or modeled as a 2.2cm thick object of density 1.02. A constant dose-rate was assumed during the planning process and a Varian 600CD with 120 leaf millennium MLC at 6MV used. Calculated and measured plans were compared using a gamma criteria of 3%/3mm. Using data from a prostate cancer patient, single and dual arc plans were created using both constant and variable dose-rate IMAT without the couch outlined and then recalculated with the couch outlined. Results: When not considering the treatment couch, the percentage of pixels passing the 3%/3mm gamma criteria was 77.62% and 70.65% for single and dual arc IMAT plans respectively. When the couch was included, the percentage of pixels passing increased to 96.54% and 99.18% for single and dual arc plans. IMAT prostate plans created without the couch but recalculated with the couch resulted in plans that were not considered to be clinically acceptable. Conclusion: It is essential that the treatment couch is properly accounted for in the IMAT planning process otherwise the delivered treatment will not match the treatment as planned.

SU-GG-T-504: Design and Verification of a Phantom for Accurate Delivery of Prostate Treatment Plans to Cells In-Vitro

Purpose: In-vitro investigations are typically performed using a single beam of radiation normally at kV energies. Emerging data has raised questions over whether this methodology results in outcomes that are representative for advanced radiation oncology delivery techniques such as IMRT and VMAT. It is important that cellular studies are undertaken in clinically relevant configurations. This requires the design of suitable phantoms and careful verification of the dosimetric accuracy of the delivery of different IMRT and VMAT techniques to the cells. Method and Materials: A PMMA phantom with an insert for a media filled T75 flask was designed and constructed. Conformal, IMRT and VMAT plans were created using Oncentra MasterPlan v3.3sp1 (Nucletron, BV) such that a uniform dose encapsulated the cell layer. All plans were subsequently recalculated on a CT scan set of a) a Farmer ionization chamber and b) a 2D ionization chamber array (IBA MatriXX Evolution) positioned between slabs of 30×30cm solid water. A constant dose-rate was assumed during the planning process and a Varian 600CD with 120 leaf millennium MLC at 6MV used. Calculated and measured plans were compared using gamma criteria of 3%/3mm for the 2D array. Each plan was delivered to the phantom with Gafchromic EBT film placed at the bottom of the flask. Film measurements were compared with those calculated. Results: Ion chamber results were within 1.8% of the expected value for all plans. All plans resulted in more than 98.7% pixels passing the gamma criteria. Gafchromic EBT film measurements were within 1.7% of those planned. Conclusion: A PMMA phantom has been constructed which has minimal air gaps and ensures accurate and uniform delivery of dose to cells within a T75 flask. This phantom can be used for in-vitro studies of complex delivery modalities to cells.

Transient luteolytic effect of prostaglandin E2 in humans.

Prostaglandin E2 was administered by i.v. infusion to ten non pregnant volunteers at a constant dose rate of 4.5μg/min for 5.5 hours with a mean total dose of 1.5 mg. The infusion was accompanied by flushing, local pain, erythema, abdominal cramps and vomiting. The menstrual cycles were normal in every aspect except its earlier occurrence in some women. An acute drop in plasma progesterone occurred after half an hour in the majority of cases, but this decline was ransient, since most cases showed a clear post-infusion recovery.

The letal Dose of Gamma Irradiation on Larvae of the species Dorcus parallelopipedus (Coleoptera: Lucanidae)

Larvae of many species from the family Lucanidae feed with wood of different trees and conducted gamma irradiation on larvae of Dorcus parallelopipedus (Linnaeus 1758) had the aim to find an efficient method of wood-protection during wood-work, transport and storing. During year 2008, 62 larvae were sampled, divided into two classes considering mass and length. There were 26 larvae with an average length of 16.5 mm and mass of 355.1 mg in the first class, 36 larvae with an average length of 31.8 mm and mass of 2101.5 mg in the second class. Before the irradiation, larvae of class I were divided into 5 groups with 4 specimens each, while larvae of class II were divided into 4 groups with 8 specimens. The rest of larvae were in the control group. A standard radiotherapy unit Co-60 Alcyion Cirus (CIS Bio International) was used for gamma irradiation. Both classes of larvae were exposed to a constant dose rate of 3.6 Gy min -1 in different intervals of time, accepting doses of 360, 720, 1080, 1440 and 1800 Gy. All irradiated larvae died after 55 days from irradiation while all specimens from the control group survived. The instant lethal doses for both classes were determined: 4238.3 Gy for larvae in class I and 2784.9 Gy for class II.

Cytogenetic Damage in Cells Exposed to Ionizing Radiation under Conditions of a Changing Dose Rate

The current international paradigm on the biological effects of radiation is based mainly on the effects of dose with some consideration for the dose rate. No allowance has been made for the potential influence of a changing dose rate (second derivative of dose), and the biological effects of exposing cells to changing dose rates have never been analyzed. This paper provides evidence that radiation effects in cells may depend on temporal changes in the dose rate. In these experiments, cells were moved toward or away from an X-ray source. The speed of movement, the time of irradiation, and the temperature during exposure were controlled. Here we report the results of the first experiments with TK6 cells that were exposed at a constant dose rate, at an increasing dose rate, or at a decreasing dose rate. The average dose rate and the total dose were same for all samples. Micronuclei were scored as the end point. The results show that the level of cytogenetic damage was higher in cells exposed to a decreasing dose rate compared to both an increasing and a constant dose rate. This finding may suggest that the second derivative of dose may influence radiation risk estimates, and the results should trigger further studies on this issue.

Dose Response and Optical Properties of Dyed Poly Vinyl Alcohol-Trichloroacetic Acid Polymeric Blends Irradiated with Gamma-Rays

Problem statement: The effects of gamma irradiation on optical properties of cresol-red dyed Poly Vinyl Alcohol (PVA) blended with Trichloroacetic Acid (TCA) for possible use in dosimetry and measurement of radiation dose in gamma rays have been studied using both Raman spectroscopy and UV-Visible spectrophotometer method. Approach: The dosimeters are composed of Poly Vinyl Alcohol (PVA), Trichloroacetic Acid (TCA) at various concentrations are 20, 25, 30 and 35%, and acid-base indicator cresol-red dyed. Results: The dosimeters were irradiated to doses up 12 kGy using 60Co gamma ray source at a constant dose rate. The polymeric films undergo color change from purple to yellow due to radiation-induced acid formation. The molecular vibrational spectra were measured using Raman spectroscopy, resulting in a decrease of the Raman intensity inelastic scattering of C-Cl molecular stretching from TCA with increasing dose. The absorption spectra were measured using UV-visible spectrophotometer in the wavelength range 350-700 nm, resulting in a decrease of the absorbance at 575 nm band peak with increasing dose. The dose sensitivity D0 increases with increasing TCA concentration for both scattering and absorption methods. The optical absorption studies show that the direct and indirect optical energy band gaps and optical activation energies are dependent on dose and TCA concentration. Conclusion: The shift in the optical band gap Eg values towards lower energy with radiation dose leads to a shift of the optical activation energy DeltaE value towards the lower energy region with increasing dose. The optical band gap (Eg) and the absorption edge decrease with increasing dose attributed to the structural disorder of polymer blends due to dehydrochlorination of trichloroacetic acid with increasing dose. The energy width of the tail of localized state in the forbidden band gap was evaluated using the Urbach-edges method. It was found that the activation energy (DeltaE) is less dependent of radiation dose but strongly dependent on concentration of blends.

Tumor cell survival dependence on helical tomotherapy, continuous arc and segmented dose delivery

The temporal pattern of radiation delivery has been shown to influence the tumor cell survival fractions for the same radiation dose. To study the effect more specifically for state of the art rotational radiation delivery modalities, 2 Gy of radiation dose was delivered to H460 lung carcinoma, PC3 prostate cancer cells and MCF-7 breast tumor cells by helical tomotherapy (HT), seven-field LINAC (7F), and continuous dose delivery (CDD) over 2 min that simulates volumetric rotational arc therapy. Cell survival was measured by the clonogenic assay. The number of viable H460 cell colonies was 23.2 ± 14.4% and 27.7 ± 15.6% lower when irradiated by CDD compared with HT and 7F, respectively, and the corresponding values were 36.8 ± 18.9% and 35.3 ± 18.9% lower for MCF7 cells (p < 0.01). The survival of PC3 was also lower when irradiated by CDD than by HT or 7F but the difference was not as significant (p = 0.06 and 0.04, respectively). The higher survival fraction from HT delivery was unexpected because 90% of the 2 Gy was delivered in less than 1 min at a significantly higher dose rate than the other two delivery techniques. The results suggest that continuous dose delivery at a constant dose rate results in superior in vitro tumor cell killing compared with prolonged, segmented or variable dose rate delivery.

Variable dose rate single-arc IMAT delivered with a constant dose rate and variable angular spacing

Single-arc intensity-modulated arc therapy (IMAT) has gained worldwide interest in both research and clinical implementation due to its superior plan quality and delivery efficiency. Single-arc IMAT techniques such as the Varian RapidArc™ deliver conformal dose distributions to the target in one single gantry rotation, resulting in a delivery time in the order of 2 min. The segments in these techniques are evenly distributed within an arc and are allowed to have different monitor unit (MU) weightings. Therefore, a variable dose-rate (VDR) is required for delivery. Because the VDR requirement complicates the control hardware and software of the linear accelerators (linacs) and prevents most existing linacs from delivering IMAT, we propose an alternative planning approach for IMAT using constant dose-rate (CDR) delivery with variable angular spacing. We prove the equivalence by converting VDR-optimized RapidArc plans to CDR plans, where the evenly spaced beams in the VDR plan are redistributed to uneven spacing such that the segments with larger MU weighting occupy a greater angular interval. To minimize perturbation in the optimized dose distribution, the angular deviation of the segments was restricted to ⩽± 5°. This restriction requires the treatment arc to be broken into multiple sectors such that the local MU fluctuation within each sector is reduced, thereby lowering the angular deviation of the segments during redistribution. The converted CDR plans were delivered with a single gantry sweep as in the VDR plans but each sector was delivered with a different value of CDR. For four patient cases, including two head-and-neck, one brain and one prostate, all CDR plans developed with the variable spacing scheme produced similar dose distributions to the original VDR plans. For plans with complex angular MU distributions, the number of sectors increased up to four in the CDR plans in order to maintain the original plan quality. Since each sector was delivered with a different dose rate, extra mode-up time (xMOT) was needed between the transitions of the successive sectors during delivery. On average, the delivery times of the CDR plans were approximately less than 1 min longer than the treatment times of the VDR plans, with an average of about 0.33 min of xMOT per sector transition. The results have shown that VDR may not be necessary for single-arc IMAT. Using variable angular spacing, VDR RapidArc plans can be implemented into the clinics that are not equipped with the new VDR-enabled machines without compromising the plan quality or treatment efficiency. With a prospective optimization approach using variable angular spacing, CDR delivery times can be further minimized while maintaining the high delivery efficiency of single-arc IMAT treatment.

Synthesis and characterization of polypropylene‐grafted gelatin

AbstractModification of polypropylene (PP) has been successfully carried out by graft copolymerization of gelatin, by chemical method using benzoyl peroxide (BPO) as radical initiator. PP was preirradiated by gamma rays at a constant dose rate of 3.40 kGy/h before grafting to introduce hydroperoxide groups. Optimum conditions pertaining to maximum percentage of grafting were evaluated as a function of different reaction parameters. Maximum percentage of grafting of gelatin (115%) was obtained at BPO concentration = 4.132 × 10−2 mol/L in 120 min at 70°C using 30 mL of water. Characterization of PP‐g‐gelatin was carried out through FTIR, thermogravimetric analysis, and scanning electron microscopy. Swelling studies were carried out in pure, binary, ternary, and quaternary solvent systems comprising distilled water, ethanol, dimethylsulphoxide (DMSO), and N,N‐dimethylformamide (DMF) in different ratios. Maximum swelling of PP‐g‐gelatin (both composite and true graft) was observed in pure solvents that is, DMSO followed by DMF, ethanol, and water and was higher than that observed in the mixed solvent system. Water retention studies of unmodified PP, that is, pristine PP and PP‐g‐gelatin (both composite and true graft) were investigated at different time periods, temperatures and pH. Maximum % water retention for PP‐g‐gelatin (composite; 170%) was observed in 8 h at 50°C in neutral medium (pH = 7). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

SU-FF-T-663: Treatment Plan Comparison Generated by Volumetric Arc Modulation with Helical Tomotherapy and Conventional IMRT for Prostate Cancer Patients

Purpose: To compare the treatment plans generated by volumetric arc modulation with helical tomotherapy and conventional IMRT for prostate cancer patients. Method and Materials: This study was performed on 5 prostate patients originally planned using Tomotherapy Hi‐Art™ treatment planning system (TPS). To make the study comprehensive and to evaluate all the degrees of freedom available with volumetric modulation arc therapy (VMAT), six plans were generated other than the tomotherapy plan. These were conventional 7 fields IMRT, full and partial arc, each with constant and variable dose rate and full arc with constant dose rate having gantry spacing of 4 degrees. The VMAT and IMRT plans were developed using Philips Medical Systems Pinnacle3 TPS. The IMRT objectives were selected on the basis of standard protocols followed in the clinic. PTV coverage was prioritized so that the organs at risk (OAR) sparing can be compared uniformly. Results: The study shows that for prostate cases, VMAT can generate plans equivalent to conventional IMRT and Tomotherapy plans. However, conventional IMRT outperforms the rotational TPS when it comes to OAR sparing. VMAT treatment time typically takes about 1/5th the treatment time of a regular IMRT or Tomotherapy plan with equivalent target coverage and organ sparing. While comparing the plans within VMAT, better rectal wall sparing was achieved with either partial arc (with both constant and variable dose rate) or full arc with variable dose rate compared to full arc with constant dose rate. Conclusion: In this study we have found that in prostate, we can achieve plans comparable to Tomotherapy plans. Total treatment time is very crucial for Radiotherapy due to issues related to interfraction and intrafraction motion. Treatment time taken for VMAT is very less when compared to conventional IMRT and tomotherapy treatment times with same PTV coverage and comparable amount for OAR sparing.

SU-FF-T-649: A Comparison of Constant Dose Rate SmartArc and Helical Tomotherapy

Purpose: The purpose of this study was to compare constant dose rate and gantry rotation speed intensity modulated arc delivery planned with the new Pinnacle SmartArc planning system and delivered on a Varian 21EX with helical tomotherapy (HT). Materials and Methods: A series of thirty patients that were treated with HT were randomly selected and re‐planned for SmartArc treatment delivery on a Varian 21EX equipped with a 120‐leaf MLC. All SmartArc plans were optimized for delivery using a constant dose rate and gantry speed, which eliminated the need for special VMAT licensees. The same contours, objectives, and prescriptions were used for the HT and SmartArc plans. SmartArc dosimetric verification was performed using a cylindrical solid water phantom with radiographic film and with an ionization chamber array. The MLC log files for the SmartArc treatment deliveries were also downloaded and analyzed. Results: 25 of the test cases were planned with a single arc, while 5 cases were planned with double arcs. The mean delivery time was 2.5 minutes, and the mean number of monitor units was 547. For the prostate test cases, the SmartArc plans were remarkably similar to the tomotherapy plans. The dose uniformity of the PTV for the SmartArc cases was actually superior to the tomotherapy dose uniformity for most of the prostate cases. For both the lung cases and H&N cases, the PTV uniformity and normal tissue DVHs were slight worse in the SmarArc plans as compared to the HT plans. Conclusion: SmartArc treatment plans can be created for delivery using a constant dose rate and gantry rotation speed that are very similar to helical tomotherapy plans for prostate, lung, and simple H&N patients. These plans can be delivered in 2–3 minutes per arc without the need to purchase special VMAT licenses on most accelerators.

SU‐EE‐A2‐04: Alternative Delivery Approach for RapidArc Using Constant Dose Rate

Purpose: Varian RapidArc™ is a new treatment technique that delivers conformal dose distributions to the target in one single gantry rotation. The segments in the treatment arc are allowed to have different monitor unit (MU) weightings requiring variable dose‐rate (VDR) for delivery. An alternative delivery approach for (VDR) RapidArc plans that utilizes constant dose‐rate (CDR) was developed. Method and Materials: For four patient cases (two head‐and‐neck (HN), one brain, one prostate), RapidArc plans were generated using the Eclipse treatment planning system. The resultant VDR treatment plans were converted into CDR plans by re‐distributing the equi‐spaced segments in the VDR arc such that the segments with larger MU weighting occupy a greater angular interval. To minimize perturbation from the optimized dose distribution, the angular deviation of the segments was restricted to 5°. This restriction requires the treatment arc to be broken into multiple sectors such that the local MU fluctuation within each sector is reduced, thereby lowering the angular deviation of the segments during re‐distribution. The converted broken‐sector CDR (b‐CDR) plans were delivered with a single gantry sweep as in the VDR plans but each sector was delivered with different values of CDR. Results and Conclusion: For plans with complex angular MU distribution, the number of broken sectors may increase in the b‐CDR plans in order to maintain the original plan quality. All b‐CDR plans produced similar dose distributions to the VDR plans by dissecting the arc up to 5 sectors. On average, the delivery times of the b‐CDR plans were 1 minute longer than that in the VDR plans. Since the majority of treatment time is spent on the patient setup, these results show that VDR RapidArc plans can be delivered using CDR without significantly compromising the plan quality or treatment efficiency.Research sponsored by Varian Medical Systems.

SU‐FF‐T‐120: The Impact of Variable Gantry Rotation Speed and Dose‐Rate On the Quality of Aperture‐Modulated Arc Therapy (AMAT)

Purpose: From its early inception, DMLC‐based IMRT has differed from Binary MLC‐based helical IMRT by delivering the treatment volumetrically. As the field of IMRT evolved, the demands on its performance converge to three main criteria: quality of dose plans, speed of treatment delivery, and amount of leakage exposure. Many have ventured the approach of DMLC‐based aperture‐modulated arc (AMAT) therapies, indicating their potential to satisfy these three criteria. The traditional arc therapy is delivered with a constant gantry rotation speed and a constant dose rate. It is logical to expect that if the outputs at any transient gantry position can vary, the capacity or flexibility of dose optimization would increase, regardless of the type of aperture modulation algorithms. The technology of arc delivery with varying gantry rotating speed and dose‐rate has recently become available. We conducted a study to investigate the impact of these new arcing parameters. Method and Materials: The study is conducted on the following equipments: a Varian Trilogy linear accelerator, the Varian Eclipse planning system and the Pinnacle SmartArc module. The linac can operate in constant dose‐rate/gantry rotation mode or varying dose‐rate/gantry rotation mode. Plans were derived with constant dose‐rate/gantry rotation and varying dose‐rate/gantry rotation. The comparisons were carried out as objectively as possible based on the three criteria with set benchmarks. Results: From a limited range of examples studied, it is observed that while AMAT with constant dose rate and gantry rotation is capable of producing good dose plans, dose‐rate and gantry rotation variation allows further improvement of the dose plan and the delivery efficiency. MLC inter‐digitation also increases the flexibility of planning and the delivery efficiency. Conclusion: The quality of plan and the treatment delivery efficiency are inter‐related. While plan evaluation is often subjective, a greater technical flexibility undoubtedly increases planning capacity and improves delivery efficiency.

SU-FF-T-174: Evaluation of a New Dynamic Arc IMRT Optimization

Purpose: The quality and accuracy of delivery of both prostate and esophagus plans generated using of a new dynamic arc optimization algorithm (SmartArc) within the Pinnacle TPS have been evaluated and compared with standard step-and-shoot (SS) techniques. Method and Materials: Two types of single-arc plans have been evaluated, one that is delivered using a constant dose-rate (CDR) and gantry speed, and one that is delivered using both a variable dose-rate and gantry speed (VMAT). The former can be delivered on most linear accelerators; the latter requires an accelerator that has been licensed for VMAT treatments. Prostate plans were all optimized following the UK CHHIP trial protocol, which has three PTV dose levels. Esophagus plans were optimized following a local protocol. In both cases plans were created for Varian accelerators equipped with a 120-leaf MLC. The delivery of plans was evaluated using Delta-4 (ScandiDos). Results: In all prostate cases plans met the target coverage criteria. The CDR arc plans tended to give the highest hot-spot. For the prostate plans the CDR arc plans and SS plans gave approximately equivalent rectal and bladder DVHs, with the arc-plan femoral head DVHs being better. The VMAT plans gave improved DVHs for all outlined organs-at-risk for prostate plans. For esophagus plans, the CDR arc plans struggled to meet the target coverage criteria, whereas the VMAT plans gave acceptable coverage and improved organ-at-risk sparing. In all cases the measured quality assurance results were good with no significant differences between the different delivery methods. For the VMAT plans delivery time was 75–90 seconds and 2.5–3.0 minutes for the CDR-arc plans. Conclusion: The Pinnacle SmartArc optimizer provides a very useful tool for generating accurately deliverable IMRT plans on a wide range of accelerators. Conflict of Interest: This work was performed in collaboration with Philips Radiation Oncology Systems

Development and evaluation of an efficient approach to volumetric arc therapy planning

An efficient method for volumetric intensity modulated arc therapy (VMAT) planning was developed, where a single arc (360 degrees or less) is delivered under continuous variation of multileaf collimator (MLC) segments, dose rate, and gantry speed. Plans can be generated for any current linear accelerator that supports these degrees of freedom. MLC segments are derived from fluence maps at relatively coarsely sampled angular positions. The beam segments, dose rate, and gantry speed are then optimized using direct machine parameter optimization based on dose volume objectives and leaf motion constraints to minimize arc delivery time. The method can vary both dose rate and gantry speed or alternatively determine the optimal plan at constant dose rate and gantry speed. The method was used to retrospectively generate variable dose rate VMAT plans to ten patients (head and neck, prostate, brain, lung, and tonsil). In comparison to step-and-shoot intensity modulated radiation therapy, dosimetric plan quality was comparable or improved, estimated delivery times ranged from 70 to 160 s, and monitor units were consistently reduced in nine out of the ten cases by an average of approximately 6%. Optimization and final dose calculation took between 5 and 35 min depending on plan complexity.

Comparison of TID Effects in Space-Like Variable Dose Rates and Constant Dose Rates

The degradation of the LM193 dual voltage comparator has been studied at different TID dose rate profiles, including several different constant dose rates and a variable dose rate that simulates the behavior of a solar flare. A comparison of results following constant dose rate vs. variable dose rates is made to explore how well the constant dose rates used for typical part testing predict the performance during a simulated space-like mission. Testing at a constant dose rate equal to the lowest dose rate seen during the simulated flare provides an extremely conservative estimate of the overall amount of degradation. A constant dose rate equal to the average dose rate is also more conservative than the variable rate. It appears that, for this part, weighting the dose rates by the amount of total dose received at each rate (rather than the amount of time at each dose rate) results in an average rate that produces an amount of degradation that is a reasonable approximation to that received by the variable rate.

Experimental measurements and Monte Carlo simulations for dosimetric evaluations of intrafraction motion for gated and ungated intensity modulated arc therapy deliveries

Respiratory gated radiation therapy allows for a smaller margin expansion for the planning target volume (PTV) to account for respiratory induced motion and is emerging as a common method to treat lung and liver tumors. We investigated the dosimetric effect of free motion and gated delivery for intensity modulated arc therapy (IMAT) with experimental measurements and Monte Carlo simulations. The impact of PTV margin and duty cycle for gated delivery is studied with Monte Carlo simulations. A motion phantom is used for this study. Two sets of contours were drawn on the mid-inspiration CT scan of this motion phantom. For each set of contours, an IMAT plan to be delivered with constant dose rate was created. The plans were generated on a CT scan of the phantom in the static condition with 3 mm PTV margin and applied to the motion phantom under four conditions: static, full superior–inferior (SI) motion (A = 1 cm, T = 4 s) and gating conditions (25% and 50% duty cycles) with full SI motion. A 6 by 15 cm piece of radiographic film was placed in the sagittal plane of the phantom and then irradiated under all measurement conditions. Film calibration was performed with a step-wedge method to convert optical density to dose. Gated IMAT delivery was first validated in 2D by comparing static film with that from gating and full motion. A previously verified simulation tool for IMRT that takes the log files from the multileaf collimator (MLC) controller and the gating system were adapted to simulate the delivered IMAT treatment for full 3D dosimetric analysis. The IMAT simulations were validated against the 2D film measurements. The resultant IMAT simulations were evaluated with dose criteria, dose-volume histograms and 3D gamma analysis. We validated gated IMAT deliveries when we compared the static film with the one from gating using 25% duty cycle using 2D gamma analysis. Within experimental and setup uncertainties, film measurements agreed with their corresponding simulated plans using 2D gamma analysis. Finally, when planning with margins designed for gating with 25% duty cycle and applying 50% or no gating during treatment, the dose differences in Dmin, D99% and D95% of the clinical target volume can be up to 27 cGy, 20 cGy and 18 cGy, respectively, for a plan with 200 cGy prescription dose. We have experimentally delivered gated IMAT with constant dose rate to a motion phantom and assessed their accuracies with film dosimetry and Monte Carlo simulations. Film dosimetry demonstrated that 25% gating and static plans are within 2%, 2 mm. The Monte Carlo simulation method was employed to generate dose delivered in 3D to a motion phantom, and the dosimetric results were reported. Since our film measurements agreed well with Monte Carlo simulations, we can reliably use this simulation tool to further study the dosimetric effects of target motion and effectiveness of gating for IMAT deliveries.

Rate of elapsed polymerization of hydroxyethylacrylate gel induced by gamma irradiation

The rate of elapsed polymerization of polyhydroxyethylacrylate in gelatin has been studied to investigate the effect of co-monomers consumption at a given dose. The polymer gel dosimeters consisted of 2%-4% N,N-methyelene-bis-acrylamide cross-linker, 2%-4% 2-hydroxyethylacrylate monomer and gelatin at 3% and 5%. The dosimeters were irradiated by using 60Co teletherapy γ-ray source up to 20 Gy at a constant dose rate. The relaxation rate of water proton in the dosimeters at different doses and co-monomer concentrations were measured using a nuclear magnetic resonance spectroscopy. The rate of elapsed polymerization decreases with increasing the dose and the initial concentration of co-monomers. The rate of consumption of co-monomers increases with an increase of the polymerization and the gelatin content of the polymer gel.

TH‐C‐350‐01: Planning and Delivery of IMAT: Comparisons of Single and Multiple Arc Approaches

Purpose: To investigate Intensity Modulated Arc Therapy plans (IMAT) with variable dose rate single arc and constant dose rate multiple arc approaches; to provide a formula for the prediction of their delivery times, and a metric to evaluate IMAT treatment plan adaptability. Method and Materials: IMAT plans were generated for localized prostate and oropharynx cancers. For each site, we generated IMAT plans for delivery with variable dose rate single arc and with constant dose rate multiple arcs. Treatment delivery times were modeled mathematically. A maximum leaf motion per unit gantry angle metric is used here to measure the complexity of leaf motions in treatment deliveries, and we proposed to use this metric to assess IMAT plan adaptability. IMAT plans were compared based on dosimetric qualities (target conformality and uniformity indices, dose‐volume indices of critical organs), delivery times, and plan adaptabilities. All IMAT plans were generated on Pinnacle 8.1v (Philips Medical Systems) for Varian linacs. Results: Overall, both IMAT approaches have comparable dosimetric plan qualities, although generating variable dose rate single arc IMAT plans required increased angular sampling to accommodate larger leaf motions, especially when there are two or more target volumes. The delivery time for the single arc is limited by both the gantry rotational speed and the maximum leaf motion between field segments, whereas, the multiple arc approach is generally limited by the gantry rotational speed alone. For the prostate plan, the delivery times are 1.2 minutes and 2 minutes respectively. In the oropharynx case, the single arc and the multiple arc deliveries were similar at approximately 2 minutes. The adaptability indices reflected the complexity of leaf motion in the single arc compared to the multiple arc approach. Conclusion: IMAT plans with variable and constant dose rate deliveries were evaluated under multiple considerations for potential clinical utilizations.