Dosimetric Quality Research Articles

SU‐E‐T‐326: Repeated CT‐Scans in Pulsed Doserate Prostate Brachytherapy: Assessment of Deviations from the Treatment Plan

Purpose: Estimation of deviations between planned and delivered dose in Pulsed Doserate (PDR) brachytherapy for prostate cancer. Methods: A boost of 28.8 Gy is given with PDR brachytherapy in addition to 46 Gy delivered with External Beam RT. Brachytherapy is given in 24 pulses of 120 cGy, with an interpulse period time of 2.0 hours, resulting in a treatment time of over 46 hours. For 31 patients, additional CT‐scans were made apart from the Treatment Planning CT, i.e., one at 24 hours after start of PDR treatment and one shortly before finishing PDR treatment. On the second and third CT, the brachytherapy catheters were newly reconstructed and the treating physician delineated the PTV and organs at risk. Dwell positions and dwell times as used for the original Treatment Plan were imported into the newly reconstructed catheters and the dose distribution was recalculated. Plan comparison parameters were prostate V100 and D90 and rectum and bladder D2cc. Results: Averaged over 3 CT scans and all patients, the prostate V100 decreased 1.2% and D90 decreased 2.7%. For rectum, D2cc was within the tolerance dose (96 cGy/pulse) for all patients on the planning CT, but exceeded the tolerance dose on scan #2 in 7/31 patients with maximally 46% and in scan #3 in 5/25 patients with maximally 29%. Also for bladder D2cc was within the tolerance dose (120 cGy/pulse) for all patients on the planning CT. Here, the tolerance dose was exceeded on scan #2 in 2/31 patients with maximally 14% and in scan #3 in 3/25 patients with maximally 28%. Conclusions: In PDR prostate brachytherapy the relatively long treatment time has no clinically relevant deteriorating effect on the dosimetric quality of the treatment. PTV dose hardly deviates from planned dose, while OAR tolerance doses are rarely exceeded and only in small volumes.

SU-E-T-610: Impact of Variable Beam Spot Size on Treatment Time in Particle Therapy.

The dosimetric advantage of particle therapy comes with a much higher infrastructure investment and operation costs. Increasing patient throughput is a key factor to manage operation costs. We investigate the impact of variable beam spot sizes on treatment time and discuss the tradeoffs involved. The following realistic assumptions were used. (1) The beam traveling speed is independent of the beam spot size. (2) The beam spot is a 2D Gaussian. Changing the beam spot size implies varying the standard deviation. (3) The maximum beam intensity is a constant independent of the beam spot size. Increasing the beam spot reduces the fluence. (4) Varying the beam spot size incurs in a reset time penalty.A 2D tumor was used in the study. Dose calculations were based on pencil beam kernels from GEANT4.The total treatment time is divided into the beam travel time, the beam-on time, andthe time for changing the spot size. We found that: (1) Changing the beam spot size has no impact on the beam-on time, because the maximum beam intensity is independentof the beam spot and increasing the beam spot only reduces the fluence. (2) Larger beam spot size shortens the total travel time inversely proportional to the radius of the beam spot. (3) Plans with different beam spot sizes have similar dosimetric qualities. (4) If higher beam intensity could be used for larger beam spot size, savings in beam-on time would be inversely proportional to the intensity available. We have studied the interplay among beam intensity, travel time, and beam size reset time for a scanning beam with variable beam spot size. Our initial studies show necessary conditions for and limitations on savings in total treatment times. Further studies are being carried out to find additional time saving sources. Supported in part by NSF CBET-0853157.

Comparison of 16 mm OSU‐Nag and COMS eye plaques

OSU‐NAG eye plaques use fewer sources than COMS‐plaques of comparable size, and do not employ a Silastic seed carrier insert. Monte Carlo modeling was used to calculate 3D dose distributions for a 16 mm OSU‐NAG eye plaque and a 16 mm COMS eye plaque loaded with either Iodine‐125 or Cesium‐131 brachytherapy sources. The OSU‐NAG eye plaque was loaded with eight sources forming two squares, whereas the COMS eye plaque was loaded with thirteen sources approximating three isocentric circles. A spherical eyeball 24.6 mm in diameter and an ellipsoid‐like tumor 6 mm in height and 12 mm in the major and minor axes were used to evaluate the doses delivered. To establish a fair comparison, a water seed carrier was used instead of the Silastic seed carrier designed for the traditional COMS eye plaque. Calculations were performed on the dose distributions along the eye plaque axis and the DVHs of the tumor, as well as the 3D distribution. Our results indicated that, to achieve a prescription dose of 85 Gy at 6 mm from the inner sclera edge for a six‐day treatment, the OSU‐NAG eye plaque will need 6.16 U/source and 6.82 U/source for 125I and 131Cs, respectively. The COMS eye plaque will require 4.02 U/source and 4.43 U/source for the same source types. The dose profiles of the two types of eye plaques on their central axes are within 9% difference for all applicable distances. The OSU‐NAG plaque delivers about 10% and 12% more dose than the COMS for 125I and 131Cs sources, respectively, at the inner sclera edge, but 6% and 3% less dose at the opposite retina. The DVHs of the tumor for two types of plaques were within 6% difference. In conclusion, the dosimetric quality of the OSU‐NAG eye plaque used in eye plaque brachytherapy is comparable to the COMS eye plaque.PACS number: 87.56B, 87.55k, 87.55kh

Optimising the dosimetric quality and efficiency of post‐prostatectomy radiotherapy: A planning study comparing the performance of volumetric‐modulated arc therapy (VMAT) with an optimised seven‐field intensity‐modulated radiotherapy (IMRT) technique

The purpose of this study was to compare and evaluate radiotherapy treatment plans using volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) for post-prostatectomy radiotherapy. The quality of radiotherapy plans for 10 patients planned and treated with a seven-field IMRT technique for biochemical failure post-prostatectomy were subsequently compared with 10 prospectively planned single-arc VMAT plans using the same computed tomography data set and treatment planning software. Plans were analysed using parameters to assess for target volume coverage, dose to organs at risk (OAR), biological outcomes, dose conformity and homogeneity, as well as the total monitor units (MU), planning and treatment efficiency. The mean results for the study population are reported for the purpose of comparison. For IMRT, the median dose to the planning target volume, V(95%) and D(95%) was 71.1 Gy, 98.9% and 68.3 Gy compared with 71.2 Gy, 99.2% and 68.6 Gy for VMAT. There was no significant difference in the conformity index or homogeneity index. The VMAT plans achieved better sparing of the rectum and the left and right femora with a reduction in the median dose by 7.9, 6.3 and 3.6 Gy, respectively. The total number of monitor units (MU) was reduced by 24% and treatment delivery time by an estimated 3 min per fraction without a significant increase in planning requirements. VMAT can achieve post-prostatectomy radiotherapy plans of comparable quality to IMRT with the potential to reduce dose to OAR and improve the efficiency of treatment delivery.

Dosimetry concepts for scanner quality assurance and tissue dose assessment in micro-CT

At present, no established methods exist for dosimetry in micro computed tomography (micro-CT). The purpose of this study was therefore to investigate practical concepts for both dosimetric scanner quality assurance and tissue dose assessment for micro-CT. The computed tomography dose index (CTDI) was adapted to micro-CT and measurements of the CTDI both free in air and in the center of cylindrical polymethyl methacrylate (PMMA) phantoms of 20 and 32 mm diameter were performed in a 6 month interval with a 100 mm pencil ionization chamber calibrated for low tube voltages. For tissue dose assessment, z-profile measurements using thermoluminescence dosimeters (TLDs) were performed and both profile and CTDI measurements were compared to Monte Carlo (MC) dose calculations to validate an existing MC tool for use in micro-CT. The consistency of MC calculations and TLD measurements was further investigated in two mice cadavers. CTDI was found to be a reproducible quantity for constancy tests on the micro-CT system under study, showing a linear dependence on tube voltage and being by definition proportional to mAs setting and z-collimation. The CTDI measured free in air showed larger systematic deviations after the 6 month interval compared to the CTDI measured in PMMA phantoms. MC calculations were found to match CTDI measurements within 3% when using x-ray spectra measured at our micro-CT installation and better than 10% when using x-ray spectra calculated from semi-empirical models. Visual inspection revealed good agreement for all z-profiles. The consistency of MC calculations and TLD measurements in mice was found to be better than 10% with a mean deviation of 4.5%. Our results show the CTDI implemented for micro-CT to be a promising candidate for dosimetric quality assurance measurements as it linearly reflects changes in tube voltage, mAs setting, and collimation used during the scan, encouraging further studies on a variety of systems. For tissue dose assessment, MC calculations offer an accurate and fast alternative to TLD measurements allowing for dose calculations specific to any geometry and scan protocol.

Volumetric modulated arc therapy for nasopharyngeal carcinoma: A dosimetric comparison with TomoTherapy and step-and-shoot IMRT

PurposeVolumetric modulated arc therapy (VMAT), a novel technique, employs a linear accelerator to conduct dynamic modulation rotation radiotherapy. The goal of this study was to compare VMAT with helical tomotherapy (HT) and step-and-shoot intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC) patients with regard to the sparing effect on organs at risk (OARs), dosimetric quality, and efficiency of delivery. Materials and methodsTwenty patients with NPC treated by HT were re-planned by VMAT (two arcs) and IMRT (7–9 fields) for dosimetric comparison. The target area received three dose levels (70, 60, and 54Gy) in 33 fractions using simultaneous integrated boosts technique. The Philips Pinnacle Planning System 9.0 was adopted to design VMAT, using SmartArc as the planning algorithm. For a fair comparison, the planning target volume (PTV) coverage of the 3 plans was normalized to the same level. Dosimetric comparisons between VMAT, HT, and IMRT plans were analyzed to evaluate (1) coverage, homogeneity, and conformity of PTV, (2) sparing of OARs, (3) delivery time, and (4) monitor units (MUs). ResultsThe VMAT, HT, and IMRT plans had similar PTV coverage with an average of 96%. There was no significant difference between VMAT and HT in homogeneity, while the homogeneity indices of VMAT (1.06) and HT (1.06) were better than IMRT plans (1.07, p<0.05). HT plans provided a better conformity index (1.17) than VMAT (1.28, p=0.01) and IMRT (1.36, p=0.02). When compared with IMRT, VMAT and HT had a better sparing effect on brain stem and spinal cord (p<0.05). The effect of parotid sparing was similar between VMAT (mean=26.3Gy) and HT (mean=27.5Gy), but better than IMRT (mean=31.3Gy, p<0.01). The delivery time per fraction for VMAT (5.7min) were much lower than for HT (9.5min, p<0.01) and IMRT (9.2min, p<0.01). ConclusionsOur results indicate that VMAT provides better sparing of normal tissue, homogeneity, and conformity than IMRT, and shorter delivery time than HT.

Biological-based optimization and volumetric modulated arc therapy delivery for stereotactic body radiation therapy

To describe biological-based optimization and Monte Carlo (MC) dose calculation-based treatment planning for volumetric modulated arc therapy (VMAT) delivery of stereotactic body radiation therapy (SBRT) in lung, liver, and prostate patients. Optimization strategies and VMAT planning parameters using a biological-based optimization MC planning system were analyzed for 24 SBRT patients. Patients received a median dose of 45 Gy [range, 34-54 Gy] for lung tumors in 1-5 fxs and a median dose of 52 Gy [range, 48-60 Gy] for liver tumors in 3-6 fxs. Prostate patients received a fractional dose of 10 Gy in 5 fxs. Biological-cost functions were used for plan optimization, and its dosimetric quality was evaluated using the conformity index (CI), the conformation number (CN), the ratio of the volume receiving 50% of the prescription dose over the planning target volume (Rx/PTV50). The quality and efficiency of the delivery were assessed according to measured quality assurance (QA) passing rates and delivery times. For each disease site, one patient was replanned using physical cost function and compared to the corresponding biological plan. Median CI, CN, and Rx/PTV50 for all 24 patients were 1.13 (1.02-1.28), 0.79 (0.70-0.88), and 5.3 (3.1-10.8), respectively. The median delivery rate for all patients was 410 MU/min with a maximum possible rate of 480 MU/min (85%). Median QA passing rate was 96.7%, and it did not significantly vary with the tumor site. VMAT delivery of SBRT plans optimized using biological-motivated cost-functions result in highly conformal dose distributions. Plans offer shorter treatment-time benefits and provide efficient dose delivery without compromising the plan conformity for tumors in the prostate, lung, and liver, thereby improving patient comfort and clinical throughput. The short delivery times minimize the risk of patient setup and intrafraction motion errors often associated with long SBRT treatment delivery times.

Dosimetric characteristics and quality control tests for the collimator sectors of the Leksell Gamma Knife®PerfexionTM

The purpose of this study was to evaluate the dosimetric characteristics of each sector of the Leksell Gamma Knife Perfexion (LGK PFX) and to develop tests that can be done for the routine quality assurance checks of the sectors of the LGK PFX. The following tests were performed to evaluate the dosimetric characteristics of the sectors: (1) Flash-radiation dose for the 16 mm collimator, (2) transit-radiation dose for the 8 and 4 mm collimators, (3) sector leakage within the radiation cavity and, (4) sector output uniformity. In these tests, the Elekta ABS phantom was used. A micropoint ion-chamber Exradin A16 was placed at the center of the phantom for all measurements. With the version 8.0 of the control software of the MCU in the LGK PFX, the average flash-radiation dose per sector for the 16 mm collimator was measured to be 0.423 ± 0.003 cGy, and the average transit-radiation dose per sector for the 8 and 4 mm collimators was measured to be 0.169 ± 0.0009 and 0.147 ± 0.020 cGy, respectively. The calibration dose rate on the day of measurements was 280.8 cGy/min. Here, the authors have introduced a new concept of "equivalent-time-duration" (ETD) to represent the time duration, during which the flash-radiation or the transit-radiation dose is delivered. The ETD is a quotient of the measured dose of the flash-radiation or the transit-radiation and the respective calibrated dose rate for the 16, 8, or 4 mm collimator. The ETD constancy is an indicator of the constancy of the sector movements. The average value of ETD per sector was measured to be 724 ± 6, 313 ± 2, and 311 ± 45 ms for the 16, 8, and 4 mm collimators, respectively. During monthly spot checks, the authors have been measuring the total ETD for the flash-radiation when all eight sectors are open with the 16 mm collimator. The average value of the total ETD of the last 40 consecutive months was measured to be 642 ± 10 ms. This number is a useful quality parameter for the LGK PFX, which can be used to establish the base-line performance of the collimators of the LGK PFX. The reader is alerted that with the newly introduced version 9.0 of the control software of the MCU in the LGK PFX, the flash-radiation and the transit-radiation doses have become practically zero due to the fact that the treatment time is now corrected to compensate for these radiation doses. The sector output uniformity for the 16, 8, and 4 mm collimators was determined to be 98.9%, 97.3%, and 96.6%, respectively. The total leakage radiation with in the radiation cavity from all sectors was measured to be 0.17% of the dose output for the 16 mm collimator. Our measurements show that the dosimetric characteristics of all sectors of the LGK PFX are satisfactory. The authors recommend that the sector performance of the LGK PFX be determined initially at the time of installation of the LGK PFX and thereafter, at least annually. Similarly, our recommendation is that the total flash-radiation dose for the 16 mm collimator and the corresponding ETD value be measured on a monthly basis. Should any change in the sector movement occur, it will be revealed by the sector performance and the measurement of ETD.

Characterization and use of a 2D-array of ion chambers for brachytherapy dosimetric quality assurance

The two-dimensional (2D) ionization chamber array MatriXX Evolution is one of the 2D ionization chamber arrays developed by IBA Dosimetry (IBA Dosimetry, Germany) for megavoltage real-time absolute 2D dosimetry and verification of intensity-modulated radiation therapy (IMRT). The purpose of this study was to (1) evaluate the performance of ion chamber array for submegavoltage range brachytherapy beam dose verification and quality assurance (QA) and (2) use the end-to-end dosimetric evaluation that mimics a patient treatment procedure and confirm the primary source strength calibration agrees in both the treatment planning system (TPS) and treatment delivery console computers. The dose linearity and energy dependence of the 2D ion chamber array was studied using kilovoltage X-ray beams (100, 180 and 300 kVp). The detector calibration factor was determined using 300 kVp X-ray beams so that we can use the same calibration factor for dosimetric verification of high-dose-rate (HDR) brachytherapy. The phantom used for this measurement consists of multiple catheters, the IBA MatriXX detector, and water-equivalent slab of RW3 to provide full scattering conditions. The treatment planning system (TPS) (Oncentra brachy version 3.3, Nucletron BV, Veenendaal, the Netherlands) dose distribution was calculated on the computed tomography (CT) scan of this phantom. The measured and TPS calculated distributions were compared in IBA Dosimetry OmniPro-I‘mRT software. The quality of agreement was quantified by the gamma (γ) index (with 3% delta dose and distance criterion of 2 mm) for 9 sets of plans. Using a dedicated phantom capable of receiving 5 brachytherapy intralumenal catheters a QA procedure was developed for end-to-end dosimetric evaluation for routine QA checks. The 2D ion chamber array dose dependence was found to be linear for 100–300 kVp and the detector response (kuser) showed strong energy dependence for 100–300 kVp energy range. For the Ir-192 brachytherapy HDR source, dosimetric evaluation kuser factor determined by photon beam of energy of 300 kVp was used. The maximum mean difference between ion chamber array measured and TPS calculated was 3.7%. Comparisons of dose distribution for different test plans have shown agreement with >94.5% for γ ≤1. Dosimetric QA can be performed with the 2D ion chamber array to confirm primary source strength calibration is properly updated in both the TPS and treatment delivery console computers. The MatriXX Evolution ionization chamber array has been found to be reliable for measurement of both absolute dose and relative dose distributions for the Ir-192 brachytherapy HDR source.

Impact of Gantry Rotation Time on Plan Quality and Dosimetric Verification – Volumetric Modulated Arc Therapy (VMAT) vs. Intensity Modulated Radiotherapy (IMRT)

To compare plan quality criteria and dosimetric accuracy of step-and-shoot intensity-modulated radiotherapy (ss-IMRT) and volumetric modulated arc radiotherapy (VMAT) using two different gantry rotation times. This retrospective planning study based on 20 patients was comprised of 10 prostate cancer (PC) and 10 head and neck (HN) cancer cases. Each plan contained two target volumes: a primary planning target volume (PTV) and a boost volume. For each patient, one ss-IMRT plan and two VMAT plans at 90 s (VMAT90) and 120 s (VMAT120) per arc were generated with the Pinnacle© planning system. Two arcs were provided for the PTV plans and a single arc for boost volumes. Dosimetric verification of the plans was performed using a 2D ionization chamber array placed in a full scatter phantom. VMAT reduced delivery time and monitor units for both treatment sites compared to IMRT. VMAT120 vs. VMAT90 increased delivery time and monitor units in PC plans without improving plan quality. For HN cases, VMAT120 provided comparable organs at risk sparing and better target coverage and conformity than VMAT90. In the VMAT plan verification, an average of 97.1% of the detector points passed the 3 mm, 3% γ criterion, while in IMRT verification it was 98.8%. VMAT90, VMAT120, and IMRT achieved comparable treatment plans. Slower gantry movement in VMAT120 plans only improves dosimetric quality for highly complex targets.

On the role of the optimization algorithm of RapidArc® volumetric modulated arc therapy on plan quality and efficiency

The RapidArc volumetric modulated arc therapy (VMAT) planning process is based on a core engine, the so-called progressive resolution optimizer (PRO). This is the optimization algorithm used to determine the combination of field shapes, segment weights (with dose rate and gantry speed variations), which best approximate the desired dose distribution in the inverse planning problem. A study was performed to assess the behavior of two versions of PRO. These two versions mostly differ in the way continuous variables describing the modulated arc are sampled into discrete control points, in the planning efficiency and in the presence of some new features. The analysis aimed to assess (i) plan quality, (ii) technical delivery aspects, (iii) agreement between delivery and calculations, and (iv) planning efficiency of the two versions. RapidArc plans were generated for four groups of patients (five patients each): anal canal, advanced lung, head and neck, and multiple brain metastases and were designed to test different levels of planning complexity and anatomical features. Plans from optimization with PRO2 (first generation of RapidArc optimizer) were compared against PRO3 (second generation of the algorithm). Additional plans were optimized with PRO3 using new features: the jaw tracking, the intermediate dose and the air cavity correction options. Results showed that (i) plan quality was generally improved with PRO3 and, although not for all parameters, some of the scored indices showed a macroscopic improvement with PRO3. (ii) PRO3 optimization leads to simpler patterns of the dynamic parameters particularly for dose rate. (iii) No differences were observed between the two algorithms in terms of pretreatment quality assurance measurements and (iv) PRO3 optimization was generally faster, with a time reduction of a factor approximately 3.5 with respect to PRO2. These results indicate that PRO3 is either clinically beneficial or neutral in terms of dosimetric quality while it showed significant advantages in speed and technical aspects.

The Potential of using a Commercial Treatment Planning System for Physician Quality Reporting for Maintenance of ABR Certification and CMS Reimbursement

Purpose/Objective(s): Physicians are now asked to create quality reports for American Board of Radiology (ABR) certification and Centers for Medicare and Medicaid Services (CMS) reimbursement incentives. Analyzing multiple charts for dosimetric quality of treatment plans is difficult and time consuming. The objective of this study was to investigate if a commercial treatment planning system would be able to provide this information in an automated manner.

Semiautomatic segmentation for prostate brachytherapy: Dosimetric evaluation

To demonstrate that manual prostate segmentation in transrectal ultrasound images can be replaced with semiautomatic segmentation. Semiautomatic segmentation using a tapered ellipsoid model was applied to transrectal ultrasound images. Region-based volumetric evaluation was performed between original and physician-reviewed semiautomatic contours. For dosimetric assessment, treatment plans generated on semiautomatic contours were overlaid on physician-reviewed semiautomatic contours and dose parameters were computed. To establish a threshold for the acceptable amount of dosimetric degradation below which the adoption of semiautomatic planning is unacceptable, the range of variability in dosimetric quality attributed to manual variability was obtained and compared with that of semiautomatic contours. An average volume error (1-Dice similarity coefficient) of less than 7% between semiautomatic and manual volumes (140 cases) was obtained. The difference between the mean V(100) of plans created for semiautomatic contours then overlaid on physician-reviewed semiautomatic contours and the original V(100) values, that is, before overlaying on the physician-reviewed contours (41 cases) was lower than 5%. An average total duration of 2-4min, which includes algorithm initialization, 11.67±3.57s algorithm time, and contour modification is required per case. This algorithm is being used at the British Columbia Cancer Agency and to this date has been applied for the treatment of more than 600 patients. In terms of volumetric and dosimetric accuracy, the proposed algorithm is a suitable replacement for manual segmentation in the context of our planning technique. The benefits are shorter segmentation times; greater consistency; less reliance on user experience; and smooth, symmetric contours.

RapidArc radiotherapy planning for prostate cancer: Single-arc and double-arc techniques vs. intensity-modulated radiotherapy

RapidArc is a novel technique using arc radiotherapy aiming to achieve intensity-modulated radiotherapy (IMRT)-quality radiotherapy plans with shorter treatment time. This study compared the dosimetric quality and treatment efficiency of single-arc (SA) vs. double-arc (DA) and IMRT in the treatment of prostate cancer. Fourteen patients were included in the analysis. The planning target volume (PTV), which contained the prostate gland and proximal seminal vesicles, received 76 Gy in 38 fractions. Seven-field IMRT, SA, and DA plans were generated for each patient. Dosimetric quality in terms of the minimum PTV dose, PTV hotspot, inhomogeneity, and conformity index; and sparing of rectum, bladder, and femoral heads as measured by V70, V-40, and V20 (% of volume receiving >70 Gy, 40 Gy, and 20 Gy, respectively), treatment efficiency as assessed by monitor units (MU) and treatment time were compared. All plan objectives were met satisfactorily by all techniques. DA achieved the best dosimetric quality with the highest minimum PTV dose, lowest hotspot, and the best homogeneity and conformity. It was also more efficient than IMRT. SA achieved the highest treatment efficiency with the lowest MU and shortest treatment time. The mean treatment time for a 2-Gy fraction was 4.80 min, 2.78 min, and 1.30 min for IMRT, DA, and SA, respectively. However, SA also resulted in the highest rectal dose. DA could improve target volume coverage and reduce treatment time and MU while maintaining equivalent normal tissue sparing when compared with IMRT. SA achieved the greatest treatment efficiency but with the highest rectal dose, which was nonetheless within tolerable limits. For busy units with high patient throughput, SA could be an acceptable option.

TU‐A‐BRB‐07: VMAT and IMRT: The Best of Both Worlds

Purpose: To introduce a hybrid VMAT/IMRT optimization strategy that combines the delivery efficiency of VMAT with the superior intensity modulation possible with IMRT. Methods: A beamlet‐based inverse planning system was enhanced to combine the advantages of VMAT and IMRT into one comprehensive system. In the hybrid system, IMRT plans are created with many beams and optimized to convergence, taking note of the beam intensity map variation (BIMV) at each angle. Second, all beams are converted to VMAT segments and optimized for a set number of cycles. Then, the previous BIMV values and current cost function gradients with respect to the beamlets are used to define a gradient modulation factor (GMF). Beams with the highest GMF are converted from segments to beamlets and optimization proceeds with the mixed variable set until convergence or until additional beams are selected for conversion. This strategy was characterized on phantom and clinical cases. Comparisons were made between the hybrid plans, standard IMRT plans, and VMAT plans. Results: In a simple phantom, a hybrid VMAT/IMRT plan with 3 converted beams achieved superior dosimetric quality and required 30% fewer MU compared to 7‐beam IMRT. The hybrid plan also had a 20% reduction in final cost compared to VMAT. In a pancreas case, a 3 converted beam hybrid plan had improved and equivalent dosimetric quality compared to VMAT and 7‐beam IMRT, respectively. The hybrid and VMAT plans were more delivery efficient than IMRT. Conclusion: A hybrid VMAT/IMRT strategy was implemented to find the best compromise between gantry‐angle and intensity‐based degrees of freedom. This optimization method will allow patients to be simultaneously planned for dosimetric quality and delivery efficiency without switching between delivery techniques. Phantom and clinical examples suggest that the conversion of only 3 VMAT segments to modulated beams results in a good combination of quality and efficiency.Supported by NIH‐P01‐CA59827

SU‐E‐T‐834: Comparison of Rapidarc, IMRT and 3D Conformal Planning for Treatment of Chestwall, and Regional Nodes

Purpose: To investigate the application of Rapidarc as a technique to improve the dosimetric plan quality for the treatment of the chestwall and regional nodes when 3D conformal planning using the partially wide tangents (PWT) technique requires the irradiation of more than 3 cm of the ipsilateral lung and/or the heart in order to achieve coverage to the IMNs, and to compare it with IMRT. Methods: The PWT plan was done using the field in field technique. IMRT planning consisted of 6 beams at five gantry angles, chosen to minimize the entrance or exit through the contralateral structures. The arc range for the Rapidarc plan was similar to IMRT. Dose calculation was performed with the AAA algorithm in Eclipse V 8.9. Dose prescribed was 5040 cGy. Results: PTV D95 and V95 for both the Rapidarc and IMRT plans were 98% and 97% respectively. The same for 3D was lower by 4%–5% respectively. Rapidarc reduced the left lung mean dose compared to 3D and IMRT by as much as 19 Gy. The V20 Gy and V10 Gy were lower with Rapidarc by as much as 45% and 50%. The total lung mean dose was lower with Rapidarc by 8 Gy while the V20 Gy and V10 Gy reduced by 22% and 25%. Heart doses more than 25 Gy were identical with 3D and Rapidarc while IMRT gave a mean heart dose that was higher by 11 Gy. The mean dose to the contralateral organs was higher with Rapidarc by 3 Gy compared to 3D. Conclusions: For the same coverage, Rapidarc was able to better spare critical organs compared to IMRT. Compared to 3D, Rapidarc had better coverage and lower ipsilateral and total lung dose and identical volumes of heart irradiated above 25 Gy.

SU‐E‐T‐344: A Study of True Beam Linac Operation in a Varying Magnetic Field Environment

Purpose: To quantify the effects of transient, DC magnetic fields on the normal operation of a True Beam linac and to verify normal and accurate treatment delivery capability. This is necessary to ensure normal linac operation and accurate treatment delivery in such an environment. Methods: A data acquisition system was constructed consisting of a Lake Shore vector magnetometer and 3‐axis probe including custom software to monitor and record the real‐time vector magnetic field value at an in‐air reference point near the linac. This data was correlated with various dosimetric and beam quality measurements taken simultaneously. Long duration measurements (2–5 minutes) of the four steering servo currents (Asymmetry Radial (AR), Position Radial (PR), Asymmetry Transverse (AT), and Position Transverse (PT)) were recorded and correlated with magnetic field measurements. Measurements of machine output, dose rate, and beam asymmetry were also recorded and correlated with magnetic field readings. These measurements were repeated for various combinations of gantry orientation and photon and electron beam energies. Results: Linac output, dose rate, and beam asymmetry versus magnetic field demonstrated no dependence on magnetic field. Correlation plots of the four steering servo currents versus magnetic field indicated a strong dependence (correlation coefficient close to ±1.0) for 3 of the 4 steering servos at a gantry angle of 0° at a photon energy of 6MV. This dependence weakened for one of the servos (AT) and strengthened for one (AR) as the gantry angle was changed to 270°. Extrapolation of the correlation plots of each servo to the engineering limits were used to obtain a maximum permissible magnetic field value for the room Conclusions: Effects of magnetic field on linac operation was systematically quantified. Results indicate that the linac is fully capable of normal and accurate treatment delivery despite the ambient magnetic field environment in the room.

TU‐A‐BRB‐01: Convex Optimization Techniques for Handling Delivery Time and Plan Complexity in Multi‐Criteria VMAT Optimization

Purpose: To explore the joint trade‐offs of target coverage, organ sparing, and delivery time for volumetric modulated arc therapy (VMAT).Methods: One measure of beam‐on time in IMRT is the “sum of positive gradients” (SPG) of the intensity maps. We adapt this measure to VMAT planning. The adaptation is based on the fact that while the beam rotates through a specified arc length, a certain amount of modulation can be achieved “for free”, that is, without slowing down the gantry. The resulting convex optimization formulation is applied to minimizing or constraining delivery time in VMAT. We also consider the selection of collimator angle, and we explore the following: finding the single collimator angle which offers the quickest delivery while maintaining a specified dosimetric plan quality. Results: We demonstrate the optimization routines on three cases: a pancreas, a prostate, and a lung. We find that SPG smoothing decreases delivery time by a factor of 2 for the pancreas and lung cases and 2.7 for the prostate. For the pancreas case, we examine collimator rotation. Compared to the default angle of 0 degrees (i.e. leaf motion perpendicular to gantry rotation direction), a well‐chosen single collimator angle (23° in this case) can reduce the delivery time by an additional 8%. Conclusions: Although the VMAT problem is fundamentally non‐convex, due to the non‐convex mapping from leaf positions to voxel doses, we can use a modified SPG function, which is convex, to compute VMAT plans that correctly slow down the beam where it should. Our method is exact for leaves that move infinitely fast, and practically useful in the finite leaf speed case. It can straightforwardly be embedded into a multi‐criteria system allowing the user to explore the trade‐offs of target coverage, organ sparing, and delivery time.Funding support from NIH R01 Grant

SU‐E‐T‐859: A New Approach to Improve IMRT Delivery Efficiency by Intensity Map Discretization

Purpose: To minimize IMRT treatment monitor units and delivery time without compromising dosimetric quality in an in‐house treatment planning system. Methods: In‐house treatment planning system PLanUNC employs a two‐step process to generate a MLC‐IMRT treatment plan. First, the index‐dose gradient minimization dose optimization process produces a continuous intensity map. Second, MLC segmentation process converts the intensity map into MLC segments. This two‐step process is sensitive to MLC segmentation technique and can lead to excessive number of segments and dosimetric degradation. We developed a new method that discretizes the intensity map during optimization. The new method considered each beam in turn: its intensity map was optimized using the current dose distribution; the optimized intensity map was discretized according to its statistical distribution; the dose distribution was updated after discretization. In this process, the peaks and valleys in the continuous intensity map were converted into discrete plateaus, and the optimization of remaining beams mitigated possible degradation of dose conformity introduced by the discretization of previous beams. MLC segmentation was applied to the discretized fluence maps. This new approach is being validated using prostate and head and neck cases. Results: For the prostate, with no compromise to dose conformity, the number of segments was reduced by 40%, and the MUs reduced by ∼10%. For the head and neck, the number of segments was reduced by 20%, and a small dose escalation was observed in adjacent normal tissues; this may be due to complex constraints used in the optimization process and could be accommodated by re‐optimizing the beam weight using fixed beam apertures in the future. Conclusions: Discretization based on statistical distribution of beamlet intensity could effectively reduce IMRT beam complexity and result in shorter treatment time.

SU‐E‐E‐09: On‐Line Target Localization with Stereotactic Body Frame and Cone Beam CT for Accelerated Partial Breast Irradiation

Purpose: On‐line target localization with Cone Beam CT (CBCT) alone has the limitation that the detected offsets of patient tilt and spin cannot be corrected. This causes not only the geometric but also dosimetric quality compromise to the accelerated Partial Breast Irradiation. In this work, the stereotactic body frame was used for tuning the patient tilt and spin by means of its equipped air bags. The resultant geometric and dosimetric improvements were presented. Methods: On each treatment fraction, patient setup for accelerated Partial Breast Irradiation was initially performed based on the stereotactic body frame. CBCT images were acquired and registered with planning CT images to match the breast and the cavity. Offsets of patient position in the anterior‐posterior, lateral, longitudinal as well as the couch rotation were corrected by controlling the linear accelerator. Patient tilt and spin were corrected by justifying the air cushion of the stereotactic body frame. Geometric and dosimetric improvements were evaluated by calculating the physic depths and the depth doses of all treatment fields based on the CBCT images with and without patient tilt and spin justification. Results: The average magnitude of corrections to patient tilt and spin are 2.4 ±1.2 degree and 3.6 ±2.3 degree (2 patients, 20 fractions). Without tilt and spin justification, the average physic depths of all treatment fields were off by 12±7.5mm which causes depth doses off by 6.5±2.1%. Conclusions: Combining the stereotactic body frame with the Cone Beam CT for accelerated Partial Breast Irradiation demonstrated the ability to improve the quality of dose delivery. It can be implemented as routine procedure in clinic.