Bulent Aydogan

Verification of dose distribution for volumetric modulated arc therapy total marrow irradiation in a humanlike phantom.

PURPOSEnVolumetric modulated arc therapy (VMAT) treatment planning studies have been reported to provide good target coverage and organs at risk (OARs) sparing in total marrow irradiation (TMI). A comprehensive dosimetric study simulating the clinical situation as close as possible is a norm in radiotherapy before a technique can be used to treat a patient. Without such a study, it would be difficult to make a reliable and safe clinical transition especially with a technique as complicated as VMAT-TMI. To this end, the dosimetric feasibility of VMAT-TMI technique in terms of treatment planning, delivery efficiency, and the most importantly three dimensional dose distribution accuracy was investigated in this study. The VMAT-TMI dose distribution inside a humanlike Rando phantom was measured and compared to the dose calculated using RapidArc especially in the field junctions and the inhomogeneous tissues including the lungs, which is the dose-limiting organ in TMI.nnnMETHODSnThree subplans with a total of nine arcs were used to treat the planning target volume (PTV), which was determined as all the bones plus the 3 mm margin. Thermoluminescent detectors (TLDs) were placed at 39 positions throughout the phantom. The measured TLD doses were compared to the calculated plan doses. Planar dose for each arc was verified using mapcheck.nnnRESULTSnTLD readings demonstrated accurate dose delivery, with a median dose difference of 0.5% (range:u2009-4.3% and 6.6%) from the calculated dose in the junctions and in the inhomogeneous medium including the lungs.nnnCONCLUSIONSnThe results from this study suggest that RapidArc VMAT technique is dosimetrically accurate, safe, and efficient in delivering TMI within clinically acceptable time frame.

Technical Note: High temporal resolution characterization of gating response time

PURPOSEnLow temporal latency between a gating ON/OFF signal and the LINAC beam ON/OFF during respiratory gating is critical for patient safety. Here the authors describe a novel method to precisely measure gating lag times at high temporal resolutions.nnnMETHODSnA respiratory gating simulator with an oscillating platform was modified to include a linear potentiometer for position measurement. A photon diode was placed at linear accelerator isocenter for beam output measurement. The output signals of the potentiometer and diode were recorded simultaneously at 2500 Hz with an analog to digital converter for four different commercial respiratory gating systems. The ON and OFF of the beam signal were located and compared to the expected gating window for both phase and position based gating and the temporal lag times extracted.nnnRESULTSnFor phase based gating, a real-time position management (RPM) infrared marker tracking system with a single camera and a RPM system with a stereoscopic camera were measured to have mean gate ON/OFF lag times of 98/90 and 86/44 ms, respectively. For position based gating, an AlignRT 3D surface system and a Calypso magnetic fiducial tracking system were measured to have mean gate ON/OFF lag times of 356/529 and 209/60 ms, respectively.nnnCONCLUSIONSnTemporal resolution of the method was high enough to allow characterization of individual gate cycles and was primary limited by the sampling speed of the data recording device. Significant variation of mean gate ON/OFF lag time was found between different gating systems. For certain gating devices, individual gating cycle lag times can vary significantly.

A multi‐institutional study to assess adherence to lung stereotactic body radiotherapy planning goals

PURPOSEnA multi-institutional planning study was performed to evaluate the frequency that current guidelines established by Radiation Therapy Oncology Group (RTOG) protocols and other literature for lung stereotactic body radiotherapy (SBRT) treatments are followed.nnnMETHODSnA total of 300 patients receiving lung SBRT treatments in four different institutions were retrospectively reviewed. The treatments were delivered using Linac based SBRT (160 patients) or image guided robotic radiosurgery (140). Most tumors were located peripherally (250/300). Median fractional doses and ranges were 18 Gy (8-20 Gy), 12 Gy (6-15 Gy), and 10 Gy (5-12 Gy) for three, four, and five fraction treatments, respectively. The following planning criteria derived from RTOG trials and the literature were used to evaluate the treatment plans: planning target volumes, PTVV 100 ≥ 95% and PTVV 95 ≥ 99%; conformality indices, CI100% < 1.2 and CI50% range of 2.9-5.9 dependent on PTV; total lung-ITV: V20Gy < 10%, V12.5Gy < 15%, and V5Gy < 37%; contralateral lung V5Gy < 26%; and maximum doses for spinal cord, esophagus, trachea/bronchus, and heart and great vessels. Populations were grouped by number of fractions, and dosimetric criteria satisfaction rates (CSRs) were reported.nnnRESULTSnFive fraction regimens were the most common lung SBRT fractionation (46%). The median PTV was 27.2 cm(3) (range: 3.8-419.5 cm(3)). For all plans: mean PTVV 100 was 94.5% (±5.6%, planning CSR: 69.8%), mean PTVV 95 was 98.1% (±4.1%, CSR: 69.5%), mean CI100% was 1.14 (±0.21, CSR: 79.1%, and 16.5% within minor deviation), and mean CI50% was 5.63 (±2.8, CSR: 33.0%, and 28.0% within minor deviation). When comparing plans based on location, peripherally located tumors displayed higher PTVV 100 and PTVV 95 CSR (71.5% and 71.9%, respectively) than centrally located tumors (61.2% and 57.1%, respectively). Overall, the planning criteria were met for all the critical structure such as lung, heart, spinal cord, esophagus, and trachea/bronchus for at least 85% of the patients.nnnCONCLUSIONSnAmong the various parameters that were used to evaluate the SBRT plans, the CI100% and CI50% were the most challenging criteria to meet. Although the CSRs of organs at risk were higher among all cases, their proximity to the PTV was a significant factor.

A preplanning method for stereotactic radiosurgery to improve treatment workflow

Frame-based stereotactic radiosurgery (SRS) requires fixation of an invasive head ring to ensure accurate targeting. Minimizing waiting time with a fixed head ring is important for patient comfort and satisfaction. We report a practical preplanning solution for the Brainlab iPlan treatment planning system that reduces waiting time by expediting the planning process on treatment day. A water-filled anthropomorphic head phantom was used to acquire a surrogate CT image set for preplanning and fused with patients MRI, which was obtained before the day of treatment. Once an acceptable preplan was obtained, it was saved as a plan template and the phantom image set was removed from the Brainlab database to prevent any confusion and mix-up of image sets. On the treatment day, the patients CT and MRI were fused, and the customized beam settings of the preplan template were then applied and optimized. Up to 10-fold of reduction in treatment plan time was demonstrated by bench testing with multiple planners and a variety of cases. Loading the plan template and fine-tuning the preconfigured beam settings took only a small fraction of the preplan time to restore the conformity and dose falloff comparable to those of the preplan. For instance, preplan time was 2 hr for a two-isocenter case, whereas, it took less than 20 min for a less experienced planner to plan it on the day of treatment using the preplan method. The SRS preplanning technique implemented in this study for the Brainlab iPlan treatment planning system offers an opportunity to explore possible beam configurations thoroughly, optimize planning parameters, resolve gantry angle clearance issues, and communicate and address challenges with physicians before the treatment day. Preplanning has been proven to improve plan quality and to improve efficiency in our clinic, especially for multiple-isocenter and dosimetrically challenging cases. PACS number(s): 87.53.Ly, 87.55.D-, 87.55.Gh, 87.55.tm.Frame‐based stereotactic radiosurgery (SRS) requires fixation of an invasive head ring to ensure accurate targeting. Minimizing waiting time with a fixed head ring is important for patient comfort and satisfaction. We report a practical preplanning solution for the Brainlab iPlan treatment planning system that reduces waiting time by expediting the planning process on treatment day. A water‐filled anthropomorphic head phantom was used to acquire a surrogate CT image set for preplanning and fused with patients MRI, which was obtained before the day of treatment. Once an acceptable preplan was obtained, it was saved as a plan template and the phantom image set was removed from the Brainlab database to prevent any confusion and mix‐up of image sets. On the treatment day, the patients CT and MRI were fused, and the customized beam settings of the preplan template were then applied and optimized. Up to 10‐fold of reduction in treatment plan time was demonstrated by bench testing with multiple planners and a variety of cases. Loading the plan template and fine‐tuning the preconfigured beam settings took only a small fraction of the preplan time to restore the conformity and dose falloff comparable to those of the preplan. For instance, preplan time was 2 hr for a two‐isocenter case, whereas, it took less than 20 min for a less experienced planner to plan it on the day of treatment using the preplan method. The SRS preplanning technique implemented in this study for the Brainlab iPlan treatment planning system offers an opportunity to explore possible beam configurations thoroughly, optimize planning parameters, resolve gantry angle clearance issues, and communicate and address challenges with physicians before the treatment day. Preplanning has been proven to improve plan quality and to improve efficiency in our clinic, especially for multiple‐isocenter and dosimetrically challenging cases. PACS number(s): 87.53.Ly, 87.55.D‐, 87.55.Gh, 87.55.tm

MO‐A‐137‐11: How Successful Are We in Meeting SBRT Planning Goals? A Multi‐Institutional Study

PURPOSEnTo assess the variability in treatment planning of stereotactic body radiotherapy (SBRT) of lung lesions, a multi-institutional planning comparison study was performed.nnnMETHODSnA total of 150 patients receiving lung SBRT treatments in five different institutions were reviewed. Treatment plans were generated using Varian/Eclipse™ (82), CMS/Xio™ (53) and Cyberknife/Multiplan™ (15) systems. Most tumors were located peripherally (126/150) and in right lung (82/150). Various dose fractionations were used: 3x18Gy (5 cases), 3x20Gy (13), 4x12Gy (18), 4x12.5Gy (9), 5x10Gy (65), 5x12Gy (38), and 5x8or9Gy (2). The following planning criteria derived from RTOG trials and literature were used to evaluate the 4 or 5 fraction SBRT plans (133 cases): PTV-V100>=95°/o, PTVV95>=99%, conformality index (CI-100%) <1.2, CI-50% isodose <4 for PTV>50cc or <5 for PTV<50cc, total lung-ITV: V20Gy<10%, V12.5Gy<15%, V5Gy<37%, contralateral lung V5Gy<26%, cord: maximum<25 Gy, D0.25cc<22.5Gy, esophagus: max<30Gy, D5cc<27.5Gy, trachea/bronchus maximum<40Gy, heart & great vessels maximum<45Gy. The mean, standard deviation (±) and dosimetric criteria pass rates (PR) were reported.nnnRESULTSnThe median PTV volume was 27.0cc (range:3.8/188.9cc). The mean PTV-V100 was 96.0% (±3.6, PR:87.1%). The mean PTV-V95 was 99.4% (±2.1, PR:90.2%) and the mean CI-100% was 1.16 (±0.18, PR:74.0%). The mean CI-50% was 4.14 (±0.67, PR:46.4%) for PTV>50cc (28 cases) and 7.36 (±3.36, PR:29.4%) for PTV<50cc (102 cases). The planning criteria were met for all the critical structure such as lung, heart, cord, esophagus, and trachea for at least 90% of the patients. The mean of all OAR criteria PR was 97.7% for peripheral and 88.6% for central lesions.nnnCONCLUSIONnFive fraction regimens were the most common lung SBRT fractionation (70%). Among the various parameters that were used to evaluate the SBRT plans, the CI-100% and CI-50% were the most challenging criteria to meet. Although the OAR PRs were higher among all cases, their proximity to the PTV was a significant factor.

WE-C-BRB-03: Feasibility Study for Total Marrow Irradiation with RapidArc Volumetric Arc Therapy (VMAT-TMI)

Purpose: To investigate the dosimetric feasibility of VMAT‐TMI technique in terms of treatment planning and delivery efficiency and accuracy for a Rando phantom using Eclipse RapidArc technology. Methods: Total body irradiation (TBI) has been an integral part of pre‐conditioning regimens for the treatment of hematologic malignancies. Normal tissue toxicity is a common complication of TBI inclusive regimens especially at doses higher than 12 Gy due to inhomogeneous and excessive dose to organs at risk (OARs). Fixed gantry linear accelerator(linac) and Tomotherapy intensity modulated total bone marrow (IM‐TMI) techniques have been reported to reduce doses to OARs. However, the treatment time of more than one hour with both techniques may limit their clinical application. Volumetric modulated arc therapy (VMAT) may be able to increase the treatmentdelivery efficiency while increasing the target conformality and reducing the radiationdose to OARs. In order to treat all the bones in the Rando phantom, three sub‐plans with a total of nine arcs were used. A total of 100 thermoluminescent detectors(TLDs) were placed at 39 positions throughout the phantom. The measured TLDdoses were compared to the calculated plan doses. Planar dose for each arc was verified using MapCheck and MapPhan. Results: Excellent target coverage with the prescription dose and dose reduction to the OARs were achieved similar to the fixed gantry linac IM‐TMI technique. The number of monitor units was decreased by 50%, while the delivery efficiency increased by more than 60%. TLD readings demonstrated accurate dosedelivery, with a median dose difference of 0.5% from the calculated dose. Planar dose distributions of all arcs satisfied the 3% and 3 mm gamma criteria with an average pass rate of 97.8%. Conclusions: Results from this study suggest that RapidArc VMAT‐TMI technique is clinically feasible. This study is supported by Varian Medical Systems

SU‐E‐T‐446: The Effect of Breathing Motion on the Delivery of Linac‐Based Intensity Modulated Total Marrow Irradiation (IM‐TMI)

Purpose: To evaluate the effect of breathing motion on the delivery of linac‐based intensity modulated total marrow irradiation (IM‐TMI). Methods: An IM‐TMI treatment planning for an anthropomorphic Rando phantom was performed using the three isocenter technique that was previously reported by our group. The chest sub‐plan that targets the ribs, sternum and spine while sparing the lungs,liver, and heart was used to evaluate the effect of breathing on the treatment delivery. The breathing motion was simulated using a motorized stage. IM‐TMI dose measured in a stable phantom first and then in a phantom that was set on motion using three sinusoidal motions with peak‐to‐peak distances of 5 mm superior‐inferior (SI), 10 mm SI, and 5 mm both SI and anterior‐posterior (AP). A total of 46 thermoluminescent detectors(TLDs) were placed in five locations in the target bones and lungs. An additional 8 TLDs were used for dose calibration. The measured doses were corrected using the individual TLD sensitivities. The average TLD doses obtained with motion were compared to that of the stable delivery. In addition, a stereotactic photon diode was used to measure the dose in another target bone location. Results: The ratio of the moving to stable delivery of mean dose were 1.04 (range: 1.01–1.06), 1.06 (1.04–1.07) and 1.03 (1.00–1.04) in target bones while the ratios were 1.00 (0.94–1.09), 1.01 (0.97–1.04) and 1.00 (0.97–1.04) in the lungs for the 5 mm SI, 10 mm SI, and 5 mm SI and AP motions respectively. Conclusions: In this study, 16 out of 18 measurement points showed less than 6% difference with breathing motions in a single fraction. The delivered dose difference was as much as 7% high in bone and 9% low in lung with breathing motion.

SU‐GG‐T‐71: Dosimetric Comparison of Bone Marrow‐Sparing Intensity Modulated Radiation Theraphy Versus Conventional Techniques for the Treatment of Cervical Cancer

Purpose: To compare bone marrow‐sparing intensity modulated pelvic radiation therapy (BMS‐IMRT) to conventional (AP/PA and 4‐field box) techniques in the treatment of cervical cancer.Method and Materials: Seven cervical cancer patients treated with concurrent chemotherapy and IMRT were analyzed. We compared BMS‐IMRT to AP/PA and 4‐field box plans. All plans were normalized to cover PTV with the 99% isodose line. The clinical target volume (CTV) consisted of the pelvic and presacral lymph nodes, uterus and cervix, upper vagina, and parametrial tissue. A 1.0 cm uniform margin was added to create the PTV. Normal tissues included bowel, bladder, and pelvic bone marrow (PBM), which comprised the lumbosacral spine (LSBM), ilium (IBM), and ischium, pubis, and proximal femora (lower pelvis bone marrow — LPBM). Dose volume histograms for PTV and normal tissues were compared for BMS‐IMRT vs. 4‐field box and AP/PA plans. Results: BMS‐IMRT was superior to 4‐field box in reducing the dose to PBM, small bowel, rectum and bladder. Compared to AP/PA, BMS‐IMRT reduced the PBM volume receiving a dose above 16.4 Gy, however, that receiving below this threshold dose was increased. BMS‐IMRT reduced IBM, LPBM and bowel radiation above 27.7 Gy, 18.7 Gy, and 21.1 Gy, respectively, but increased dose below these thresholds, compared to AP/PA plans. BMS‐IMRT reduced the volume of LSBM, rectum, small bowel, and bladder at all dose levels in all 7 patients. Conclusion: BMS‐IMRT reduced irradiation of PBM compared to 4‐field box techniques. Compared to AP/PA, BMS‐IMRT reduced LSBM irradiation and reduced the volume of PBM irradiated to high doses, but increased that irradiated to low doses. BMS‐IMRT may reduce acute HT compared to conventional techniques.

SU‐EE‐A1‐05: Intensity Modulated Radiation Therapy (IMRT) as An Alternative to Adjuvant High Dose Rate (HDR) Brachytherapy in Endometrial Cancer Patients

Purpose: To evaluate the role of IMRT as an alternative to HDR brachytherapy for the adjuvant treatment of endometrial cancer patients following surgery. Method and Materials: The CT scans of five patients previously treated with adjuvant HDR were used in this study. All patients were scanned with a vaginal cylinder in place, and HDR planning was performed using the BrachyVision (Varian Medical Systems) software. In all cases, a dose of 700 cGy/fraction was prescribed at a distance of 0.5 cm from the cylinder surface. The same planning CT scans were then used for IMRT planning (Eclipse, Varian Medical Systems). In this paradigm, the vaginal cylinder is a component of a hypothetical immobilization system that would be indexed to the linactreatment table. The goal of IMRT planning was to deliver the prescription dose to the clinical target volume (CTV) while minimizing the volume of rectum and bladder irradiated. Dose volume histograms (DVHs) of the bladder, rectum and CTV were recorded and compared for both HDR and IMRT plans. Results: The mean bladder doses (as a percentage of the prescription dose) were 31.8% and 32.2% (p = 0.93) for the HDR and IMRT plans, respectively. However, in 4 of the 5 plans, IMRT produced lower maximum bladder doses compared to HDR (averages: IMRT = 99.1% vs. HDR = 158.4%). IMRT plans also resulted in lower mean rectal doses (21.6%) than HDR plans (30.9%) (p = 0.01). Moreover, the maximum rectal doses were lower in all IMRT plans compared to the HDR plans (95.1% vs. 165.8%). On average, the minimum dose to CTV was slightly higher using IMRT (97.8% vs. 95.5%). Conclusion: These results suggest than when used in conjunction with a suitable immobilization system, IMRT may provide an alternative to HDR brachytherapy in women with endometrial cancer following hysterectomy.