H Chen

Synchrony – Cyberknife Respiratory Compensation Technology

Studies of organs in the thorax and abdomen have shown that these organs can move as much as 40 mm due to respiratory motion. Without compensation for this motion during the course of external beam radiation therapy, the dose coverage to target may be compromised. On the other hand, if compensation of this motion is by expansion of the margin around the target, a significant volume of normal tissue may be unnecessarily irradiated. In hypofractionated regimens, the issue of respiratory compensation becomes an important factor and is critical in single-fraction extracranial radiosurgery applications. CyberKnife is an image-guided radiosurgery system that consists of a 6-MV LINAC mounted to a robotic arm coupled through a control loop to a digital diagnostic x-ray imaging system. The robotic arm can point the beam anywhere in space with 6 degrees of freedom, without being constrained to a conventional isocenter. The CyberKnife has been recently upgraded with a real-time respiratory tracking and compensation system called Synchrony. Using external markers in conjunction with diagnostic x-ray images, Synchrony helps guide the robotic arm to move the radiation beam in real time such that the beam always remains aligned with the target. With the aid of Synchrony, the tumor motion can be tracked in three-dimensional space, and the motion-induced dosimetric change to target can be minimized with a limited margin. The working principles, advantages, limitations, and our clinical experience with this new technology will be discussed.

Dosimetric evaluations of the interplay effect in respiratory‐gated intensity‐modulated radiation therapy

The interplay between a mobile target and a dynamic multileaf collimator can compromise the accuracy of intensity-modulated radiation therapy (IMRT). Our goal in this study is to investigate the dosimetric effects caused by the respiratory motion during IMRT. A moving phantom was built to simulate the typical breathing motion. Different sizes of the gating windows were selected for gated deliveries. The residual motions during the beam-on period ranged from 0.5 to 3 cm. An IMRT plan with five treatment fields from different gantry angles were delivered to the moving phantom for three irradiation conditions: Stationary condition, moving with the use of gating system, and moving without the use of gating system. When the residual motion was 3 cm, the results showed significant differences in dose distributions between the stationary condition and the moving phantom without gating beam control. The overdosed or underdosed areas enclosed about 33% of the treatment area. In contrast, the dose distribution on the moving phantom with gating window set to 0.5 cm showed no significant differences from the stationary phantom. With the appropriate setting of the gating window, the deviation of dose from the respiratory motion can be minimized. It appeals that limiting the residual motion to less than 0.5 cm is critical for the treatments of mobile structures.

Four-dimensional computed tomography-based respiratory-gated whole-abdominal intensity-modulated radiation therapy for ovarian cancer: a feasibility study.

This study assesses the feasibility and implementation of respiratory-gated whole-abdominal intensity-modulated radiation therapy (RG-WAIMRT). Three patients were treated with RG-WAIMRT. The planning target volume (PTV1) included the entire peritoneal cavity and a pelvic boost field was created (PTV2). The dose prescribed was 30 Gy to PTV1 and 14.4 Gy to PTV2. For comparison, a conventional three-dimensional (3D) plan was generated for each patient. In the WAIMRT plan, an average of 90% of PTV1 received 30 Gy compared to 70% for the conventional 3D plan. The percent volume receiving 30 Gy (V30) for liver averaged 54% (WAIMRT) vs 43% (3D). The percent volume receiving 20 Gy (V20) for kidneys averaged 19% vs 0%, and the mean V20 for bone marrow was 74% vs 83%, respectively. Major acute toxicities were anemia (grade 2: 1/3), leukopenia (grade 3: 2/3 patients), and thrombocytopenia (grade 2: 1/3 patients, grade 3: 1/3 patients). One patient could not complete the whole-abdomen field after 19.5 Gy because of persistent nausea. No major subacute toxicity has been reported. WAIMRT demonstrated superior target coverage and reduced dose to bone marrow, with a slightly increased dose to liver and kidneys. WAIMRT is a novel and feasible technique for ovarian cancer treatment.

TU‐FF‐A4‐04: Experimental Confirmation of Near Parabolic Shape of Dose Profile in Cylindrical Phantom for Dual Source CT

Purpose: To measure the radial dose distribution in a cylindrical CT phantom for both a single and dual source CT and to characterize its shape in order to determine whether the near parabolic shape used to justify CTDI volume calculations with equal weighting of the center and peripheral CTDI values is applicable for a two tube device. Method and Materials: A cylindrical phantom, the same diameter (32 cm) as the standard CTDI phantom was made with acrylic plastic. When assembled, it consisted of a cylinder with a cut in the transverse plane. Landauer optically stimulated luminescence(OSL) dots and Kodak X‐OMAT V films were sandwiched in the transversal slit. The assembled cylinder was scanned using a clinical protocol over a length that extended well beyond its endpoints at 120 kVp using a Siemens Dual Source Definition CT.Results: For a single tube, the radial dose distribution as measured using both the OSL detectors and film is close to parabolic. (The drop off in scatter close to the surface is not well accounted for by this simple curve.)Somewhat surprisingly,deviation from a parabolic shape near the surface when two tubes are used is only marginally different than for one tube. Under the conditions of the scan, equal weighting results in errors of only a few %, due mostly to the drop off at the surface. The 1/3*center+2/3*peripheral weighting used in standard calculations stems from a linear fit and results in errors of up to 11%. Conclusion: Measurements of the dose profile in a cylindrical phantom show that the shape is close to parabolic for both single and dual source machines except for drop off at the edges. A parabolic shape results in equal weighting coefficients for volumetric integral dose calculations using only center and peripheral CTDI measurements.

SU‐GG‐T‐88: A Multi‐Institutional Retrospective Study On Clinical IMRT Treatment Delivery Efficiency

Purpose: To better understand IMRT delivery efficiency in daily clinical operation we conducted a five‐institution retrospective study on clinical IMRTtreatment delivery time and IMRT MUs as functions of a number of variables — the number of fields per IMRTtreatment,treatment site, total MUs, and total number of segment fields for MLC‐IMRT treatment. The goal is to identify major contributing factor(s) for IMRT delivery time for each IMRT approach for improvement. Method and Materials: Data from more than 350 patients were extracted from IMPAC/LANTIS R&V system. A 5‐day average value was taken for any time data. The IMRTtreatment delivery time represents the portion of total patient delivery time that is specific to IMRT delivery technique and is defined as the time elapsed between the beam‐ON of the first field/segment and the beam‐OFF of the last field/segment in an IMRTtreatment. The accelerators studied are Siemens, Varian, and Elekta. Treatment planning systems used are PLanUNC, ADAC Pinnacle, and CMS XiO. The four IMRT delivery approaches studied are: segmental MLC‐IMRT on Siemens accelerator, segmental MLC‐IMRT on Elekta accelerator, dynamic MLC‐IMRT on Varian accelerator, and compensator‐IMRT on Siemens accelerators. Results: Our initial results show that for MLC‐IMRT treatments the IMRTtreatment delivery time is closely correlated with the number of segment fields and less correlated with the total number of MUs and treatment site. There is a large variation in IMRTtreatment delivery time for IMRTtreatment of a given number of fields, depending on the accelerator and IMRT approach used. Conclusion: Our five‐institution retrospective study on clinical treatment delivery data shows that manual compensator‐IMRT treatment is among the fastest of the five IMRT delivery approaches studied. In average all IMRT delivery approaches spent only approximately 20% of the IMRT delivery time (as defined) on actual radiation delivery.

SU‐FF‐J‐56: Patient Dose From Kilo‐Voltage Cone Beam Computed Tomography (kV‐CBCT) Imaging

Purpose: To investigate patient dose from on‐board imager‐based kV‐CBCT. Method and Materials: Radiation doses from kV‐CBCT were measured using TLDs at different locations in three anthropomorphic‐phantoms (H&N, chest and pelvis) and patients retrospectively. kV‐CBCT scans were performed in standard settings (125 kV, 80 mA and 25 ms) using a Varian Trilogy linear accelerator. Both full‐fan (FOV=24 cm) and half‐fan (FOV=40 cm) modes were evaluated for H&N case while only half‐fan (FOV=45 cm) technique was studied for chest and pelvic cases. The skin dose in both phantoms and patients were measured at 4 locations: anterior, posterior, Rt‐Lat, and Lt‐Lat. Doses measured in the phantoms included different critical organs. The dosimeters used were high sensitivity TLD‐100H and only those with standard‐deviations less than 3% and sensitivity within ±3% were selected for this study. Each TLD was individually calibrated using an ion chamber under the irradiation condition. Phantom data was averaged from 3 separate measurements and patient data was averaged from 5 measurements in each category. Results: The skin dose for H&N cases were 9–10cGy for half‐fan mode in both the phantom and patients. The dose for brain and brainstem were 7.1cGy and 7.6cGy, respectively. The doses in same locations were 2–3cGy lower if the full‐fan mode is used. The skin dose for chest cases was 8–10cGy and were same for the phantom and patient measurements. Measured mean lung dose was 8.5cGy and spinal cord dose was 6.2cGy. For pelvis, measured skin dose was 2.9–4.2cGy and the prostate and rectum dose were 2.9cGy. Conclusions: For pelvic cases, kV‐CBCT dose was comparable or less than that from portal imaging. For chest and H&N cases the dose can be two times higher than that for the pelvis cases. Daily CBCT may lead to extra 400cGy to skin and 250cGy to spinal cord in 40 fractions.

SU-FF-J-77: Image Quality Assessment for An Investigational Megavoltage Cone-Beam CT Device

Purpose: Megavoltage Cone‐Beam CT (MVCBCT) is an essential image guided radiation therapy(IGRT) device to acquire patients daily treatmentCT for accurate localization of treatment targets. The objective of this research was to assess its image qualities. Method and Materials: The image quality of MVCBCT was assessed by four indicators: noise, contrast, spatial resolution, and CT intensity stability. A CT electron density phantom and a Siemens calibration phantom were used. The images were acquired under various MU settings. The Siemens Syngo image processing software was used to sample and analysis the data. Results: The noise factor was used and found that the more MU to acquire the images produced less noise. 6 MU is the cut‐off value for noise factor of less than 5%. For contrast of the outer ring of the CT phantom, the electron density range of 0.976 were visible on all MUs. For the inner ring, we only see 1.052 on MU 1.043 for MU > 15. For the CT intensity stability, if the CT number differences has to be criteria of 0.25. Conclusion: The images from MVCBCT device were assessed for quality indicators; we conclude that the MU of 6 or above would have satisfactory results. For the future application of dose calculation on the MVCBCT images, the CT intensity stability is important, and we found that for 6 MU and above would have stable CT numbers.

SU‐F‐J‐35: Moving Towards Isocentric Prone Breast Setup with Contralateral Leveling Tattoo and Couch Move Assistant (CMA)

PURPOSEnLarge setup variability has been observed for prone breast patients due to rotation error. To reduce this uncertainty, we propose a novel implementation of isocentric prone breast method on Elekta linac with couch move assistant (CMA) and on-line KV CBCT.nnnMETHODSnDaily CBCT is used to evaluate the prone breast patient positioning uncertainty of proposed isocentric technique against our routine manual clinical setup. Clinical setup involves a manual AP shift from the ipsilateral torso tattoos (2PT) to the treatment iso directly underneath and SSD and flash check. While the proposed isocentric prone breast method features an additional contralateral leveling tattoo positioned at mid-level of torso during CT simulation to determine correct patient obliqueness, and an automatic couch shift using Elekta CMA to correct both known distance from 2PT to the iso and daily setup uncertainty. Summary statistics were calculated for a cohort of prone breast patients from our clinic (n=5), and will be updated as more patients get administrated.nnnRESULTSnSmall field of view partial CBCT acquisition is optimized to achieve clearance and minimize nominal imaging dose to 0.6cGy per scan to a 16cm phantom. Routine clinical setup uncertainty is 0.3±0.2cm, 0.7±0.4cm and 0.4±0.3cm in S/I, L/R and A/P respectively. Using the isocentric prone breast method, the setup uncertainty could be reduced to 0±0.3cm, 0.3±0.4cm and 0.1±0.3cm if the average couch shift in the 1st week of treatment is applied by CMA to the rest of treatment days. The uncertainty could be further reduced to 0.3±0.1cm, 0±0.1cm and 0±0.2cm with additional contralateral leveling tattoo. The improvement in precision is found to be statistically significant (p<0.05) using unpaired student t-tests.nnnCONCLUSIONnContralateral leveling tattoo is essential to determine correct patient obliqueness. More consistent and accurate isocentric prone breast patient positioning is achievable on Elekta linac with CMA and CBCT.

SU-E-T-250: Determining VMAT Machine Limitations of An Elekta Linear Accelerator with Agility MLC for Accurate Modeling in RayStation and Robust Delivery

Purpose: To implement VMAT in RayStation with the Elekta Synergy linac with the new Agility MLC, and to utilize the same vendor softwares to determine the optimum Elekta VMAT machine parameters in RayStation for accurate modeling and robust delivery. Methods: iCOMCat is utilized to create various beam patterns with user defined dose rate, gantry, MLC and jaw speed for each control point. The accuracy and stability of the output and beam profile are qualified for each isolated functional component of VMAT delivery using ion chamber and Profiler2 with isocentric mounting fixture. Service graphing on linac console is used to verify the mechanical motion accuracy. The determined optimum Elekta VMAT machine parameters were configured in RayStation v4.5.1. To evaluate the system overall performance, TG-119 test cases and nine retrospective VMAT patients were planned on RayStation, and validated using both ArcCHECK (with plug and ion chamber) and MapCHECK2. Results: Machine output and profile varies 1.5% output and 7% profile deviation is seen with >2.5cm/s leaf motion. All clinical cases achieved comparable plan quality as treated IMRT plans. The gamma passing rate is 99.5±0.5% on ArcCheck (<3% iso center dose deviation) and 99.1±0.8% on MapCheck2 using 3%/3mm gamma (10% lower threshold). Mechanical motion accuracy in all VMAT deliveries is <1°/1mm. Conclusion: Accurate RayStation modeling and robust VMAT delivery is achievable on Elekta Agility for <2.5cm/s leaf motion and full range of dose rate and gantry speed determined by the same vendor softwares. Our TG-119 and patient results have provided us with the confidence to use VMAT clinically.

SU-F-BRA-07: Dosimetric Verification of the Valencia Skin Applicator Using Gafchromic EBT3 Film

Purpose: The Valencia applicators have recently been introduced for HDR treatment of small and shallow superficial skin lesions (< 20 mm diameter and 3-mm depth). Per AAPM TG 56, any HDR applicator internal dimensions must be verified prior to clinical use. However radiographic and tomographic imaging to validate the Valencia applicators is impractical due to the Tungsten alloy housing and flattening filter. In this study, we propose to use EBT3 film to indirectly confirm the physical integrity of the Valencia applicators. Methods: Treatment plans were created using the Oncentra MasterPlan TPS v4.5 for the H2 (20-mm dia.) and H3 (30-mm dia.) Valencia Applicators. A virtual CT phantom (2-mm slice thickness) was created with one source position in water. The published effective depth method was used for each applicator to delivery 500 cGy to a 3-mm depth using the TG-43 formalism. Film measurements (n=3) at 3-mm depth and vertical plane in solid water were performed for each applicator to verify the prescribed dose calculated by the TPS. Percent depth dose curves and off-axis profiles (phantom surface and 3-mm depth) were measured and compared to published data. Films were analyzed using an in-house written program and RIT113 v6 software. Film calibration was performed per TG-55 protocol using the Ir-192 source with NIST-traceable calibration. Results: The prescription absolute dose difference was 1% for the Valencia H2 applicator and 4% for the Valencia H3 applicator. The measured percent depth dose curves and off-axis dose profiles measured for the H2 and H2 Valencia applicators are in excellent agreement with the Granero et al. Monte Carlo results1. Conclusion: Gafchromic EBT3 film can be used to indirectly verify the internal components of special HDR skin applicators constructed from high Z materials.1Granero et al. “Design and evaluation of a HDR skin applicator with flattening filter”, Med. Phys. 35(2), 495–503, 2008