I Yeo

Dose tolerance limits and dose volume histogram evaluation for stereotactic body radiotherapy

Almost 20 years ago, Emami et al. presented a comprehensive set of dose tolerance limits for normal tissue organs to therapeutic radiation, which has proven essential to the field of radiation oncology. The paradigm of stereotactic body radiotherapy (SBRT) has dramatically different dosing schemes but, to date, there has still been no comprehensive set of SBRT normal organ dose tolerance limits. As an initial step toward that goal, we performed an extensive review of the literature to compare dose limits utilized and reported in existing publications. The impact on dose tolerance limits of some key aspects of the methods and materials of the various authors is discussed. We have organized a table of 500 dose tolerance limits of normal structures for SBRT. We still observed several dose limits that are unknown or not validated. Data for SBRT dose tolerance limits are still preliminary and further clinical trials and validation are required. This manuscript presents an extensive collection of normal organ dose tolerance limits to facilitate both clinical application and further research. PACS numbers: 87.53.Ly, 87.55.dk

A quality assurance method with submillimeter accuracy for stereotactic linear accelerators

The Stereotactic Alignment for Linear Accelerator (S. A. Linac) system is developed to conveniently improve the alignment accuracy of a conventional linac equipped with stereotactic cones. From the Winston‐Lutz test, the SAlinac system performs three‐dimensional (3D) reconstruction of the quality assurance (QA) ball coordinates with respect to the radiation isocenter, and combines this information with digital images of the laser target to determine the absolute position of the room lasers. A handheld device provides near‐real‐time repositioning advice to enable the user to align the QA ball and room lasers to within 0.25 mm of the centroid of the radiation isocenter. The results of 37 Winston‐Lutz tests over 68 days showed that the median 3D QA ball alignment error was 0.09 mm, and 97% of the time the 3D error was ≤0.25u2009mm. All 3D isocentric errors in the study were 0.3 mm or less. The median x and y laser alignment coordinate error was 0.09 mm, and 94% of the time the x and y laser error was ≤0.25u2009mm. A phantom test showed that the system can make submillimeter end‐to‐end accuracy achievable, making a conventional linac a “Submillimeter Knife”. PACS numbers: 87.53.Ly, 87.55.Qr

SU‐FF‐T‐182: Validation of Amplitude and Phase Gating Against Potential Irreproducibility of Motion

In gated radiation therapy, reproducibility of patients internal motion between imaging and delivery is essential. However, changes in period, amplitude, shape of motion, and baseline shift occur. In this study, we investigated the dosimetric impact of the changes on the coverage of clinical and internal target volumes for phase and amplitude gating. We used conventional and intensity‐modulated beams that are designed to cover internal target volumes. We assumed a duty cycle of 40‐to‐60% phases and equivalent amplitudes, a period of 4.5sec, and an amplitude of 4cm as conditions used in imaging. We introduced the above changes from these and used actual patients breathing motion in the measurement and computational simulation on a diode array (double precision). When a baseline shift of ‐1cm was assumed, only a part (67%) of clinical target volume (CTV) received a prescribed does for phase gating; a similar profile shift in delivereddose was observed for the amplitude gating (when the amplitude window was stationary). As the amount of the shift increased, the impact increased. An amplitude change by a few centimeters caused a shift in delivereddose profile and underdose in CTV for phase gating. The underdose was not observed for amplitude gating. The change in breathing periods did not affect the delivereddose profile. The change in breathing pattern from sinusoidal into linear shapes showed underdose in 12% of CTV and profile change for phase gating and no profile change and underdose for amplitude gating. The simulations agreed with the measurements, and used for the amplitude gating study. Results based on actual breathing patterns and intensity‐modulated fields will also be presented. This study has demonstrated that unless the amplitude opening is not adapting to the movement and extent of CTV, gated therapy is susceptible to the irreproducibility. Partly supported by Varian Medical Systems, Inc.

SU-D-202-05: Evaluation of Four-Dimensional Dose Reconstruction Under Breathing Irregularity

PURPOSEnA method of four dimensional dose reconstruction (4D-DR) using a cine mode of an EPID to determine the delivered 4D dose distribution has been suggested. This method, however, has not been tested under irregular breathing. This study investigated their effects on 4D-DR.nnnMETHODSnThe 4D-DR attempts to find the patients breathing phase associated with each EPID image by comparing it to pre-generated-EPID-quality DRRs of every breathing phases. A lung phantom with a tumor object (3cm diameter cylinder) on a moving platform was used for this test under conditions of amplitude reduction by 1/2 during (1) the entire delivery and (2) the duration when MLC blocked the tumor (hindering the phase determination). The dose delivered to the phantom was inversely reconstructed in the associated phase image of the phantom from each EPID image based on our documented inverse method of the 4D-DR; the phase-specific dose was integrated, generating the 4D reconstructed dose. Forward 4D Monte Carlo calculations were used as the 4D dose of a ground truth.nnnRESULTSnThe reconstructed dose showed 98.1% gamma pass rate for 3%/3mm under regular breathing (i.e. no amplitude reduction) when compared with the forward 4D dose. When the irregular condition (#1) was adopted for treatment, the pass rate became 94.2%. The irregularity under the MLC block (#2) showed the pass rate of 91.3%. The two conditions did not affect the reconstruction noticeably in DVH plots of the tumor and lung-tumor.nnnCONCLUSIONnAs long as the extent of on-treatment motion is smaller than that of the planning CT, the 4D-DR can be done accurately. Otherwise, ontreatment 4D CT images are necessary.

WE-D-BRA-03: Four-Dimensional Dose Reconstruction Through Retrospective Phase Determination Using Cine Images of Electronic Portal Imaging Device

Purpose: To test a method to reconstruct a four-dimensional (4D) dose distribution using the correlation of pre-calculated 4D electronic portal imaging device (EPID) images and measured cine-EPID images. Methods: 1. A phantom designed to simulate a tumor in lung (a polystyrene block with 3.0 cm diameter embedded in cork) was placed on a sinusoidally moving platform with 2 cm amplitude and 4 sec/cycle. Ten-phase 4D CT images were acquired for treatment planning and dose reconstruction. A 6MV photon beam was irradiated on the phantom with static (field size=5×8.5 cm2) and dynamic fields (sliding windows, 10×10 cm2, X1 MLC closing in parallel with the tumor movement). 2. 4D and 3D doses were calculated forwardly on PTV (1 cm margin). 3. Dose images on EPID under the fields were calculated for 10 phases. 4. Cine EPID images were acquired during irradiation. 5. Their acquisition times were correlated to the phases of the phantom at which irradiation occurred by inter-comparing calculated “reference” EPID images with measured images (2D gamma comparison). For the dynamic beam, the tumor was hidden under MLCs during a portion of irradiation time; the correlation performed when the tumor was visible was extrapolated. 6. Dose for each phase was reconstructed on the 4D CT images and summed over all phases. The summation was compared with forwardly calculated 4D and 3D dose distributions. Monte Carlo methods were used for all calculations. Results: For the open and dynamic beams, the 4D reconstructed doses showed the pass rates of 92.7 % and 100 %, respectively, at the isocenter plane given 3% / 3 mm criteria. The better agreement of the dynamic beam was from its dose gradient which blurred the otherwise sharp difference between forward and reconstructed doses. This also contributed slightly better agreement in DVH of PTV. Conclusion: The feasibility of 4D reconstruction was demonstrated.

SU-E-T-428: Feasibility Study of 4D Image Reconstruction by Organ Motion Vector Extension Based On Portal Images

Purpose: To develop and to test a method to generate a new 4D CT images of the treatment day from the old 4D CT and the portal images of the day when the motion extent exceeded from that represented by plan CTs. Methods: A motion vector of a moving tumor in a patient may be extended to reconstruct the tumor position when the motion extent exceeded from that represented by plan CTs. To test this, 1. a phantom that consists of a polystyrene cylinder (tumor) embedded in cork (lung) was placed on a moving platform with 4 sec/cycle and amplitudes of 1 cm and 2 cm, and was 4D-scanned. 2. A 6MV photon beam was irradiated on the moving phantoms and cineEPID images were obtained. 3. A motion vector of the tumor was acquired from 4D CT images of the phantom with 1 cm amplitude. 4. From cine EPID images of the phantom with the 2 cm amplitude, various motion extents (0.3 cm, 0.5 cm, etc) were acquired and programmed into the motion vector, producing CT images at each position. 5. The reconstructed CT images were then compared with pre-acquired “reference” 4D CT images at each position (i.e. phase). Results: The CT image was reconstructed and compared with the reference image, showing a slight mismatch in the transition direction limited by voxel size (slice thickness) in CT image. Due to the rigid nature of the phantom studied, the modeling the displacement of the center of object was sufficient. When deformable tumors are to be modeled, more complex scheme is necessary, which utilize cine EPID and 4D CT images. Conclusion: The new idea of CT image reconstruction was demonstrated. Deformable tumor movements need to be considered in the future.

SU-E-T-02: 3D Dose Reconstruction of Volumetric Modulated Arc Therapy From Delivered EPID Dose Image

PURPOSEnTo evaluate dose reconstruction of volumetric modulated arc therapy (VMAT) from EPID image prediction.nnnMETHODSnA test VMAT plan with dynamic MLCs was developed on a water equivalent cubic phantom with dimension of 31.4×34×34 cm3 . From the initial MLC opening of 5×5 cm2 at gantry 0°, they moved with a constant speed as the gantry rotates, until they reached X1 at -2.5 cm and X2 at = -2.237 cm at gantry 90°. The plan was delivered while cine EPID images were acquired. For the 84 gantry angle segments, doses to the phantom were calculated by XVMC and reconstructed from EPID based on fast density-scaled model and non-iterative dose reconstruction method we have previously developed. The reconstructed dose was compared with the forwardly calculated dose individually over 84 gantry angle segments and cumulatively in terms of gamma evaluation and dose volume histogram for PTV (3×3×3 cm3 ) positioned at the center of the phantom.nnnRESULTSn3D-gamma pass rate was 97.0 % cumulatively with 3 %/3 mm criteria and maximum 100% individually. The DVHs of PTV were indistinguishable except in the region of high dose (>95% of prescribed dose) where the difference of a few percent in volume was found.nnnCONCLUSIONnShowing good agreement with the forwardly calculated dose, we have extended our model of dose reconstruction successfully to the verification of VMAT. Although more clinical demonstration is to be followed, our study presents a promising tool for time-resolved and cumulative VMAT validation.

SU‐E‐T‐410: Clinical Application of Dose Reconstruction Based on Full‐Scope Monte Carlo Calculations

Purpose: To reconstruct dose delivered to a deformable phantom from images acquired by an electronic portal imaging device(EPID) using a full‐scope Monte Carlo based non‐iterative reconstruction method we have developed and to evaluate the delivered dose to the deformed organs using the resulting reconstructed dose. Methods: A deformable prostate phantom was embedded into a 20cm‐deep and 40cm‐wide water phantom. The phantom was CT scanned and the anatomical models of prostate, seminal vesicles, and rectum were contoured. A coplanar 4‐field IMRT plan was used for this study. The XVMC Monte Carlo code, XVMC, was used for verification of planned dose in phantom and calculation of dose response by EPID.Organ deformation was simulated by inserting a “transrectal” balloon containing 20ml of water and air. A new CT scan was obtained and the deformed structures were contoured. The IMRT plan was delivered to the two phantoms and integrated EPIDimages were respectively acquired. Dose reconstruction was performed using these images with calculated EPID responses. The deformed phantom was registered to the original phantom using an in‐house developed software based on the Demons algorithm. The transfer matrix for each voxel was obtained and used to correlate the two sets of the reconstructed dose to generate a cumulative reconstructed dose on the original phantom. Results: Forwardly calculated planning dose in the original phantom was compared to the cumulative reconstructed dose from EPID in the original phantom. The prescribed 200 cGy isodose lines showed little difference with respect to the “prostate” and “seminal vesicles”, but appreciable difference (3%) was observed at 210cGy dose level. Conclusions: The results validate our dose reconstruction method for EPID dosimetry. Organ deformation resulted small but observable dose changes in the target and critical structure. This study was in part supported by Varian Medical Systems, Inc.

SU‐E‐T‐520: Post‐Treatment Evaluation of Dose Delivery by Gated Radiation Therapy Considering the Impact of Breathing Irreproducibility

Purpose: To evaluate the dosedelivery retrospectively after treatment using breathing sessions recorded during the treatment. Methods: Two lungcancer patients were given 22 and 25 respiratory sessions for imaging and treatment. A computational model was developed in past for calculating dosedelivered to a moving target, given a planned dose distribution. We have determined ITV from the breathing traces acquired during CT using 30‐to‐70 and 35‐to‐75% phase windows, respectively, for the two patients. We used the same window within the traces acquired during treatment for dose reconstruction (the windows were given as tagged traces). The ITV margins were determined using first the difference between the maximum and minimum amplitudes within the windows and second the 95%‐ confidence interval of the difference from the entire patterns from CT. We added the second option as we could not locate the portion of the breathing traces that corresponds to the CT slices of tumor, while these slices are used for moving‐target contouring. So, using the entire pattern tends to overestimate internal margins. The magnitude of CTV was assumed to be 3 cm; the planned dose was justly covering PTV boundaries. The delivereddose profiles calculated for treatment sessions were compared to the planned distribution.Results: Using the determined ITV from the CT session, the CTV and normal tissue coverage were calculated. The CTV coverage was 100% for all sessions except one for patient A. For the patient B, the sessions which deliver the prescription dose to 100% of the CTV volume was 25% of the entire sessions. Almost all (23/24) sessions delivered 95% of the prescription dose to 95% of the CTV volume, affected by less reproducible breathing patterns than the patient A. Conclusions: We reconstructed the dose coverage in CTV and normal tissue within ITV. This study could help determine PTV margins. In part supported by Varian medical systems, Inc and in part supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MEST) (No. 2009‐0085999) and by the Industrial Strategic technology development program : 10035527 funded by the Ministry of Knowledge Economy(MKE, Korea)

MO‐EE‐A3‐01: Feasibility Study on Four‐Dimensional Dose Reconstruction of Radiation Delivery

Purpose: When an intensity modulated radiation beam is used to a moving target, the two time variables due to miss‐synchronization can cause unpredicted dose delivery. Exit dosimetry possesses information on delivered radiation to the target. Using a continuous scan (cine) mode of electronic portal imaging device(EPID), our goal is to use the temporal images related to the target motion and the beam, and perform dose reconstruction.Method: To evaluate this hypothesis, firstly, cine mode acquisition was compared with integration mode acquisition on EPID (IAS3 and As 1000). We have delivered open and clinical beams using 6MV at 300MU/min to EPID. Secondly, in‐phantom film and exit EPIDdosimetry for the two modes was performed on a moving platform using a forwardly‐advancing beam that generates a pyramid dose shape. Beam delivery was gated for ten steps of 10% phase duration. Film was left for ten deliveries, but EPIDimages were acquired ten times on integration mode which effectively simulates cine acquisition. While a documented integration mode was used, the cine acquisition can be used clinically. In‐phantom dose was reconstructed from the EPIDimages of each phase and the reconstructed dose was summed and compared with the in‐phantom film dose. For dose reconstruction, we used our Monte Carlo response‐based algorithm documented previously. Results: The sum of the cine acquisition agreed with (1%) the integrated acquisition during step‐and‐shoot delivery. The discrepancy was variable with delivered MUs and number of segments due to beam instability at start of acquisition and reading loss for cine mode during MLC movement. For sliding‐window delivery, the cine agreed with the other (0.5%). The reconstructed dose agreed with the measurement within 1.4% at the isocenter. Further evaluation with more results will be presented. Conclusion: Feasibility of 4D dose reconstruction was demonstrated. In part supported by Varian Medical Systems.