Conventional PTV Research Articles

Dosimetric Impact of Simulated Daily Adaptive Radiotherapy with Significantly Reduced Setup Margins in the Definitive Treatment of Head and Neck Cancer

<h3>Purpose/Objective(s)</h3> Even when treating head and neck cancer (HNC) with daily image-guided radiation therapy, setup margins of 3-5 mm are required to ensure adequate coverage of disease due to setup error. These margins lead to additional normal tissue dose that may compromise acute and long-term quality-of-life. The use of daily adaptive radiotherapy (ART) using an online adaptive system significantly reduces the need for a PTV margin, since the targets and organs-at-risk (OAR) are recontoured each fraction. In this study, we hypothesized using daily ART (simulated) with 1 mm margins based on our involved nodal radiotherapy (INRT) paradigm would improve coverage and reduce OAR dose. <h3>Materials/Methods</h3> Ten patients with HNC treated on our institutional phase II INRT trial were selected for this study. Cone-beam CT scans were uploaded to the ETHOS emulator for daily adaptive replanning which was simulated using fractions 5, 10, 15, 20, 25, 30, and 35. Two plans were generated with a 1 mm (reduced) and a 5 mm (conventional) PTV expansion. The dose grid from the delivered, conventional margin plan was transposed onto the CBCT and adapted contours. Separately, a reduced target margin plan optimized on the online adaptive system was created. PTV coverage (70 Gy-gross/primary disease, 66.5 Gy-suspicious nodes, 63 Gy-extended volume surrounding the primary disease) and D1 were calculated for target structures along with the dose to OARs. Comparisons performed via paired two tailed t-test. <h3>Results</h3> Six patients with oropharynx cancer and four patients with larynx cancer were included. Median V100% coverage for 70 Gy (96% vs 95.6%), 66.5 Gy (98.5% vs 76.5%), and 63 Gy (98.9% vs 74.9%) were improved with the adaptive planning (all p-values<0.03). Plan homogeneity was improved in the ART plan (e.g., PTV70 median D1% 72.8 Gy vs 74.8 Gy, all p-values<0.02). The total median dose reduction was 10.8 Gy (interquartile range, IQR 8.0-12.6 Gy) for the ipsilateral submandibular gland (SMG), 12.2 Gy (IQR 7.3-23.5 Gy) for the contralateral SMG, 8.3 Gy (IQR 3.8-12.6 Gy) for the ipsilateral parotid, 8.1 Gy (IQR 4.4-11.0 Gy) for the contralateral parotid, 7.0 Gy (IQR 4.5-9.1 Gy) for larynx, 6.6 Gy (IQR 2.7-8.4 Gy) for oral cavity, and 8.6 Gy (IQR 5.9-12.1 Gy) for constrictors. <h3>Conclusion</h3> Simulated daily ART using a 1 mm PTV margin provided significant improvement over conventional PTV margins in coverage and critical OAR dosimetry. This small proof-of-concept study suggests this approach may lead to improved quality-of-life while preserving or improving tumor coverage. Further evaluation is needed to assess the resources needed to implement daily adaptations. A phase II randomized trial of daily ART is underway to determine if dosimetric improvements lead to similar gains in patient-reported outcomes.

Implementation and experimental validation of a robust hybrid direct aperture optimization approach for mixed-beam radiotherapy.

PurposeThe objectives of the work presented in this paper were to (1) implement a robust‐optimization method for deliverable mixed‐beam radiotherapy (MBRT) plans within a previously developed MBRT planning framework; (2) perform an experimental validation of the delivery of robust‐optimized MBRT plans; and (3) compare PTV‐based and robust‐optimized MBRT plans in terms of target dose robustness and organs at risk (OAR) sparing for clinical head and neck and brain patient cases.MethodsA robust‐optimization method, which accounts for translational setup errors, was implemented within a previously developed treatment planning framework for MBRT. The framework uses a hybrid direct aperture optimization method combining column generation and simulated annealing. A robust plan was developed and then delivered to an anthropomorphic head phantom using the Developer Mode of a TrueBeam linac. Planar dose distributions were measured and compared to the planned dose. Robust‐optimized and PTV‐based plans were developed for three clinical patient cases consisting of two head and neck cases and one brain case. The plans were compared in terms of the robustness to 5 mm shifts of the target volume dose as well as in terms of OAR sparing.ResultsUsing a gamma criterion of 3%/2 mm and a dose threshold of 10%, the agreement between film measurements and dose calculations was better than 97.7% for the total plan and better than 95.5% for the electron component of the plan. For the two head and neck patient cases, the average clinical target volume (CTV) dose homogeneity index (V95%–V107%) over all the considered setup error scenarios was on average 19% lower for the PTV‐based plans and it had a larger standard deviation. The robust‐optimized plans achieved, on average, a 20% reduction in the OAR doses compared to the PTV‐based plans. For the brain patient case, the CTV dose homogeneity index was similar for the two plans, while the OAR doses were 22% lower, on average, for the robust‐optimized plan. No clear trend in terms of electron contributions was found across the three patient cases, although robust‐optimized plans tended toward higher electron beam energies.ConclusionsA framework for robust optimization of deliverable MBRT plans has been developed and validated. PTV‐based MBRT were found to not be robust to setup errors, while the dose delivered by the robust‐optimized plans were clinically acceptable for all considered error scenarios and had better OAR sparing. This study shows that the robust optimization is a promising alternative to conventional PTV margins for MBRT.

In situ color-to-depth calibration: toward practical three-dimensional color particle tracking velocimetry

We propose a seeding particle-based color-to-depth calibration methodology for three-dimensional color particle tracking velocimetry (3D color PTV) using a single camera and volumetric rainbow gradient illumination. The use of sheet-color illumination from a liquid crystal display projector enables in situ calibration, namely the color-to-depth relationships of particles seeded in a fluid are determined without inserting any calibration equipment or taking a different optical setup. That is, in this methodology, the calibration and application can be performed using the same optical configuration, and only the digital illumination patterns need to be changed. Adopting this calibration allows evaluating actual color-to-depth relationships of the particles in measurements. The calibration is conducted regarding the relationship between spatially distributed particle colors and their depth coordinates by support of an artificial neural network. By combining conventional PTV with the depth estimated by the color, particle trajectories in 3D real space can be reconstructed from the calibration. The performance of the proposed method was evaluated using a rotating flow in a cylindrical tank by comparing its results with the flow fields measured by conventional particle image velocimetry. Good accordance in the comparison at the highly 3D flow suggests the applicability of the present methodology for various flow configurations.

Holographic astigmatic particle tracking velocimetry (HAPTV)

The formation of twin images in digital inline holography (DIH) prevents the placement of the focal plane in the center of a sample volume for DIH-based particle tracking velocimetry (DIH-PTV) with a single camera. As a result, it is challenging to apply DIH-PTV for flow measurements in large-scale laboratory facilities or many field applications where it would otherwise be desirable due to the low cost and compact setup. Here we introduce holographic astigmatic PTV (HAPTV) by inserting a cylindrical lens in the optical setup of DIH-PTV, generating distorted holograms. Such distortion is subsequently utilized in a customized reconstruction algorithm to distinguish tracers positioned on different sides of the focal plane which can in turn be placed in the middle of a sample volume. Our HAPTV approach is calibrated under high (1 µm pixel−1) and low (10 µm pixel−1) magnifications with an error standard deviation of 4.2 µm (one particle diameter) and 120.7 µm (~5 times the particle diameter), respectively. We compare the velocity field of a laminar jet flow obtained using HAPTV and conventional PIV to illustrate the accuracy of the technique when applied to practical flow measurement applications. The work demonstrates that HAPTV improves upon the depth of field of conventional astigmatic PTV and enables the implementation of DIH-based PTV for in situ applications.

Clinical implementation in proton therapy of multi-field optimization by a hybrid method combining conventional PTV with robust optimization

To implement a robust multi-field optimization (MFO) technique compatible with the application of a Monte Carlo (MC) algorithm and to evaluate its robustness. Nine patients (three brain, five head-and-neck, one spine) underwent proton treatment generated by a novel robust MFO technique. A hybrid (hMFO) approach was implemented, planning dose coverage on isotropic PTV compensating for setup errors, whereas range calibration uncertainties are incorporated into PTV robust optimization process. hMFO was compared with single-field optimization (SFO) and full robust multi-field optimization (fMFO), both on the nominal plan and the worst-case scenarios assessed by robustness analysis. The SFO and the fMFO plans were normalized to hMFO on CTV to obtain iso-D95 coverage, and then the organs at risk (OARs) doses were compared. On the same OARs, in the normalized nominal plans the potential impact of variable relative biological effectiveness (RBE) was investigated. hMFO reduces the number of scenarios computed for robust optimization (from twenty-one in fMFO to three), making it practicable with the application of a MC algorithm. After normalizing on D95 CTV coverage, nominal hMFO plans were superior compared to SFO in terms of OARs sparing (p < 0.01), without significant differences compared to fMFO. The improvement in OAR sparing with hMFO with respect to SFO was preserved in worst-case scenarios (p < 0.01), confirming that hMFO is as robust as SFO to physical uncertainties, with no significant differences when compared to the worst case scenarios obtained by fMFO. The dose increase on OARs due to variable RBE was comparable to the increase due to physical uncertainties (i.e. 4–5 Gy(RBE)), but without significant differences between these techniques. hMFO allows improving plan quality with respect to SFO, with no significant differences with fMFO and without affecting robustness to setup, range and RBE uncertainties, making clinically feasible the application of MC-based robust optimization.

Evaluation of Dynamic Tumor-tracking Intensity-modulated Radiotherapy for Locally Advanced Pancreatic Cancer

Intensity-modulated radiotherapy (IMRT) is now regarded as an important treatment option for patients with locally advanced pancreatic cancer (LAPC). To reduce the underlying tumor motions and dosimetric errors during IMRT as well as the burden of respiratory management for patients, we started to apply a new treatment platform of the dynamic tumor dynamic tumor-tracking intensity-modulated radiotherapy (DTT-IMRT) using the gimbaled linac, which can swing IMRT toward the real-time tumor position under patients’ voluntary breathing. Between June 2013 and March 2015, ten patients were treated, and the tumor-tracking accuracy and the practical benefits were evaluated. The mean PTV size in DTT-IMRT was 18% smaller than a conventional ITV-based PTV. The root-mean-squared errors between the predicted and the detected tumor positions were 1.3, 1.2, and 1.5 mm in left-right, anterior-posterior, and cranio-caudal directions, respectively. The mean in-room time was 24.5 min. This high-accuracy of tumor-tracking with reasonable treatment time are promising and beneficial to patients with LAPC.

Robust optimization in lung treatment plans accounting for geometric uncertainty.

Robust optimization generates scenario‐based plans by a minimax optimization method to find optimal scenario for the trade‐off between target coverage robustness and organ‐at‐risk (OAR) sparing. In this study, 20 lung cancer patients with tumors located at various anatomical regions within the lungs were selected and robust optimization photon treatment plans including intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) plans were generated. The plan robustness was analyzed using perturbed doses with setup error boundary of ±3 mm in anterior/posterior (AP), ±3 mm in left/right (LR), and ±5 mm in inferior/superior (IS) directions from isocenter. Perturbed doses for D99, D98, and D95 were computed from six shifted isocenter plans to evaluate plan robustness. Dosimetric study was performed to compare the internal target volume‐based robust optimization plans (ITV‐IMRT and ITV‐VMAT) and conventional PTV margin‐based plans (PTV‐IMRT and PTV‐VMAT). The dosimetric comparison parameters were: ITV target mean dose (Dmean), R95(D95/Dprescription), Paddick's conformity index (CI), homogeneity index (HI), monitor unit (MU), and OAR doses including lung (Dmean, V20 Gy and V15 Gy), chest wall, heart, esophagus, and maximum cord doses. A comparison of optimization results showed the robust optimization plan had better ITV dose coverage, better CI, worse HI, and lower OAR doses than conventional PTV margin‐based plans. Plan robustness evaluation showed that the perturbed doses of D99, D98, and D95 were all satisfied at least 99% of the ITV to received 95% of prescription doses. It was also observed that PTV margin‐based plans had higher MU than robust optimization plans. The results also showed robust optimization can generate plans that offer increased OAR sparing, especially for normal lungs and OARs near or abutting the target. Weak correlation was found between normal lung dose and target size, and no other correlation was observed in this study.

Effects of Postmastectomy Radiation Therapy on Immediate Tissue Expander and Acellular Dermal Matrix Reconstruction

PTV was modified accordingly. Cosmesis was scored according to the Harvard scale at the 3 month follow up visit. Results: Since February 2014, ten patients were enrolled on this prospective registry. Median age was 57.4 (range: 42 e 77). Mean PTV volume was 76.4 cc (range: 131 e 54.6 cc), representing a 64% reduction in volume when compared to a conventional PTV margin. Mean time in treatment room was 36 min, of which 15 min was spent on patient set up and MR imaging. 100 online volumetric MR images were available for evaluation. The mean 90% and 95% prescription dose covered 99.7% (range: 99.1-100%) and 97.6% (range: 90.5 e 99.9%) of PTV respectively. Mean inter-fraction standard deviation in 90% and 95% prescription coverage was less than 1% of PTV (90% prescription coverage: 0.26% PTV, and 95% prescription coverage: 0.63% PTV). Evaluation of cine MR during treatment did not demonstrate significant intra-fractional motion. Maximum skin dose was less than prescription in all cases. Nine of the ten patients had 3 month cosmetic data collected per the Harvard scale for breast cosmesis. All nine patients had excellent (7/9) or good (2/9) cosmetic results. Conclusion: Delivery of MR-guided APBI with reduced margins achieved appropriate target coverage based on assessment of actual delivered dose with favorable short term cosmesis. To our knowledge, this is the first series of APBI patients to be treated using MR-IGRT with minimal margins, thereby sparing a greater proportion of normal breast. Efforts to collect local control and long term cosmetic data are currently underway. Author Disclosure: S. Acharya: None. B.W. Fischer-Valuck: None.M.C. Roach: None. R. Kashani: None. L.L. Ochoa: None. M. Yang: None. S. Mutic: Honoraria; Philips Healthcare. Consultant; ViewRay. Travel Expenses; American Association of Medical Dosimetrists, American Association of Physicists in Medicine, American Society for Therapeutic Radiation Oncology. Stock; TreatSafely, Radialogica. I. Zoberi: None. M.A. Thomas: None.

SU-E-T-288: Dose Volume Population Histogram (DVPH): A New Method to Evaluate Intensity Modulated Proton Therapy Plans With Geometrical Uncertainties

Purpose: In Proton therapy, especially intensity modulated proton therapy(IMPT), the dose distribution shape is very sensitive to errors due to sharp dose gradients at the Bragg peaks. The concept of the conventional margin is based on the assumption that dose distribution is shifted rather than deformed due to geometrical uncertainties. The goal of this study is to access the validity of the margin concept as well as propose a new approach using Dose Volume Population Histogram (DVPH) in evaluating IMPT plans. Methods: For a prostate case, an intensity modulated proton therapy is optimized based on the conventional PTV based objective function. The plan is evaluated based on the PTV DVH and CTV DVPH (dose volume population histogram) which explicitly taking into account geometric uncertainties. The DVPH is calculated based on 2197 dose distributions at different CTV possible positions for both random and systematic errors. The DVPH with a 90% confidence level is used for the comparison. Results: The minimum dose of the CTV DVPH with a 90% confidence level is only about 18% of the prescribed dose, while the minimum dose of the PTV is 95%. For bladder DVHs, the D50 and D35 is 26% and 30%, compared to 65% and 70% of the prescribed dose from the bladder DVPH with 90% confidence level. Conclusion: The results showed that the PTV concept for ensuring the prescribed dose actually delivered to the CTV is invalid in proton therapy. The good PTV DVH might Result in an underdose to the target and should not be used for IMPT optimization. For OARs, the conventional evaluation approach underestimates dose volume end points. The new concept DVPH has been proved to provide a more accurate DVH evaluation in proton therapy.

SU‐E‐J‐206: Adaptive Radiotherapy for Gynecological Malignancies with MRIGuided Cobolt‐60 Radiotherapy

Purpose:Even in the IMRT era, bowel toxicity and bone marrow irradiation remain concerns with pelvic irradiation. We examine the potential gain from an adaptive radiotherapy workflow for post‐operative gynecological patients treated to pelvic targets including lymph nodes using MRI‐guided Co‐60 radiation therapy.Methods:An adaptive workflow was developed with the intent of minimizing time overhead of adaptive planning. A pilot study was performed using retrospectively analyzed images from one patient's treatment. The patient's treated plan was created using conventional PTV margins. Adaptive treatment was simulated on the patient's first three fractions. The daily PTV was created by removing non‐target tissue, including bone, muscle and bowel, from the initial PTV based on the daily MRI. The number of beams, beam angles, and optimization parameters were kept constant, and the plan was re‐optimized. Normal tissue contours were not adjusted for the re‐optimization, but were adjusted for evaluation of plan quality. Plan quality was evaluated based on PTV coverage and normal tissue DVH points per treatment protocol. Bowel was contoured as the entire bowel bag per protocol at our institution. Pelvic bone marrow was contoured per RTOG protocol 1203.Results:For the clinically treated plan, the volume of bowel receiving 45 Gy was 380 cc, 53% of the rectum received 30 Gy, 35% of the bladder received 45 Gy, and 28% of the pelvic bone marrow received 40 Gy. For the adaptive plans, the volume of bowel receiving 45 Gy was 175–201 cc, 55–62% of the rectum received 30 Gy, 21– 27% of the bladder received 45 Gy, and 13–17% of the pelvic bone marrow received 40 Gy.Conclusion:Adaptive planning led to a large reduction of bowel and bone marrow dose in this pilot study. Further study of on‐line adaptive techniques for the radiotherapy of pelvic lymph nodes is warranted.Dr. Low is a member of the scientific advisory board of ViewRay, Inc.

SU‐F‐BRD‐05: Robustness of Dose Painting by Numbers in Proton Therapy

Purpose:Proton range uncertainties may cause important dose perturbations within the target volume, especially when steep dose gradients are present as in dose painting. The aim of this study is to assess the robustness against setup and range errors for high heterogeneous dose prescriptions (i.e., dose painting by numbers), delivered by proton pencil beam scanning.Methods:An automatic workflow, based on MATLAB functions, was implemented through scripting in RayStation (RaySearch Laboratories). It performs a gradient‐based segmentation of the dose painting volume from 18FDG‐PET images (GTVPET), and calculates the dose prescription as a linear function of the FDG‐uptake value on each voxel. The workflow was applied to two patients with head and neck cancer. Robustness against setup and range errors of the conventional PTV margin strategy (prescription dilated by 2.5 mm) versus CTV‐based (minimax) robust optimization (2.5 mm setup, 3% range error) was assessed by comparing the prescription with the planned dose for a set of error scenarios.Results:In order to ensure dose coverage above 95% of the prescribed dose in more than 95% of the GTVPET voxels while compensating for the uncertainties, the plans with a PTV generated a high overdose. For the nominal case, up to 35% of the GTVPET received doses 5% beyond prescription. For the worst of the evaluated error scenarios, the volume with 5% overdose increased to 50%. In contrast, for CTV‐based plans this 5% overdose was present only in a small fraction of the GTVPET, which ranged from 7% in the nominal case to 15% in the worst of the evaluated scenarios.Conclusion:The use of a PTV leads to non‐robust dose distributions with excessive overdose in the painted volume. In contrast, robust optimization yields robust dose distributions with limited overdose.RaySearch Laboratories is sincerely acknowledged for providing us with RayStation treatment planning system and for the support provided.

PlanJury: probabilistic plan evaluation revisited

Purpose: Over a decade ago, the 'Van Herk margin recipe paper' introduced plan evaluation through DVH statistics based on population distributions of systematic and random errors. We extended this work for structures with correlated uncertainties (e.g. lymph nodes or parotid glands), and considered treatment plans containing multiple (overlapping) dose distributions (e.g. conventional lymph node and hypo-fractionated tumor doses) for which different image guidance protocols may lead to correlated errors.Methods: A command-line software tool 'PlanJury' was developed which reads 3D dose and structure data exported from a treatment planning system. Uncertainties are specified by standard deviations and correlation coefficients. Parameters control the DVH statistics to be computed: e.g. the probability of reaching a DVH constraint, or the dose absorbed at given confidence in a (combined) volume. Code was written in C++ and parallelized using OpenMP. Testing geometries were constructed using idealized spherical volumes and dose distributions.Results: Negligible stochastic noise could be attained within two minutes computation time for a single target. The confidence to properly cover both of two targets was 90% for two synchronously moving targets, but decreased by 7% if the targets moved independently. For two partially covered organs at risk the confidence of at least one organ below the mean dose threshold was 40% for synchronous motion, 36% for uncorrelated motion, but only 20% for either of the organs separately. Two abutting dose distributions ensuring 91% confidence of proper target dose for correlated motions led to 28% lower confidence for uncorrelated motions as relative displacements between the doses resulted in cold spots near the target.Conclusions: Probabilistic plan evaluation can efficiently be performed for complicated treatment planning situations, thus providing important plan quality information unavailable in conventional PTV based evaluations.

SU-E-J-206: Delivered Dose to Organs From CBCT-Based IGRT of the Prostate

Purpose: To estimate the actual delivered dose to organs from CBCT-based image-guided radiation therapy for localized prostate cancer treatments. Methods: Seven localized prostate cancer treatments were retrospectively selected, each with ∼40 CBCT images. All patients were treated with 9 IMRT beams. The PTV margin was 6 mm in the posterior direction, and 10 mm otherwise around the prostate. For each patient, the original plan was used to calculate treatment dose on each CBCT image at treatment position. The calculated daily dose was then warped back to the original planning CT via an intensity-based B-spline deformable image registration (DIR) algorithm. The DIR algorithm employed mutual information with a multi-resolution and multi-stage scheme. From the transferred total cumulative dose, dosimetric parameters were recorded for comparison with the original plan. The quality of prostate registration was quantified using three implanted fiducial markers. Only those cases with < 2mm error were included in the dose analysis. Results: The overall marker displacement was 1.8±1.8 mm after registrations. 75 of 281 registrations (27%) exceeded the 2 mm error threshold. The primary causes were the excessive noise level on CBCT images especially for large patients, presence of rectum gas, and lower abdominal motion artifacts. Excluding these cases, a total of 206 CBCT images were included in dosimetric analysis. Changes in dose coverage to the prostate was minimal (<1%). Dose deviations for rectum (Dmax, V50, V65, V75) were (0.5±1.0%, 2.6±2.5%, 3.5±3.7%, 2.3±4.2%) and (−1.1−0.9%, −2.5±5.3%, −3.0±5.3%, −3.8±4.9%) for bladder, respectively. Conclusion: With the conventional PTV margins, deviations in dose coverage of the prostate in the face of daily anatomic changes were minor However, relatively larger deviations were observed for rectum and bladder (1SD=∼4.3%). Reduced margins may lower dose to critical organs while maintaining target coverage. However, margin reduction must be viewed cautiously and with consideration of clinical outcomes.

Adaptive radiotherapy for head and neck cancer—Dosimetric results from a prospective clinical trial

PurposeTo conduct a clinical trial evaluating adaptive head and neck radiotherapy (ART). MethodsPatients with locally advanced oropharyngeal cancer were prospectively enrolled. Daily CT-guided setup and deformable image registration permitted mapping of dose to avoidance structures and CTVs. We compared four planning scenarios: (1) original IMRT plan aligned daily to marked isocenter (BB); (2) original plan aligned daily to bone (IGRT); (3) IGRT with one adaptive replan (ART1); and (4) actual treatment received by each study patient (IGRT with one or two adaptive replans, ART2). ResultsAll 22 study patients underwent one replan (ART1); eight patients had two replans (ART2). ART1 reduced mean dose to contralateral parotid by 0.6Gy or 2.8% (paired t-test; p=0.003) and ipsilateral parotid by 1.3Gy (3.9%) (p=0.002) over the IGRT alone. ART2 further reduced the mean contralateral parotid dose by 0.8Gy or 3.8% (p=0.026) and ipsilateral parotid by 4.1Gy or 9% (p=0.001). ART significantly reduced integral body dose. ConclusionsThis pilot trial suggests that head and neck ART dosimetrically outperforms IMRT. IGRT that leverages conventional PTV margins does not improve dosimetry. One properly timed replan delivers the majority of achievable dosimetric improvement. The clinical impact of ART must be confirmed by future trials.

TH‐A‐213AB‐05: Robust Plan Optimization for Calypso Gated Hypo‐Fractionated Prostate Treatment Using Volumetric Modulated Arc Therapy

Purpose: To develop a robust optimization algorithm which incorporates a Calypso‐guided gating strategy into the optimization process to maximize clinical target volume (CTV) coverage and organs at risk (OAR) sparing in the presence of intra‐fractional motion. Method: In a computer simulation, intra‐fractional motion traces from prior treatments with Calypso were converted to a probability distribution function (PDF), truncated using a patient specific gating window, and renormalized to yield a new PDF with Calypso‐gated interventions. In lieu of a conventional PTV, multiple instances of CTV and OARs were replicated and displaced to extreme positions inside the gating window representing possible delivery scenarios. When optimizing the volumetric modulated arc therapy plan, doses to the CTV and OARs were calculated as a sum of doses to the replicas weighted by the PDF to account for motion. A robust plan meeting the clinical constraints was produced and compared to the counterpart conventional margin (PTV) plan. Calypso traces from a separate testing database served to simulate motion during gated delivery. Dosimetric end points extracted from 4D dose accumulations for each trace were utilized to evaluate potential clinical benefit. Results: Five prostate cases from a hypofractionated protocol (42.5 Gy in 5 fractions) were retrospectively investigated. The patient specific gating window resulted in tight anterior and inferior action levels (∼1mm) to protect rectal wall and bladder wall, and resulted in an average of four beam interruptions per fraction in the simulation. The robust‐optimized plans achieved the same average CTV D95 coverage of 40.5Gy as the PTV‐optimized plans, but with reduced patient‐averaged rectum wall D1cc by 2.2 Gy and bladder wall mean dose by 2.9Gy. Conclusion: Integration of an intra‐fractional motion management strategy into the robust optimization process is feasible and may yield improved OAR sparing compared to the standard margin approach.

하이브리드 PTV-PIV알고리듬에 의한 고정밀 와도 추정

Abstract. A PTV algorithm was constructed using a linear transformation, in which the merits of the con-ventional PIV and PTV were adopted. In PIV calculations, the obtained velocity vectors are affected bythe filtering effects by its calculation principle. PTV techniques are widely used for their excellences ofmeasuring small scaled flows, such as nano and bio flows. However, PTVs produce vector errors due tointerpolation process. To overcome these problems, a hybrid PTV algorithm was constructed by combiningPTVs' and PIVs' benefits using a linear transformation. The Taylor-Green vortex flows were generatedfor the tests of vorticity calculations. The conventional gray-level cross-correlation PIV technique and 2-Frame PTV technique were tested for the same flows for comparisons with those obtained by the con-structed hybrid algorithm. The excellence of the constructed hybrid algorithm was validated through anactual experiment on the cylinder wake. Key Words: PIV(입자영상유속계), PTV(입자추적유속계), Hybrid Algorithm(하이브리드알고리듬 ), CylinderWake( 실린더후류), Vorticity(와도)

WE-A-BRA-02: A Novel Beam-Specific PTV Design to Account for Setup Error and Range Uncertainties for Scanning Beam Proton Therapy

Purpose: Conventional PTV is purely geometric concept that is inadequate for proton therapy since it does not take into account range errors that arise from CT number uncertainties and interplay effect of setup error and tissue heterogeneity. Especially for spot‐scanning proton therapy, we can't use the passively scattered proton planning approach, such as compensator smearing. This study investigates a novel method of designing PTV that guarantees adequate target coverage to CTV under such uncertainties. Methods and Materials: The CTV was expanded laterally from the axis of beam's eye view (BEV) to account for internal organ motion and setup error. Ray tracing algorithm is used to calculate range uncertainty at distal and proximal surfaces. Due to lateral setup error and organ motion, range uncertainties were analyzed within the lateral motion breadth to construct the maximum (distal) and minimum (proximal) range surfaces along each ray line. This is equivalent to compensator smearing. Subsequently, these water‐equivalent ranges were converted to physical margins using local density information along each ray line. Mathematical phantom was constructed and planned using conventional PTV and new beam‐specific PTV under single field uniform dose treatment technique. Dose distributions were compared under different simulated setup error and motion. Results: The plans using conventional PTV showed significant cold spots and hot spots in the presence of setup error and motion. The minimum percent dose of CTV coverage dropped from 96.7.9% to 38.0% when both setup error and motion is applied. However, plans using beam‐specific PTV only showed minimum dose drop from 96.9% to 87.4%. The resulted dose distribution showed significant advantage in covering the edge of the CTV using beam‐specific PTV. Conclusions: This preliminary study indicates the feasibility of proton beam‐specific PTV to guarantee adequate target coverage under anticipated setup error in heterogeneous human tissue.

MO‐EE‐A1‐03: A 4D IMRT Planning Method Using Deformable Image Registration for Lung Cancer

Purpose: 4D CT is increasingly used to define the ITV, the envelope of the CTV as it moves during breathing, and PTV (ITV + set up margin). IMRT plans are designed to provide uniform dose to the PTV. We propose a planning method to design true 4D IMRT plans in which the PTVs of the individual phases of the 4D CT as well as the conventional PTV may receive non‐uniform doses, but the cumulative doses to the PTVs of individual phases, computed using deformable image registration (DIR), are uniform. Methods: The non‐uniform dose prescription for conventional PTV was obtained by solving linear equations by requiring motion‐convolved 4D dose to be uniform to the PTV for the end exhale phase (PTV50%) and by constraining maximum inhomogeneity to be 30%. A plug‐in code to Pinnacle was developed to perform the IMRT optimization based on this non‐uniform PTV dose prescription. The 4D dose was obtained by summing the mapped doses from individual phases of the 4D CT using DIR. This 4D dose distribution was compared with that of the ITV method. The robustness of 4D plans over the course of radiotherapy was evaluated by computing the sum of the 4D dose distributions for each weekly CT mapped to the planning 4D CT using DIR. Results: The 4D dose distribution provided additional lung sparing by 5% for V5, V10, V20 and V30 compared to the use of the ITV method. The dose volume histograms of PTV50%, CTV, lung, spinal cord, and heart for the cumulative dose over the course of IMRT were similar to those for 4D dose at the time of original planning. Conclusion: The proposed 4D planning method may increase lung sparing compared to the ITV method used commonly in the clinics and is robust against inter fractional set‐up and anatomy changes.

An improved PTV system for large-scale physical river model

To measure the surface flow in a physical river model, an improved system of Large-Scale Particle Tracking Velocimetry (LSPTV) was proposed and the elements of the PTV system were described. Usually the tracer particles of a PTV system seeded on water surface tend to form conglomerates due to surface tension of water. In addition, they can not float on water surface when water flow is shallow. Ellipsoid particles were used to avoid the above problems. Another important issue is particle recognition. In order to eliminate the influence of noise, particles were recognized by the processing of multi-frame images. The kernel of the improved PTV system is the algorithm for particle tracking. A new 3-frame PTV algorithm was developed. The performance of this algorithm was compared with the conventional 4-frame PTV algorithm and 2-frame PTV algorithm by means of computer simulation using synthetically generated images. The results show that the new 3-frame PTV algorithm can recover more velocity vectors and have lower relative error. In addition, in order to attain the whole flow field from individual flow fields, the method of stitching individual flow fields by obvious marks was worked out. Then the improved PTV system was applied to the measurement of surface flow field in Model Yellow River and shows good performance.

Image-guided intensity-modulated radiation therapy for gallbladder carcinoma

Purpose Clinical and technical parameter analysis of patients treated with ultrasound-based image-guided tomotherapeutic IMRT for gallbladder cancer. Methods and materials Between 8/2001 and 5/2005, 10 patients with primary tumors of the gallbladder were treated by image-guided IMRT to median doses of 59 Gy. To analyze normal tissue radiation exposure reduction using this novel approach, a virtual plan comparison between actually delivered IMRT plans and re-computed plans with identical inverse planning parameters but more conventional PTV safety margins was conducted. Results Average CTV was 379 cm 3, with a mean initial PTV of 834 cm 3. In 9/10 patients, a boost was delivered to a mean CTV boost of 171 cm 3 and average PTV boost of 241 cm 3. One patient reported RTOG grade 3 acute toxicity. All other patients exhibited Grade 2 or lower acute toxicity. Preliminary median overall survival was 16.7 months (range 3.2–34.9 months), with 5/10 patients alive at analysis. Virtual plan comparison revealed significant organ-at-risk sparing by the enabled PTV margin reduction. Conclusion Ultrasound-based image-guided IMRT is a feasible mechanism of delivering conformal radiation doses to tumors of the gallbladder with acceptable toxicity. Early outcome data with this novel radiation planning and delivery technique are encouraging and comparable to previously reported literature.