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Title: SU-G-TeP1-09: Modality-Specific Dose Gradient Modeling for Prostate IMRT Using Spherical Distance Maps of PTV and Isodose Contours

Abstract

Purpose: Overlapping volume histogram (OVH) and distance-to-target histogram (DTH) calculations rely on the assumption that dose gradients are symmetric with respect to primary target volume (PTV) expansion and minimum distance to PTV surface, respectively. It is desirable to lift this assumption and instead account for achievable modality-specific dose gradients (MSDG) for a given PTV shape. Methods: From a library of 96 prostate 7-beam IMRT plans, we computed spherical distance maps (SDMs) for PTVs and 3 iso-dose contours. Each SDM contains the minimum distances between plan isocenter and the object’s surface for a fixed set of azimuthal and polar angles. We performed principal component analysis (PCA)-based missing data recovery with PTV SDM as input and a single iso-dose contour SDM as output. Repeating this process for the set of iso-dose contours sparsely reconstructed the MSDG for a given PTV. DVH points were computed from the MSDG for bladder and rectum (OARs) as a natural way of casting patient-specific geometric information into modality-specific dose space (vs. OVH and DTH where data is purely geometric). For comparison, we implemented a cumulative (c)DTH-based prediction algorithm, and produced DVH for both OARs separately. We then computed linear regressions between each method and the DVH ofmore » the original patient plan for three different OAR DVH points. Results: R-squared for MSD-Gcomputed DVH vs. original plan DVH at V90%, V80% and V60% of max dose were 0.86, 0.83, and 0.78, for bladder and 0.36, 0.52, and 0.55 for rectum, respectively; R-squared for cDTH-based predicted DVH vs. original plan DVH were 0.67, 0.81, and 0.83, for bladder and 0.44, 0.50, and 0.51, for rectum, respectively. Conclusion: By simply modeling the MSDG about a given PTV, we are able to reproduce DVHs consistent with a validated cDTH-based DVH prediction. Regression analysis suggests MSDG could further improve predictions.« less

Authors:
 [1];  [2]; ; ;  [1];  [3]
  1. University of Texas Southwestern Medical Center, Dallas, TX (United States)
  2. (United States)
  3. Rensselaer Polytechnic Institute, Troy, NY (United States)
Publication Date:
OSTI Identifier:
22649349
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; BLADDER; DISTANCE; FORECASTING; PROSTATE; RADIATION DOSES; RADIOTHERAPY; RECTUM; REGRESSION ANALYSIS; SIMULATION; SPHERICAL CONFIGURATION

Citation Formats

Folkerts, MM, University of California San Diego, La Jolla, CA, Gu, X, Lu, W, Jiang, SB, and Radke, RJ. SU-G-TeP1-09: Modality-Specific Dose Gradient Modeling for Prostate IMRT Using Spherical Distance Maps of PTV and Isodose Contours. United States: N. p., 2016. Web. doi:10.1118/1.4956999.
Folkerts, MM, University of California San Diego, La Jolla, CA, Gu, X, Lu, W, Jiang, SB, & Radke, RJ. SU-G-TeP1-09: Modality-Specific Dose Gradient Modeling for Prostate IMRT Using Spherical Distance Maps of PTV and Isodose Contours. United States. doi:10.1118/1.4956999.
Folkerts, MM, University of California San Diego, La Jolla, CA, Gu, X, Lu, W, Jiang, SB, and Radke, RJ. 2016. "SU-G-TeP1-09: Modality-Specific Dose Gradient Modeling for Prostate IMRT Using Spherical Distance Maps of PTV and Isodose Contours". United States. doi:10.1118/1.4956999.
@article{osti_22649349,
title = {SU-G-TeP1-09: Modality-Specific Dose Gradient Modeling for Prostate IMRT Using Spherical Distance Maps of PTV and Isodose Contours},
author = {Folkerts, MM and University of California San Diego, La Jolla, CA and Gu, X and Lu, W and Jiang, SB and Radke, RJ},
abstractNote = {Purpose: Overlapping volume histogram (OVH) and distance-to-target histogram (DTH) calculations rely on the assumption that dose gradients are symmetric with respect to primary target volume (PTV) expansion and minimum distance to PTV surface, respectively. It is desirable to lift this assumption and instead account for achievable modality-specific dose gradients (MSDG) for a given PTV shape. Methods: From a library of 96 prostate 7-beam IMRT plans, we computed spherical distance maps (SDMs) for PTVs and 3 iso-dose contours. Each SDM contains the minimum distances between plan isocenter and the object’s surface for a fixed set of azimuthal and polar angles. We performed principal component analysis (PCA)-based missing data recovery with PTV SDM as input and a single iso-dose contour SDM as output. Repeating this process for the set of iso-dose contours sparsely reconstructed the MSDG for a given PTV. DVH points were computed from the MSDG for bladder and rectum (OARs) as a natural way of casting patient-specific geometric information into modality-specific dose space (vs. OVH and DTH where data is purely geometric). For comparison, we implemented a cumulative (c)DTH-based prediction algorithm, and produced DVH for both OARs separately. We then computed linear regressions between each method and the DVH of the original patient plan for three different OAR DVH points. Results: R-squared for MSD-Gcomputed DVH vs. original plan DVH at V90%, V80% and V60% of max dose were 0.86, 0.83, and 0.78, for bladder and 0.36, 0.52, and 0.55 for rectum, respectively; R-squared for cDTH-based predicted DVH vs. original plan DVH were 0.67, 0.81, and 0.83, for bladder and 0.44, 0.50, and 0.51, for rectum, respectively. Conclusion: By simply modeling the MSDG about a given PTV, we are able to reproduce DVHs consistent with a validated cDTH-based DVH prediction. Regression analysis suggests MSDG could further improve predictions.},
doi = {10.1118/1.4956999},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Purpose: Fast and reliable comprehensive quality assurance tools are required to validate the safety and accuracy of complex intensity-modulated radiotherapy (IMRT) plans for prostate treatment. In this study, we evaluated the performance of the COMPASS system for both off-line and potential online procedures for the verification of IMRT treatment plans. Methods and Materials: COMPASS has a dedicated beam model and dose engine, it can reconstruct three-dimensional dose distributions on the patient anatomy based on measured fluences using either the MatriXX two-dimensional (2D) array (offline) or a 2D transmission detector (T2D) (online). For benchmarking the COMPASS dose calculation, various dose-volume indicesmore » were compared against Monte Carlo-calculated dose distributions for five prostate patient treatment plans. Gamma index evaluation and absolute point dose measurements were also performed in an inhomogeneous pelvis phantom using extended dose range films and ion chamber for five additional treatment plans. Results: MatriXX-based dose reconstruction showed excellent agreement with the ion chamber (<0.5%, except for one treatment plan, which showed 1.5%), film ({approx}100% pixels passing gamma criteria 3%/3 mm) and mean dose-volume indices (<2%). The T2D based dose reconstruction showed good agreement as well with ion chamber (<2%), film ({approx}99% pixels passing gamma criteria 3%/3 mm), and mean dose-volume indices (<5.5%). Conclusion: The COMPASS system qualifies for routine prostate IMRT pretreatment verification with the MatriXX detector and has the potential for on-line verification of treatment delivery using T2D.« less
  • A course of one to three large fractions of high dose rate (HDR) interstitial brachytherapy is an attractive alternative to intensity modulated radiation therapy (IMRT) for delivering boost doses to the prostate in combination with additional external beam irradiation for intermediate risk disease. The purpose of this work is to quantitatively compare single-fraction HDR boosts to biologically equivalent fractionated IMRT boosts, assuming idealized image guided delivery (igIMRT) and conventional delivery (cIMRT). For nine prostate patients, both seven-field IMRT and HDR boosts were planned. The linear-quadratic model was used to compute biologically equivalent dose prescriptions. The cIMRT plan was evaluated asmore » a static plan and with simulated random and setup errors. The authors conclude that HDR delivery produces a therapeutic ratio which is significantly better than the conventional IMRT and comparable to or better than the igIMRT delivery. For the HDR, the rectal gBEUD analysis is strongly influenced by high dose DVH tails. A saturation BED, beyond which no further injury can occur, must be assumed. Modeling of organ motion uncertainties yields mean outcomes similar to static plan outcomes.« less
  • A simplified method of verifying intensity modulated radiation therapy (IMRT) fields using a Varian aS500 amorphous silicon electronic portal imaging device (EPID) is demonstrated. Unlike previous approaches, it does not involve time consuming or complicated analytical processing of the data. The central axis pixel response of the EPID, as well as the profile characteristics obtained from images acquired with a 6 MV photon beam, was examined as a function of field size. Ion chamber measurements at various depths in a water phantom were then collected and it was found that at a specific depth d{sub ref}, the dose response andmore » profile characteristics closely matched the results of the EPID analysis. The only manipulation required to be performed on the EPID images was the multiplication of a matrix of off axis ratio values to remove the effect of the flood field calibration. Similarly, d{sub ref} was found for 18 MV. Planar dose maps at d{sub ref} in a water phantom for a bar pattern, a strip pattern, and 14 clinical IMRT fields from two patient cases each being from a separate anatomical region, i.e., head and neck as well as the pelvis, for both energies were generated by the Pinnacle planning system (V7.4). EPID images of these fields were acquired and converted to planar dose maps and compared directly with the Pinnacle planar dose maps. Radiographic film dosimetry and a MapCHECK dosimetry device (Sun Nuclear Corporation, Melbourne, FL) were used as an independent verification of the dose distribution. Gamma analysis of the EPID, film, and Pinnacle planar dose maps generated for the clinical IMRT fields showed that approximately 97% of all points passed using a 3% dose/3mm DTA tolerance test. Based on the range of fields studied, the author's results appear to justify using this approach as a method to verify dose distributions calculated on a treatment planning system, including complex intensity modulated fields.« less
  • The aim of this study is to evaluate the impact of the patient dose due to the kilovoltage cone beam computed tomography (kV-CBCT) in a prostate intensity-modulated radiation therapy (IMRT). The dose distributions for the five prostate IMRTs were calculated using the Pinnacle3 treatment planning system. To calculate the patient dose from CBCT, phase-space beams of a CBCT head based on the ELEKTA x-ray volume imaging system were generated using the Monte Carlo BEAMnrc code for 100, 120, 130, and 140 kVp energies. An in-house graphical user interface called DOSCTP (DOSXYZnrc-based) developed using MATLAB was used to calculate the dosemore » distributions due to a 360 deg. photon arc from the CBCT beam with the same patient CT image sets as used in Pinnacle3. The two calculated dose distributions were added together by setting the CBCT doses equal to 1%, 1.5%, 2%, and 2.5% of the prescription dose of the prostate IMRT. The prostate plan and the summed dose distributions were then processed in the CERR platform to determine the dose-volume histograms (DVHs) of the regions of interest. Moreover, dose profiles along the x- and y-axes crossing the isocenter with and without addition of the CBCT dose were determined. It was found that the added doses due to CBCT are most significant at the femur heads. Higher doses were found at the bones for a relatively low energy CBCT beam such as 100 kVp. Apart from the bones, the CBCT dose was observed to be most concentrated on the anterior and posterior side of the patient anatomy. Analysis of the DVHs for the prostate and other critical tissues showed that they vary only slightly with the added CBCT dose at different beam energies. On the other hand, the changes of the DVHs for the femur heads due to the CBCT dose and beam energy were more significant than those of rectal and bladder wall. By analyzing the vertical and horizontal dose profiles crossing the femur heads and isocenter, with and without the CBCT dose equal to 2% of the prescribed dose, it was found that there is about a 5% increase of dose at the femur head. Still, such an increase in the femur head dose is well below the dose limit of the bone in our IMRT plans. Therefore, under these dose fractionation conditions, it is concluded that, though CBCT causes a higher dose deposited at the bones, there may be no significant effect in the DVHs of critical tissues in the prostate IMRT.« less
  • Purpose: Varian's On-Board Imager is a linac-integrated cone-beam CT (CBCT) system used at the authors' institution to acquire images prior to delivering each fraction of prostate intensity modulated radiotherapy. The images are used to determine a couch shift that realigns the tumor with the position obtained in the planning CT. However, this manual image-guided radiotherapy (IGRT) technique is operator dependent, time consuming, offers limited degrees of freedom, and requires significant imaging dose over the course of treatment. To overcome these problems, the authors propose two fully automatic IGRT techniques that require significantly less imaging dose. Methods: Dose is reduced bymore » lowering the x-ray tube mA s during CBCT acquisition at the cost of increasing image noise. In ''forward'' IGRT, the CBCT image is automatically registered to the planning CT to obtain the necessary couch shift. The ''reverse'' technique offers additional degrees of freedom as it involves nonrigid registration of the planning CT to the CBCT. Both techniques were evaluated using images of an anthropomorphic phantom with simulated motion and by retrospectively analyzing data from ten prostate cancer patients. Results: IGRT error for the phantom data at 100% relative imaging dose was 8.2{+-}3.7, 3.5{+-}1.2, and 2.1{+-}0.6 mm for setup only, forward, and reverse techniques, respectively. For patient images acquired at 100% relative imaging dose, the errors were 5.4{+-}1.7, 5.0{+-}1.6, 5.0{+-}2.0, and 4.0{+-}1.6 mm for setup only, manual forward (performed clinically), automatic forward, and reverse IGRT, respectively. Furthermore, imaging dose could be reduced to 20% without a significant loss in image guidance accuracy. Conclusions: The presented image guidance methods are accurate while requiring only 20% of the standard imaging dose. The combination of low dose, automation, and accuracy enables frequent corrections during treatment, possibly leading to reduced margins and improved treatment outcomes.« less