A three-source model for the calculation of head scatter factors.
ABSTRACT Accurate determination of the head scatter factor Sc is an important issue, especially for intensity modulated radiation therapy, where the segmented fields are often very irregular and much less than the collimator jaw settings. In this work, we report an Sc calculation algorithm for symmetric, asymmetric, and irregular open fields shaped by the tertiary collimator (a multileaf collimator or blocks) at different source-to-chamber distance. The algorithm was based on a three-source model, in which the photon radiation to the point of calculation was treated as if it originated from three effective sources: one source for the primary photons from the target and two extra-focal photon sources for the scattered photons from the primary collimator and the flattening filter, respectively. The field mapping method proposed by Kim et al. [Phys. Med. Biol. 43, 1593-1604 (1998)] was extended to two extra-focal source planes and the scatter contributions were integrated over the projected areas (determined by the detector's eye view) in the three source planes considering the source intensity distributions. The algorithm was implemented using Microsoft Visual C/C++ in the MS Windows environment. The only input data required were head scatter factors for symmetric square fields, which are normally acquired during machine commissioning. A large number of different fields were used to evaluate the algorithm and the results were compared with measurements. We found that most of the calculated Sc's agreed with the measured values to within 0.4%. The algorithm can also be easily applied to deal with irregular fields shaped by a multileaf collimator that replaces the upper or lower collimator jaws.
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ABSTRACT: Volumetric modulated arc therapy (VMAT) has recently emerged as a new clinical modality for conformal radiation therapy. The aim of this work is to establish a methodology and procedure for retrospectively reconstructing the actual dose delivered in VMAT based on the pre-treatment cone-beam computed tomography (CBCT) and dynamic log files. CBCT was performed before the dose delivery and the system's log files were retrieved after the delivery. Actual delivery at a control point including MLC leaf positions, gantry angles and cumulative monitor units (MUs) was recorded in the log files and the information was extracted using in-house developed software. The extracted information was then embedded into the original treatment DICOM-radiation therapy (RT) file to replace the original control point parameters. This reconstituted DICOM-RT file was imported into the Eclipse treatment planning system (TPS) and dose was computed on the corresponding CBCT. A series of phantom experiments was performed to show the feasibility of dose reconstruction, validate the procedure and demonstrate the efficacy of this methodology. The resultant dose distributions and dose-volume histograms (DVHs) were compared with those of the original treatment plan. The studies indicated that CBCT-based VMAT dose reconstruction is readily achievable and provides a valuable tool for monitoring the dose actually delivered to the tumor target as well as the sensitive structures. In the absence of setup errors, the reconstructed dose shows no significant difference from the original pCT-based plan. It is also elucidated that the proposed method is capable of revealing the dosimetric changes in the presence of setup errors. The method reported here affords an objective means for dosimetric evaluation of VMAT delivery and is useful for adaptive VMAT in future.Physics in Medicine and Biology 07/2010; 55(13):3597-610. · 2.70 Impact Factor
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ABSTRACT: This Idea Award (DAMD17-03-1-0023, entitled "Intensity Modulated Radiation Treatment of Prostate Cancer Guided by High Field MR Spectroscopic Imaging") was awarded to the principal investigator (PI) for the period of May 1, 2003 - April 30, 2006. This is the final report for the grant. The goal of this project is to establish biologically conformal -- as opposed to anatomically conformal -- IMRT as a viable modality through integration with 3T magnetic resonance spectroscopic imaging (MRSI) to more effectively kill prostate tumor cells. The underlying hypothesis driving this work is that the MRSI-guided IMRT will provide substantially improved dose distributions required to achieve greater local tumor control while maintaining, or reducing, complications to sensitive structures. The specific aims of the project are: (1) To establish a robust procedure for registering and mapping of MR spectroscopic data to CT/MRI images for prostate irradiation. (2) To develop an inverse planning system for MRSI-guided IMRT prostate treatment and demonstrate the feasibility of concurrent dose escalation to intraprostatic lesion(s) through a set of phantom studies and at least two previously treated prostate cases who had undergone CT/MRSI scans. Under the generous support from the U.S. Army Medical Research and Materiel Command (AMRMC), the PI has contributed significantly to prostate cancer research by applying physics and engineering knowledge to prostate cancer research. A number of significant conference abstracts and refereed papers have resulted from the support. The preliminary data obtained under the support of the grant has also enabled the PI to start new research initiatives, in particularly, in adaptive prostate radiation therapy. The past year's research activities of the PI are highlighted in the following.04/2006;
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ABSTRACT: Independent monitor unit verification of intensity-modulated radiation therapy (IMRT) plans requires detailed 3-dimensional (3D) dose verification. The aim of this study was to investigate using a 3D dose engine in a second commercial treatment planning system (TPS) for this task, facilitated by in-house software. Our department has XiO and Pinnacle TPSs, both with IMRT planning capability and modeled for an Elekta-Synergy 6MV photon beam. These systems allow the transfer of computed tomography (CT) data and RT structures between them but do not allow IMRT plans to be transferred. To provide this connectivity, an in-house computer programme was developed to convert radiation therapy prescription (RTP) files as generated by many planning systems into either XiO or Pinnacle IMRT file formats. Utilization of the technique and software was assessed by transferring 14 IMRT plans from XiO and Pinnacle onto the other system and performing 3D dose verification. The accuracy of the conversion process was checked by comparing the 3D dose matrices and dose volume histograms (DVHs) of structures for the recalculated plan on the same system. The developed software successfully transferred IMRT plans generated by 1 planning system into the other. Comparison of planning target volume (TV) DVHs for the original and recalculated plans showed good agreement; a maximum difference of 2% in mean dose, - 2.5% in D95, and 2.9% in V95 was observed. Similarly, a DVH comparison of organs at risk showed a maximum difference of +7.7% between the original and recalculated plans for structures in both high- and medium-dose regions. However, for structures in low-dose regions (less than 15% of prescription dose) a difference in mean dose up to +21.1% was observed between XiO and Pinnacle calculations. A dose matrix comparison of original and recalculated plans in XiO and Pinnacle TPSs was performed using gamma analysis with 3%/3mm criteria. The mean and standard deviation of pixels passing gamma tolerance for XiO-generated IMRT plans was 96.1 ± 1.3, 96.6 ± 1.2, and 96.0 ± 1.5 in axial, coronal, and sagittal planes respectively. Corresponding results for Pinnacle-generated IMRT plans were 97.1 ± 1.5, 96.4 ± 1.2, and 96.5 ± 1.3 in axial, coronal, and sagittal planes respectively.Medical dosimetry: official journal of the American Association of Medical Dosimetrists 06/2013; · 1.26 Impact Factor