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Title: Noncoplanar beams improve dosimetry quality for extracranial intensity modulated radiotherapy and should be used more extensively


No abstract prepared.

 [1];  [2]
  1. Radiation Oncology, David Geffen School of Medicine at UCLA, Los Angeles, California 90095 (United States)
  2. Medical Physics, Swedish Cancer Institute, Seattle, Washington 98104 (United States)
Publication Date:
OSTI Identifier:
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 42; Journal Issue: 2; Other Information: (c) 2015 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States

Citation Formats

Sheng, Ke, E-mail:, and Shepard, David M., E-mail: Noncoplanar beams improve dosimetry quality for extracranial intensity modulated radiotherapy and should be used more extensively. United States: N. p., 2015. Web. doi:10.1118/1.4895981.
Sheng, Ke, E-mail:, & Shepard, David M., E-mail: Noncoplanar beams improve dosimetry quality for extracranial intensity modulated radiotherapy and should be used more extensively. United States. doi:10.1118/1.4895981.
Sheng, Ke, E-mail:, and Shepard, David M., E-mail: 2015. "Noncoplanar beams improve dosimetry quality for extracranial intensity modulated radiotherapy and should be used more extensively". United States. doi:10.1118/1.4895981.
title = {Noncoplanar beams improve dosimetry quality for extracranial intensity modulated radiotherapy and should be used more extensively},
author = {Sheng, Ke, E-mail: and Shepard, David M., E-mail:},
abstractNote = {No abstract prepared.},
doi = {10.1118/1.4895981},
journal = {Medical Physics},
number = 2,
volume = 42,
place = {United States},
year = 2015,
month = 2
  • Accurate measurements of the penumbra region are important for the proper modeling of the radiation beam for linear accelerator-based intensity modulated radiation therapy. The usual data collection technique with a standard ionization chamber artificially broadens the measured beam penumbrae due to volume effects. The larger the chamber, the greater is the spurious increase in penumbra width. This leads to inaccuracies in dose calculations of small fields, including small fields or beam segments used in IMRT. This source of error can be rectified by the use of film dosimetry for penumbra measurements because of its high spatial resolution. The accuracy ofmore » IMRT calculations with a pencil beam convolution model in a commercial treatment planning system was examined using commissioning data with and without the benefit of film dosimetry of the beam penumbrae. A set of dose-spread kernels of the pencil beam model was calculated based on commissioning data that included beam profiles gathered with a 0.6-cm-i.d. ionization chamber. A second set of dose-spread kernels was calculated using the same commissioning data with the exception of the penumbrae, which were measured with radiographic film. The average decrease in the measured width of the 80%-20% penumbrae of various square fields of size 3-40 cm, at 5 cm depth in water-equivalent plastic was 0.27 cm. Calculations using the pencil beam model after it was re-commissioned using film dosimetry of the penumbrae gave better agreement with measurements of IMRT fields, including superior reproduction of high dose gradient regions and dose extrema. These results show that accurately measuring the beam penumbrae improves the accuracy of the dose distributions predicted by the treatment planning system and thus is important when commissioning beam models used for IMRT.« less
  • Radiation sensitive gels have been used as dosimeters for clinical dose verification of different radiation therapy modalities. However, the use of gels is not widespread, because careful techniques are required to achieve the dose precision and accuracy aimed for in clinical dose verification. Here, the introduction of gel dosimetry in a clinical environment is described, including the whole chain of customizations and preparations required to introduce magnetic resonance (MR) based gel dosimetry into clinical routine. In order to standardize gel dosimetry in dose verifications for radiosurgery and intensity modulated radiotherapy (IMRT), we focused on both the customization of the gelmore » composition and of the MR imaging parameters to increase its precision. The relative amount of the components of the normoxic, methacrylic acid based gel (MAGIC) was changed to obtain linear and steep dose response relationships. MR imaging parameters were customized for the different dose ranges used in order to lower the relative standard deviation of the measured transversal relaxation rate (R{sub 2}). An optimization parameter was introduced to quantify the change in the relative standard deviation of R{sub 2} ({sigma}{sub R2,rel}) taking the increase in MR time into account. A 9% methacrylic acid gel customized for radiosurgery was found to give a linear dose response up to 40 Gy with a slope of 0.94 Gy{sup -1} s{sup -1}, while a 6% methacrylic acid gel customized for IMRT had a linear range up to 3 Gy with a slope of 1.86 Gy{sup -1} s{sup -1}. With the help of an introduced optimization parameter, the mean {sigma}{sub R2,rel} was improved by 13% for high doses and by 55% for low doses, without increasing MR time to unacceptable values. A mean dose resolution of less than 0.13 Gy has been achieved with the gel and imaging parameters customized for IMRT and a dose resolution from 0.97 Gy (at 5 Gy) to 2.15 Gy (at 40 Gy) for the radiosurgery dose range. The comparisons of calculated and measured relative 3D dose distributions performed for radiosurgery and IMRT showed an acceptable overall correlation. The gamma criterion for the radiosurgery verification with a voxel size of 1.5x1.5x1.5 mm{sup 3} was passed by 96.8% of the voxels (1.5 mm distance, 8% in dose). For the IMRT verification using a voxel size of 1.25x1.25x5 mm{sup 3} the gamma criterion was passed by 50.3% of the voxels (3 mm distance, 3% dose uncertainty). Using dedicated data analysis and visualization software, MR based normoxic gel dosimetry was found to be a valuable tool for clinically based dose verification, provided that customized gel compositions and MR imaging parameters are used. While high dose precision was achieved, further work is required to achieve clinically acceptable dose accuracy.« less
  • There is an increasing interest in the use of inhomogeneity corrections for lung, air, and bone in radiotherapy treatment planning. Traditionally, corrections based on physical density have been used. Modern algorithms use the electron density derived from CT images. Small fields are used in both conformal radiotherapy and IMRT, however, their beam characteristics in inhomogeneous media have not been extensively studied. This work compares traditional and modern treatment planning algorithms to Monte Carlo simulations in and near low-density inhomogeneities. Field sizes ranging from 0.5 cm to 5 cm in diameter are projected onto a phantom containing inhomogeneities and depth dosemore » curves are compared. Comparisons of the Dose Perturbation Factors (DPF) are presented as functions of density and field size. Dose Correction Factors (DCF), which scale the algorithms to the Monte Carlo data, are compared for each algorithm. Physical scaling algorithms such as Batho and Equivalent Pathlength (EPL) predict an increase in dose for small fields passing through lung tissue, where Monte Carlo simulations show a sharp dose drop. The physical model-based collapsed cone convolution (CCC) algorithm correctly predicts the dose drop, but does not accurately predict the magnitude. Because the model-based algorithms do not correctly account for the change in backscatter, the dose drop predicted by CCC occurs farther downstream compared to that predicted by the Monte Carlo simulations. Beyond the tissue inhomogeneity all of the algorithms studied predict dose distributions in close agreement with Monte Carlo simulations. Dose-volume relationships are important in understanding the effects of radiation to the lung. The dose within the lung is affected by a complex function of beam energy, lung tissue density, and field size. Dose algorithms vary in their abilities to correctly predict the dose to the lung tissue. A thorough analysis of the effects of density, and field size on dose to the lung and how modern dose calculation algorithms compare to Monte Carlo data is presented in this research project. This work can be used as a basis to further refine an algorithm's accuracy in low-density media or to correct prior dosimetric results.« less
  • Purpose: To investigate the accuracy, sensitivity and constancy of integral quality monitor (IQM), a new system for in vivo dosimetry of conventional intensity modulated radiation therapy (IMRT) or rotational volumetric modulated arc therapy (VMAT) Methods: A beta-version IQM system was commissioned on an Elekta Infinity LINAC equipped with 160-MLCs Agility head. The stationary and rotational dosimetric constancy of IQM was evaluated, using five-field IMRT and single-or double-arc VMAT plans for prostate and head-and-neck (H&N) patients. The plans were delivered three times over three days to assess the constancy of IQM response. Picket fence (PF) fields were used to evaluate themore » sensitivity of detecting MLC leaf errors. A single leaf offset was intentionally introduced during delivery of various PF fields with segment apertures of 3×1, 5×1, 10×1, and 24×1cm2. Both 2mm and 5mm decrease in the field width were used. Results: Repeated IQM measurements of prostate and H&N IMRT deliveries showed 0.4 and 0.5% average standard deviation (SD) for segment-by-segment comparison and 0.1 and 0.2% for cumulative comparison. The corresponding SDs for VMAT deliveries were 6.5, 9.4% and 0.7, 1.3%, respectively. Statistical analysis indicates that the dosimetric differences detected by IQM were significant (p < 0.05) in all PF test deliveries. The largest average IQM signal response of a 2 mm leaf error was found to be 2.1% and 5.1% by a 5mm leaf error for 3×1 cm2 field size. The same error in 24×1 cm2 generates a 0.7% and 1.4% difference in the signal. Conclusion: IQM provides an effective means for real-time dosimetric verification of IMRT/ VMAT treatment delivery. For VMAT delivery, the cumulative dosimetry of IQM needs to be used in clinical practice.« less
  • This study presents a dosimetric optimization effort aiming to compare noncoplanar field (NCF) on 3 dimensions conformal radiotherapy (3D-CRT) and coplanar field (CF) on intensity-modulated radiotherapy (IMRT) planning for postocular invasion tumor. We performed a planning study on the computed tomography data of 8 consecutive patients with localized postocular invasion tumor. Four fields NCF 3D-CRT in the transverse plane with gantry angles of 0-10 deg., 30-45 deg., 240-270 deg., and 310-335 deg. degrees were isocentered at the center of gravity of the target volume. The geometry of the beams was determined by beam's eye view. The same constraints were preparedmore » with between CF IMRT optimization and NCF 3D-CRT treatment. The maximum point doses (D max) for the different optic pathway structures (OPS) with NCF 3D-CRT treatment should differ in no more than 3% from those with the NCF IMRT plan. Dose-volume histograms (DVHs) were obtained for all targets and organ at risk (OAR) with both treatment techniques. Plans with NCF 3D-CRT and CF IMRT constraints on target dose in homogeneity were computed, as well as the conformity index (CI) and homogeneity index (HI) in the target volume. The PTV coverage was optimal with both NCF 3D-CRT and CF IMRT plans in the 8 tumor sites. No difference was noted between the two techniques for the average D{sub max} and D{sub min} dose. NCF 3D-CRT and CF IMRT will yield similar results on CI. However, HI was a significant difference between NCF 3D-CRT and CF IMRT plan (p < 0.001). Physical endpoints for target showed the mean target dose to be low in the CF IMRT plan, caused by a large target dose in homogeneity (p < 0.001). The impact of NCF 3D-CRT versus CF IMRT set-up is very slight. NCF3D-CRT is one of the treatment options for postocular invasion tumor. However, constraints for OARs are needed.« less