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Title: 2-Step IMAT and 2-Step IMRT in three dimensions

Abstract

In two dimensions, 2-Step Intensity Modulated Arc Therapy (2-Step IMAT) and 2-Step Intensity Modulated Radiation Therapy (IMRT) were shown to be powerful methods for the optimization of plans with organs at risk (OAR) (partially) surrounded by a target volume (PTV). In three dimensions, some additional boundary conditions have to be considered to establish 2-Step IMAT as an optimization method. A further aim was to create rules for ad hoc adaptations of an IMRT plan to a daily changing PTV-OAR constellation. As a test model, a cylindrically symmetric PTV-OAR combination was used. The centrally placed OAR can adapt arbitrary diameters with different gap widths toward the PTV. Along the rotation axis the OAR diameter can vary, the OAR can even vanish at some axis positions, leaving a circular PTV. The width and weight of the second segment were the free parameters to optimize. The objective function f to minimize was the root of the integral of the squared difference of the dose in the target volume and a reference dose. For the problem, two local minima exist. Therefore, as a secondary criteria, the magnitude of hot and cold spots were taken into account. As a result, the solution with a largermore » segment width was recommended. From plane to plane for varying radii of PTV and OAR and for different gaps between them, different sets of weights and widths were optimal. Because only one weight for one segment shall be used for all planes (respectively leaf pairs), a strategy for complex three-dimensional (3-D) cases was established to choose a global weight. In a second step, a suitable segment width was chosen, minimizing f for this global weight. The concept was demonstrated in a planning study for a cylindrically symmetric example with a large range of different radii of an OAR along the patient axis. The method is discussed for some classes of tumor/organ at risk combinations. Noncylindrically symmetric cases were treated exemplarily. The product of width and weight of the additional segment as well as the integral across the segment profile was demonstrated to be an important value. This product was up to a factor of 3 larger than in the 2-D case. Even in three dimensions, the optimized 2-Step IMAT increased the homogeneity of the dose distribution in the PTV profoundly. Rules for adaptation to varying target-OAR combinations were deduced. It can be concluded that 2-Step IMAT and 2-Step IMRT are also applicable in three dimensions. In the majority of cases, weights between 0.5 and 2 will occur for the additional segment. The width-weight product of the second segment is always smaller than the normalized radius of the OAR. The width-weight product of the additional segment is strictly connected to the relevant diameter of the organ at risk and the target volume. The derived formulas can be helpful to adapt an IMRT plan to altering target shapes.« less

Authors:
 [1]
  1. Klinik und Poliklinik fuer Strahlentherapie, Universitaet Wuerzburg, Josef-Schneider-Str. 11, D-97080 Wuerzburg (Germany)
Publication Date:
OSTI Identifier:
20726883
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 32; Journal Issue: 12; Other Information: DOI: 10.1118/1.2134928; (c) 2005 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
61 RADIATION PROTECTION AND DOSIMETRY; BOUNDARY CONDITIONS; GEOMETRY; HEALTH HAZARDS; MODULATION; NEOPLASMS; OPTIMIZATION; ORGANS; PLANNING; RADIATION DOSE DISTRIBUTIONS; RADIATION DOSES; RADIOTHERAPY; SHAPE; THREE-DIMENSIONAL CALCULATIONS

Citation Formats

Bratengeier, Klaus. 2-Step IMAT and 2-Step IMRT in three dimensions. United States: N. p., 2005. Web. doi:10.1118/1.2134928.
Bratengeier, Klaus. 2-Step IMAT and 2-Step IMRT in three dimensions. United States. doi:10.1118/1.2134928.
Bratengeier, Klaus. Thu . "2-Step IMAT and 2-Step IMRT in three dimensions". United States. doi:10.1118/1.2134928.
@article{osti_20726883,
title = {2-Step IMAT and 2-Step IMRT in three dimensions},
author = {Bratengeier, Klaus},
abstractNote = {In two dimensions, 2-Step Intensity Modulated Arc Therapy (2-Step IMAT) and 2-Step Intensity Modulated Radiation Therapy (IMRT) were shown to be powerful methods for the optimization of plans with organs at risk (OAR) (partially) surrounded by a target volume (PTV). In three dimensions, some additional boundary conditions have to be considered to establish 2-Step IMAT as an optimization method. A further aim was to create rules for ad hoc adaptations of an IMRT plan to a daily changing PTV-OAR constellation. As a test model, a cylindrically symmetric PTV-OAR combination was used. The centrally placed OAR can adapt arbitrary diameters with different gap widths toward the PTV. Along the rotation axis the OAR diameter can vary, the OAR can even vanish at some axis positions, leaving a circular PTV. The width and weight of the second segment were the free parameters to optimize. The objective function f to minimize was the root of the integral of the squared difference of the dose in the target volume and a reference dose. For the problem, two local minima exist. Therefore, as a secondary criteria, the magnitude of hot and cold spots were taken into account. As a result, the solution with a larger segment width was recommended. From plane to plane for varying radii of PTV and OAR and for different gaps between them, different sets of weights and widths were optimal. Because only one weight for one segment shall be used for all planes (respectively leaf pairs), a strategy for complex three-dimensional (3-D) cases was established to choose a global weight. In a second step, a suitable segment width was chosen, minimizing f for this global weight. The concept was demonstrated in a planning study for a cylindrically symmetric example with a large range of different radii of an OAR along the patient axis. The method is discussed for some classes of tumor/organ at risk combinations. Noncylindrically symmetric cases were treated exemplarily. The product of width and weight of the additional segment as well as the integral across the segment profile was demonstrated to be an important value. This product was up to a factor of 3 larger than in the 2-D case. Even in three dimensions, the optimized 2-Step IMAT increased the homogeneity of the dose distribution in the PTV profoundly. Rules for adaptation to varying target-OAR combinations were deduced. It can be concluded that 2-Step IMAT and 2-Step IMRT are also applicable in three dimensions. In the majority of cases, weights between 0.5 and 2 will occur for the additional segment. The width-weight product of the second segment is always smaller than the normalized radius of the OAR. The width-weight product of the additional segment is strictly connected to the relevant diameter of the organ at risk and the target volume. The derived formulas can be helpful to adapt an IMRT plan to altering target shapes.},
doi = {10.1118/1.2134928},
journal = {Medical Physics},
number = 12,
volume = 32,
place = {United States},
year = {Thu Dec 15 00:00:00 EST 2005},
month = {Thu Dec 15 00:00:00 EST 2005}
}
  • The implementation of intensity-modulated radiation therapy (IMRT) in the clinic necessitates commissioning for all systems involved. This paper describes work carried out for the treatment planning system (Helax-TMS version 6.1) and the treatment delivery equipment (Siemens Primus) available at our center. Particular regard was paid to small monitor units (MUs) and small field segments typical of step-and-shoot IMRT plans. The beam profile stability of the Siemens Primus accelerators when delivering small MU was examined with a linear detector array. Dose monitor linearity and intersegment variations were measured with a 0.6-cm{sup 3} ionization chamber. Treatment planning system calculated total scatter factorsmore » (S{sub cp}) and beam profiles for symmetric and asymmetric small fields for 6- and 15MV beams were compared against measurements in water using a 0.125-cm{sup 3} ionization chamber and a diamond detector. The 6- and 15MV beams from the Primus accelerators were found to be stable at MUs less than 10. Dose monitor linearity for small exposures under 10 MU was within {+-} 2% for 6 MV, but found to be not so initially for 15 MV. This could be remedied by an adjustment of a soft spot on the Siemens Primus. The delivery of small MU segments as part of an IMRT sequence was found to be consistent down to segment sizes of 1 MU. The treatment planning system pencil-beam convolution model agreed with measurement within {+-} 5% for fields collimated down to 3 x 3 cm. The collapsed cone point-kernel model better predicted the output for the smallest field, but displayed some unpredictable shifts in the position of the penumbra. The startup characteristics of Siemens Primus accelerators were found suitable for step-and-shoot IMRT. The diminution in accuracy of Helax-TMS dose calculation models for multileaf collimated fields of less than 2 x 2 cm has led us to avoid these in IMRT treatments at our center.« less
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  • Purpose: The goal of this planning study was to compare step-and-shoot intensity-modulated radiotherapy (IMRT) plans with helical dynamic IMRT plans for oropharynx patients on the basis of dose distribution. Methods and Materials: Five patients with oropharynx cancer had been previously treated by step-and-shoot IMRT at University Medical Centre Utrecht, The Netherlands, applying five fields and approximately 60-90 segments. Inverse planning was carried out using Plato, version 2.6.2. For each patient, an inverse IMRT plan was also made using Tomotherapy Hi-Art System, version 2.0, and using the same targets and optimization goals. Statistical analysis was performed by a paired t test.more » Results: All tomotherapy plans compared favorably with the step-and-shoot plans regarding sparing of the organs at risk and keeping an equivalent target dose homogeneity. Tomotherapy plans in particular realized sharper dose gradients compared with the step-and-shoot plans. The mean dose to all parotid glands (n = 10) decreased on average 6.5 Gy (range, -4 to 14; p = 0.002). The theoretical reduction in normal tissue complication probabilities in favor of the tomotherapy plans depended on the parotid normal tissue complication probability model used (range, -3% to 32%). Conclusion: Helical tomotherapy IMRT plans realized sharper dose gradients compared with the clinically applied step-and shoot plans. They are expected to be able to reduce the parotid normal tissue complication probability further, keeping a similar target dose homogeneity.« less
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