A pencil beam algorithm for intensity modulated proton therapy derived from Monte Carlo simulations

Soukup, Martin; Fippel, Matthias; Alber, M.

doi:10.1088/0031-9155/50/21/010

Cited by 178 publications

(237 citation statements)

References 29 publications

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“…[8][9][10] Previous dose algorithms for SSPT normally used a single Gaussian (SG) function to describe the shape of an in-air lateral profile of an individual pencil beam. 4,11 However, our recent works have demonstrated that a SG function could not describe in-air lateral profiles well for an individual pencil beam from our scanning nozzle. [12][13][14][15] The TPS vendor, therefore, implemented a double Gaussian (DG) fluence model to account for the spot fluence due to contributions of large angle scattering from the devices within the scanning nozzle.…”

Section: Introductionmentioning

confidence: 81%

“…The low-dose envelope due to nuclear interactions changes with depth for a given proton energy, building up and reaching the maximum at an intermediate depth, and then receding. 11,14,15 The TPS must accurately model this variation. The observed larger differences at some intermediate depths for high proton energies (e.g., 23.2 cm for 221.8 MeV, as shown in Fig.…”

Section: Discussionmentioning

confidence: 99%

“…Pencil beam algorithms similar to Eq. (2) have been reported, 4,11 although normally only two Gaussians are used, one for multiple Coulomb scattering and the other for nuclear interactions, accounting for the "halo" dose.…”

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confidence: 99%

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Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system

et al. 2013

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Purpose: To present our method and experience in commissioning dose models in water for spot scanning proton therapy in a commercial treatment planning system (TPS). Methods: The input data required by the TPS included in-air transverse profiles and integral depth doses (IDDs). All input data were obtained from Monte Carlo (MC) simulations that had been validated by measurements. MC-generated IDDs were converted to units of Gy mm 2 /MU using the measured IDDs at a depth of 2 cm employing the largest commercially available parallel-plate ionization chamber. The sensitive area of the chamber was insufficient to fully encompass the entire lateral dose deposited at depth by a pencil beam (spot). To correct for the detector size, correction factors as a function of proton energy were defined and determined using MC. The fluence of individual spots was initially modeled as a single Gaussian (SG) function and later as a double Gaussian (DG) function. The DG fluence model was introduced to account for the spot fluence due to contributions of large angle scattering from the devices within the scanning nozzle, especially from the spot profile monitor. To validate the DG fluence model, we compared calculations and measurements, including doses at the center of spread out Bragg peaks (SOBPs) as a function of nominal field size, range, and SOBP width, lateral dose profiles, and depth doses for different widths of SOBP. Dose models were validated extensively with patient treatment field-specific measurements. Results: We demonstrated that the DG fluence model is necessary for predicting the field size dependence of dose distributions. With this model, the calculated doses at the center of SOBPs as a function of nominal field size, range, and SOBP width, lateral dose profiles and depth doses for rectangular target volumes agreed well with respective measured values. With the DG fluence model for our scanning proton beam line, we successfully treated more than 500 patients from March 2010 through June 2012 with acceptable agreement between TPS calculated and measured dose distributions. However, the current dose model still has limitations in predicting field size dependence of doses at some intermediate depths of proton beams with high energies. Conclusions: We have commissioned a DG fluence model for clinical use. It is demonstrated that the DG fluence model is significantly more accurate than the SG fluence model. However, some deficiencies in modeling the low-dose envelope in the current dose algorithm still exist. Further improvements to the current dose algorithm are needed. The method presented here should be useful for commissioning pencil beam dose algorithms in new versions of TPS in the future.

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Section: Introductionmentioning

confidence: 81%

Section: Discussionmentioning

confidence: 99%

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Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system

et al. 2013

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“…8 and 9͒, and generate beam data for commissioning treatment planning software. 10 The last application requires a general purpose model that is accurate inside and outside the primary field to interpolate between measured data and/or estimated corrections due to nuclear interactions in the superposition of pencil beams. 11 Second, we use the model to determine the source of the radiation and the contributions from neutrons, which have high linear energy transfer ͑LET͒, and photons and scattered primary protons, which have low LET.…”

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confidence: 99%

Assessment of out‐of‐field absorbed dose and equivalent dose in proton fields

et al. 2009

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Purpose:In proton therapy, as in other forms of radiation therapy, scattered and secondary particles produce undesired dose outside the target volume that may increase the risk of radiation-induced secondary cancer and interact with electronic devices in the treatment room. The authors implement a Monte Carlo model of this dose deposited outside passively scattered fields and compare it to measurements, determine the out-of-field equivalent dose, and estimate the change in the dose if the same target volumes were treated with an active beam scanning technique. Methods: Measurements are done with a thimble ionization chamber and the Wellhofer MatriXX detector inside a Lucite phantom with field configurations based on the treatment of prostate cancer and medulloblastoma. The authors use a GEANT4 Monte Carlo simulation, demonstrated to agree well with measurements inside the primary field, to simulate fields delivered in the measurements. The partial contributions to the dose are separated in the simulation by particle type and origin. Results: The agreement between experiment and simulation in the out-of-field absorbed dose is within 30% at 10-20 cm from the field edge and 90% of the data agrees within 2 standard deviations. In passive scattering, the neutron contribution to the total dose dominates in the region downstream of the Bragg peak ͑65%-80% due to internally produced neutrons͒ and inside the phantom at distances more than 10-15 cm from the field edge. The equivalent doses using 10 for the neutron weighting factor at the entrance to the phantom and at 20 cm from the field edge are 2.2 and 2.6 mSv/Gy for the prostate cancer and cranial medulloblastoma fields, respectively. The equivalent dose at 15-20 cm from the field edge decreases with depth in passive scattering and increases with depth in active scanning. Therefore, active scanning has smaller out-of-field equivalent dose by factors of 30-45 in the entrance region and this factor decreases with depth. Conclusions: The dose deposited immediately downstream of the primary field, in these cases, is dominated by internally produced neutrons; therefore, scattered and scanned fields may have similar risk of second cancer in this region. The authors confirm that there is a reduction in the out-of-field dose in active scanning but the effect decreases with depth. GEANT4 is suitable for simulating the dose deposited outside the primary field. The agreement with measurements is comparable to or better than the agreement reported for other implementations of Monte Carlo models. Depending on the position, the absorbed dose outside the primary field is dominated by contributions from primary protons that may or may not have scattered in the brass collimating devices. This is noteworthy as the quality factor of the low LET protons is well known and the relative dose risk in this region can thus be assessed accurately.

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“…The pencil beam kernel represents the dose distribution in water created by an elementary pencil beam of unit fluence with kinetic energy corresponding to the beam's full range, and can be calculated using various combinations of physics models and/or measured data. [13][14][15] Because pencil beam algorithms are based on dose kernels computed in water, an important consideration is how to properly account for tissue heterogeneities that occur within the patient. Most pencil beam algorithms use path length scaling with or without proton transport theory to model the effects of material heterogeneities on the treatment beam.…”

Section: Introductionmentioning

confidence: 99%

A generalized 2D pencil beam scaling algorithm for proton dose calculation in heterogeneous slab geometries

Westerly¹,

Mo²,

Mackie³

et al. 2013

Medical Physics

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Purpose: Pencil beam algorithms are commonly used for proton therapy dose calculations. Szymanowski and Oelfke ["Two-dimensional pencil beam scaling: An improved proton dose algorithm for heterogeneous media," Phys. Med. Biol. 47, 3313-3330 (2002)] developed a two-dimensional (2D) scaling algorithm which accurately models the radial pencil beam width as a function of depth in heterogeneous slab geometries using a scaled expression for the radial kernel width in water as a function of depth and kinetic energy. However, an assumption made in the derivation of the technique limits its range of validity to cases where the input expression for the radial kernel width in water is derived from a local scattering power model. The goal of this work is to derive a generalized form of 2D pencil beam scaling that is independent of the scattering power model and appropriate for use with any expression for the radial kernel width in water as a function of depth. Methods: Using Fermi-Eyges transport theory, the authors derive an expression for the radial pencil beam width in heterogeneous slab geometries which is independent of the proton scattering power and related quantities. The authors then perform test calculations in homogeneous and heterogeneous slab phantoms using both the original 2D scaling model and the new model with expressions for the radial kernel width in water computed from both local and nonlocal scattering power models, as well as a nonlocal parameterization of Molière scattering theory. In addition to kernel width calculations, dose calculations are also performed for a narrow Gaussian proton beam. Results: Pencil beam width calculations indicate that both 2D scaling formalisms perform well when the radial kernel width in water is derived from a local scattering power model. Computing the radial kernel width from a nonlocal scattering model results in the local 2D scaling formula under-predicting the pencil beam width by as much as 1.4 mm (21%) at the depth of the Bragg peak for a 220 MeV proton beam in homogeneous water. This translates into a 32% dose discrepancy for a 5 mm Gaussian proton beam. Similar trends were observed for calculations made in heterogeneous slab phantoms where it was also noted that errors tend to increase with greater beam penetration. The generalized 2D scaling model performs well in all situations, with a maximum dose error of 0.3% at the Bragg peak in a heterogeneous phantom containing 3 cm of hard bone. Conclusions:The authors have derived a generalized form of 2D pencil beam scaling which is independent of the proton scattering power model and robust to the functional form of the radial kernel width in water used for the calculations. Sample calculations made with this model show excellent agreement with expected values in both homogeneous water and heterogeneous phantoms.

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A pencil beam algorithm for intensity modulated proton therapy derived from Monte Carlo simulations

Cited by 178 publications

References 29 publications

Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system

Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system

Assessment of out‐of‐field absorbed dose and equivalent dose in proton fields

A generalized 2D pencil beam scaling algorithm for proton dose calculation in heterogeneous slab geometries

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