Pencil Beam Dose Calculation Algorithm Research Articles

A 3D superposition pencil beam dose calculation algorithm for a 60Co therapy unit and its verification by MC simulation

The MCNP Monte Carlo code was used to simulate the collimating system of the 60 Co therapy unit to calculate the primary and scattered photon fluences as well as the electron contamination incident to the isocentric plane as the functions of the irradiation field size. Furthermore, a Monte Carlo simulation for the polyenergetic Pencil Beam Kernels (PBKs) generation was performed using the calculated photon and electron spectra. The PBK was analytically fitted to speed up the dose calculation using the convolution technique in the homogeneous media. The quality of the PBK fit was verified by comparing the calculated and simulated 60 Co broad beam profiles and depth dose curves in a homogeneous water medium. The inhomogeneity correction coefficients were derived from the PBK simulation of an inhomogeneous slab phantom consisting of various materials. The inhomogeneity calculation model is based on the changes in the PBK radial displacement and on the change of the forward and backward electron scattering. The inhomogeneity correction is derived from the electron density values gained from a complete 3D CT array and considers different electron densities through which the pencil beam is propagated as well as the electron density values located between the interaction point and the point of dose deposition. Important aspects and details of the algorithm implementation are also described in this study. • Monte Carlo simulation of photon and electron spectra for a 60 Co therapy unit. • Monte Carlo simulation of a polyenergetic Pencil Beam Kernel (PBK). • Inhomogeneity correction based on Monte Carlo simulation. • Implementation of dose calculation algorithm on the CT matrix with 3D inhomogeneity correction.

Accuracy assessment of pencil beam and Monte Carlo dose calculation algorithms during IMRT and VMAT delivery

Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) are techniques that have been widely used over the last years for treating cancer due to better target coverage and the lower dose to the critical organs. This study assesses the accuracy of the Pencil Beam (PB) and the Monte Carlo (MC) calculation algorithms commonly used to determine the dose distribution in such applications. IMRT and VMAT plans were created for ten prostate patients using a commercially available treatment planning system (Monaco, Elekta Inc.). All plans were optimized to be clinically acceptable and the final dose calculation was performed using a) the PB and b) the MC calculation algorithm. The plans were evaluated in the central coronal plane using the I'mRT Matrixx (IBA Dosimetry GmbH) 2D-array placed in a solid water phantom a) by setting all control points to zero gantry degree and b) by leaving the beam orientation as planned. Evaluation was performed with OmniPro-I'mRT software using the gamma index method with acceptance criteria of 3% dose difference and 3 mm distance to agreement taking into account all points with dose values greater than the 10% of the maximum dose. Results show that PB calculation algorithm can accurately determine the dose distribution applied to the I'mRT Matrixx 2D array when all control points were set to zero gantry degree for both IMRT and VMAT plans (average passing rates of 96.6 and 94.7% of the points passed the 3%/3 mm acceptance criteria, for IMRT and VMAT, were observed, respectively). On the other hand, considerable differences between the different calculation algorithms were observed when the actual (i.e. the planned) beam orientation was used for the plan verification. In this case, the PB algorithm was found adequate for dose determination only for the IMRT plan verification (an average passing rate of 91% was observed). For the VMAT plans, MC was found significant superior compared to the PB dose calculation algorithm (average passing rates of 73.5 and 93% for the PB and the MC algorithms, were observed, respectively).

Influence of different dose calculation algorithms on the estimate of NTCP for lung complications

Due to limitations and uncertainties in dose calculation algorithms, different algorithms can predict different dose distributions and dose‐volume histograms for the same treatment. This can be a problem when estimating the normal tissue complication probability (NTCP) for patient‐specific dose distributions. Published NTCP model parameters are often derived for a different dose calculation algorithm than the one used to calculate the actual dose distribution. The use of algorithm‐specific NTCP model parameters can prevent errors caused by differences in dose calculation algorithms. The objective of this work was to determine how to change the NTCP model parameters for lung complications derived for a simple correction‐based pencil beam dose calculation algorithm, in order to make them valid for three other common dose calculation algorithms. NTCP was calculated with the relative seriality (RS) and Lyman‐Kutcher‐Burman (LKB) models. The four dose calculation algorithms used were the pencil beam (PB) and collapsed cone (CC) algorithms employed by Oncentra, and the pencil beam convolution (PBC) and anisotropic analytical algorithm (AAA) employed by Eclipse. Original model parameters for lung complications were taken from four published studies on different grades of pneumonitis, and new algorithm‐specific NTCP model parameters were determined. The difference between original and new model parameters was presented in relation to the reported model parameter uncertainties. Three different types of treatments were considered in the study: tangential and locoregional breast cancer treatment and lung cancer treatment. Changing the algorithm without the derivation of new model parameters caused changes in the NTCP value of up to 10 percentage points for the cases studied. Furthermore, the error introduced could be of the same magnitude as the confidence intervals of the calculated NTCP values. The new NTCP model parameters were tabulated as the algorithm was varied from PB to PBC, AAA, or CC. Moving from the PB to the PBC algorithm did not require new model parameters; however, moving from PB to AAA or CC did require a change in the NTCP model parameters, with CC requiring the largest change. It was shown that the new model parameters for a given algorithm are different for the different treatment types.PACS numbers: 87.53.‐j, 87.53.Kn, 87.55.‐x, 87.55.dh, 87.55.kd

SU-E-T-512: Monte Carlo Dose Verification of Pencil Beam Scanning Proton Therapy

Purpose: To verify a clinical pencil (PB) beam dose calculation algorithm for scanned beam intensity modulated proton therapy (IMPT), using TOPAS (TOol for PArticle Simulation), a GEANT4 based Monte Carlo (MC) simulation system. Methods: Seven patients, previously treated with IMPT for various treatment sites and prescriptions, were selected from our patient database. Proton fluence maps of the treated plans were exported for each field from our clinical treatment planning system (ASTROID) and imported to TOPAS along with the patient and beam geometry. The absolute dose distribution of each individual beam was calculated and compared to the PB algorithm‐based calculation from ASTROID. Results: The differences observed in mean and median target doses were less than ±1% for all cases, while D02 and D98 (surrogates for maximum and minimum dose values respectively) differed by less than ±3% for the majority of beams. Differences in the mean dose for the organs at risk (OARs) ranged from −8.9% to 3.7%, with reference to MC calculation, with an average over all the OARs of −0.1%, indicating no systematic over‐or under‐estimation of the dose by the PB algorithm. 3D gamma analysis (2%/2mm) for the PB to MC dose comparison resulted in an average 95.2% (±5.0) of the target volume having an absolute gamma value equal or less than 1 and 99.2% (±1.2%) equal or less than 2. For the healthy tissue receiving at least 5% of the target mean dose, the corresponding percentages were 99.6% (±0.3%) and 99.9% (±0.1%). Conclusion: We have clinically implemented MC for IMPT plan recalculation. Our PB calculation algorithm for IMPT was found to be in overall good agreement with MC calculations. Clinically significant deviations in OAR mean dose can be attributed to lung tissue or bone anatomy in the beam path.

Effect of the embolization material in the dose calculation for stereotactic radiosurgery of arteriovenous malformations

It is reported in the literature that the material used in an embolization of an arteriovenous malformation (AVM) can attenuate the radiation beams used in stereotactic radiosurgery (SRS) up to 10% to 15%. The purpose of this work is to assess the dosimetric impact of this attenuating material in the SRS treatment of embolized AVMs, using Monte Carlo simulations assuming clinical conditions. A commercial Monte Carlo dose calculation engine was used to recalculate the dose distribution of 20 AVMs previously planned with a pencil beam dose calculation algorithm. Dose distributions were compared using the following metrics: average, minimal and maximum dose of AVM, and 2D gamma index. The effect in the obliteration rate was investigated using radiobiological models. It was found that the dosimetric impact of the embolization material is less than 1.0Gy in the prescription dose to the AVM for the 20 cases studied. The impact in the obliteration rate is less than 4.0%. There is reported evidence in the literature that embolized AVMs treated with SRS have low obliteration rates. This work shows that there are dosimetric implications that should be considered in the final treatment decisions for embolized AVMs.

SU-E-T-513: Clinical Implementation of a GPU Accelerated Pencil Beam Dose Calculation Algorithm.

This work reports a clinical implementation of a GPU accelerated pencil beam dose calculation algorithm (GPU-PB). Model parameters were determined using in-house scripts written in MATLAB. Dose distributions in a water phantom were calculated using a Pinnacle TPS for various open field sizes. Lateral profiles at 2-mm incremental depths were used to calculate PB kernel parameters. Weighted sum of squares were used for least-squares parameter fitting utilizing a Levenberg-marquardt algorithm. Weightings were adjusted based on goodness of fitting and in accordance with field size to suppress deviations due to horn effects. The scale factor was fitted iteratively. The calculated doses for two patient cases were analyzed with a 3D gamma method (3%/3mm). Excellent agreement between Pinnacle calculation and GPU-PB calculation was achieved regarding PDD, profiles and output factor. For a head-neck 10-field step-and-shoot IMRT case, gamma passing rate was over 99% and maximum absolute dose value is 75.6 Gy vs. 73.9 Gy, differing by 2.3%. Similar results were obtained for a SBRT lung case. Gamma passing rate is a function of PB kernel cut-off distance and beamlet resolution. It appears that if the highest accuracy is desired, a resolution of 10 mm or better in the direction parallel to MLC travel and a cutoff distance of 10 cm or better should be used. The calculation time increases with both cut-off distance and beamlet resolution. For example, GPU calculation time is 1.36 seconds for 5 cm cut-off distance, and increases to 4.84 s for 15 cm cut-off distance. A GPU accelerated PB dose calculation algorithm has been implemented using clinical measurement data. Excellent agreement with Pinnacle TPS has been achieved. Beamlet size and PB cut-off distance should be chosen according to desired dose calculation accuracy and speed.

SU-E-T-495: Monte Carlo Dose Verification of Passive Scattering Proton Therapy for Prostate Cancer.

To verify the clinical pencil beam dose calculation algorithm for passive scattering proton therapy using field with large range in tissue, i.e. in prostate cancer, using a Monte Carlo (MC) simulation system. Previously treated prostate cancer cases were randomly selected from our patient database. All patients received the same dose prescription of 50Gy (25 fractions) to planning treatment volume including the seminal vesicles (PTV1), followed by 28Gy (14 fractions) boost to the prostate gland only (PTV2). Patient and beam geometry were imported to our MC simulation platform (TOPAS - TOol for PArticle Simulation) and the dose of each individual beam, as well as their weighted sum, were calculated and compared to pencil beam algorithm-based calculation from the clinical treatment planning system (XiO). Preliminary results from four patient cases show overall good agreement between the pencil beam and MC calculations. However, a small but systematic overestimation of the dose, as calculated by the pencil beam calculation algorithm, was noticed for the target structures (<2% difference in D95 for PTV1 and PTV2), compared to the MC calculation. The inverse was observed for the OARs (rectum and bladder) for which the dose seems to be somewhat underestimated by the pencil beam calculation algorithm (up to 3.75% difference in the volume covered by the 70Gy and 75Gy isodose lines). Furthermore, systematic difference in the range calculation was noticed: the pencil beam calculation algorithm results in larger proton range, in the order of 3-4mm, compared to the MC calculations for all beams and patients studied. This can be attributed to the bone anatomy in the path of the beams (femoral heads). Routine MC dose calculation has the potential to improve delivery accuracy in proton therapy of prostate cancer and influence the analysis of currently ongoing clinical trials of protons versus IMRT. Funded by NIH/NCI R01 CA140735.

SU-E-T-535: Proton Dose Calculations in Homogeneous Media.

To develop a pencil beam dose calculation algorithm for scanned proton beams that improves modeling of scatter events. Our pencil beam algorithm (PBA) was developed for calculating dose from monoenergetic, parallel proton beams in homogeneous media. Fermi-Eyges theory was implemented for pencil beam transport. Elastic and nonelastic scatter effects were each modeled as a Gaussian distribution, with root mean square (RMS) widths determined from theoretical calculations and a nonlinear fit to a Monte Carlo (MC) simulated 1mm × 1mm proton beam, respectively. The PBA was commissioned using MC simulations in a flat water phantom. Resulting PBA calculations were compared with results of other models reported in the literature on the basis of differences between PBA and MC calculations of 80-20% penumbral widths. Our model was further tested by comparing PBA and MC results for oblique beams (45 degree incidence) and surface irregularities (step heights of 1 and 4 cm) for energies of 50-250 MeV and field sizes of 4cm × 4cm and 10cm × 10cm. Agreement between PBA and MC distributions was quantified by computing the percentage of points within 2% dose difference or 1mm distance to agreement. Our PBA improved agreement between calculated and simulated penumbral widths by an order of magnitude compared with previously reported values. For comparisons of oblique beams and surface irregularities, agreement between PBA and MC distributions was better than 99%. Our algorithm showed improved accuracy over other models reported in the literature in predicting the overall shape of the lateral profile through the Bragg peak. This improvement was achieved by incorporating nonelastic scatter events into our PBA. The increased modeling accuracy of our PBA, incorporated into a treatment planning system, may improve the reliability of treatment planning calculations for patient treatments. This research was supported by contract W81XWH-10-1-0005 awarded by The U.S. Army Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702-5014. This report does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.

TH-F-211-04: A Fast Finite Size Pencil Beam Algorithm for Dose Calculation Using GPUs

Purpose: To facilitate real-time dose calculation for online adaptive radiation therapy, we have implemented the Monte Carlo-based finite size pencil beam (FSPB) dose calculation algorithm on Graphics Processing Units (GPUs). Methods: The FSPB dose calculation algorithm is based on the analytical kernel calculated from Monte Carlo simulations using the EGSnrc Monte Carlo code. This provides the benefit of Monte Carlo accuracy while the GPU implementation and the exponential kernel significantly reduces execution time. The GPU-based FSPB implementation was executed on the NVidia GTX480 GPU card. Both CPU and GPU versions of the FSPB algorithm were implemented operationally identically and benchmarked for accuracy and execution time. The dose calculation was performed in heterogeneous phantoms consisting of water, lung and bone slabs, and in clinical cases. The GPU-based FSPB implementation was compared with the CPU version and with a commercial treatment planning system for different situations including IMRT plans. Results: The FSPB algorithm with heterogeneity correction and anisotropic analytical algorithm (AAA) in heterogeneous phantom and clinical treatment planning cases agree within 2%. The GPU implementation was found to be up to 477 times faster than the single threaded CPU while achieving identical result in terms of dosimetric accuracy. The GPU-based FSPB was close to six times faster than the AAA, with single beam calculations completed within 0.45 s for typical scenarios. Conclusions: This work describes the implementation of a GPU-based FSPB algorithm for dose calculation in radiation therapy. The results demonstrated that the GPU-based FSPB algorithm can provide accurate 3D dose distributions in heterogeneous media in less than a second per beam for clinically relevant cases. This research is supported by CPRIT Individual Investigator Award RP110329.

SU-GG-T-101: Dosimetric Analysis of Matching 6 MV Photon and Electron Fields in Chestwall Treatment

Purpose: To investigate dose distribution for matching 6MV photon and electron fields in chestwall treatment. Method and Materials: A rectangular water equivalent phantom was used to investigate dose distribution at junction of adjacent photon and electron fields. Both 3D and simple IMRT plans have been created for photon fields. Electron plans with different electron energy were created and normalized to 90% iso‐dose line. Photon and electron fields were matched based on skin markings. Monte Carlo simulation has been applied for photon plans. We compared plans with a single photon‐electron match line to “mixed treatment” plans using 2 photon‐electron match lines (ie day1/day2) Results: Hot spots are observed at junction of photon field for all energy electron fields. The highest dose are 129%, 134%, 137%, 139% and 139% of dose for 6, 9, 12, 16 and 20 MeV electrons, respectively, with 6MV photon fields. The depths of these hot spots from skin surface are 1.5, 2.5, 3.4, 4.6 and 5.5cm, as compared with dmax for the electron fields individually (1.4, 2.2, 3.0, 3.0 and 2.0 cm, respectively). The hot spots for “mixed treatment” arrangement (day 1 and day 2) are 120%, 123%, 126%, 129% and 130% of dose, much lower than for the single match‐line fields. The depth for hot spot for day 1 and day 2 arrangement stays similar with day1 treatment. Results from Monte Carlo simulation agree with pencil beam dose calculation algorithm in skin region. Conclusion: Hot spots are reduced to clinically acceptable level by mixed treatment arrangement with two or even more matching lines between photon and electron fields. Further study is warranted to compare these results with intensity and/or energy modulated electron treatment, which may provide a more homogeneous dose distribution for chestwall treatment.

SU‐GG‐T‐297: Measurements of Proton Dose Distributions Using a Thin Oxide MOSFET Detector

Purpose: In proton dosimetry, the MOSFET response depends strongly on Linear Energy Transfer (LET). To improve the LET dependence, we focused on the SiO2 thickness of the MOSFET detector. We compared the response of a new MOSFET detector with thinner oxide thickness, TN‐252RD, to the standard MOSFET, TN‐502RD, already investigated. Furthermore, in order to measure proton dose distributions using the MOSFET, a correction method of the MOSFET response to proton beams has been developed. Method and Materials: Dose reproducibility and LET dependence of the MOSFET response were evaluated experimentally using 190 MeV proton beam. A Bragg curve measured by an ionization chamber (IC) was used to evaluate the correction factors (IC/MOSFET) as a function of proton penetrating depth, i.e. residual range. Here, the residual proton range is successfully calculated by the pencil beam dose calculation algorithm at an arbitrary point. Proton dose distributions formed by an L‐shaped bolus were measured by MOSFETs and IC. The MOSFET doses were compensated by those correction factors. Results: The dose reproducibility of the thinner oxide MOSFET, TN‐252RD, was within 2%, almost similar to that of the standard MOSFET. The new MOSFET response at the Bragg peak improved by 10% in comparison to the standard MOSFET. For proton dose distributions measured with the L‐shaped bolus, a deviation between the IC and raw MOSFET measurements was observed. On the other hand, the corrected MOSFET data agree well with the IC results. Conclusion: The MOSFET of thinner SiO2 thickness improved the LET dependence for proton dosimetry, although the accurate depth‐dose estimation is still to be achieved due to the MOSFET LET dependence. However, with a new correction method of the MOSFET response to proton beams, we succeeded in measuring the proton dose distribution using the MOSFET.

SU-FF-T-190: Dosimetry Verification for Pencil Beam, Clarkson and Equivalent Square Dose Calculation Algorithms

Purpose: To study the accuracy of Brainlab pencil beam and Clarkson algorithms in homogeneous phantoms and heterogeneous geometries. Materials and method: In this study PMMA was used as the homogenous phantom. Different heterogeneous geometries were designed by introducing different materials between the PMMA slabs. In each situation one material is used with different thicknesses: (a) thickness of 2.7 cm and 8.1cm low‐density material and (b) thickness of 3.8 cm and 7.6 cm of high‐density material. The low‐ and high‐density material is placed beneath 6 cm of PMMA. All the phantoms were CT scanned. Six different irregularly shaped beams were designed using a Brainlab multileaf collimator. Calculations were performed for these beams in the homogenous phantom using the Brainlab pencil beam, Clarkson, Modified Clarkson and equivalent square dose calculation algorithms. The percentage depth doses based on different models were compared to measurements. A dose of 150 cGy was specified to a point beneath the inhomogeneous material for the heterogeneous cases. Modified Clarkson, pencil beam and equivalent square algorithms were used to calculate the dose and then compared to measurements. Results: All the models agreed well with measurements; the equivalent square method showed the highest average deviations of 0.55% from measured dose values. In heterogeneous geometries the equivalent square method gave very large deviations (up to 17 %). Clarkson and pencil beam algorithms disagreed with measurements by 4.8 % and 3.8 % for the low‐density material of 2.7 cm thickness, and by 8 % and 6.9 % for the low‐density material of 8.7 cm thickness, respectively. Conclusion: All Brainlab dose calculation models are reliable for homogeneous phantom but some may differ significantly from measurements in heterogeneous geometries, especially in low‐density issues.

Quantification of the impact of MLC modeling and tissue heterogeneities on dynamic IMRT dose calculations

This study quantifies the dose prediction errors (DPEs) in dynamic IMRT dose calculations resulting from (a) use of an intensity matrix to estimate the multi-leaf collimator (MLC) modulated photon fluence (DPE(IGfluence) instead of an explicit MLC particle transport, and (b) handling of tissue heterogeneities (DPE(hetero)) by superposition/convolution (SC) and pencil beam (PB) dose calculation algorithms. Monte Carlo (MC) computed doses are used as reference standards. Eighteen head-and-neck dynamic MLC IMRT treatment plans are investigated. DPEs are evaluated via comparing the dose received by 98% of the GTV (GTV D 98%), the CTV D 95%, the nodal D 90%, the cord and the brainstem D 02%, the parotid D 50%, the parotid mean dose (D (Mean)), and generalized equivalent uniform doses (gEUDs) for the above structures. For the MC-generated intensity grids, DPE(IGfluence) is within +/- 2.1% for all targets and critical structures. The SC algorithm DPE(hetero) is within +/- 3% for 98.3% of the indices tallied, and within +/- 3.4% for all of the tallied indices. The PB algorithm DPE(hetero) is within +/- 3% for 92% of the tallied indices. Statistical equivalence tests indicate that PB DPE(hetero) requires a +/- 3.6% interval to state equivalence with the MC standard, while the intervals are < 1.5% for SC DPE(hetero) and DPE(IGfluence). Overall, these results indicate that SC and MC IMRT dose calculations which use MC-derived intensity matrices for fluence prediction do not introduce significant dose errors compared with full Monte Carlo dose computations; however, PB algorithms may result in clinically significant dose deviations.

Accuracy of inhomogeneity correction algorithm in intensity-modulated radiotherapy of head-and-neck tumors

We examined the degree of calculated-to-measured dose difference for nasopharyngeal target volume in intensity-modulated radiotherapy (IMRT) based on the observed/expected ratio using patient anatomy with humanoid head-and-neck phantom. The plans were designed with a clinical treatment planning system that uses a measurement-based pencil beam dose-calculation algorithm. Two kinds of IMRT plans, which give a direct indication of the error introduced in routine treatment planning, were categorized and evaluated. The experimental results show that when the beams pass through the oral cavity in anthropomorphic head-and-neck phantom, the average dose difference becomes significant, revealing about 10% dose difference to prescribed dose at isocenter. To investigate both the physical reasons of the dose discrepancy and the inhomogeneity effect, we performed the 10 cases of IMRT quality assurance (QA) with plastic and humanoid phantoms. Our result suggests that the transient electronic disequilibrium with the increased lateral electron range may cause the inaccuracy of dose calculation algorithm, and the effectiveness of the inhomogeneity corrections used in IMRT plans should be evaluated to ensure meaningful quality assurance and delivery.

14 Evaluation of Pencil Beam, Collapsed Cone and Monte Carlo IMRT dose calculation algorithms for dual target sites

14 Evaluation of Pencil Beam, Collapsed Cone and Monte Carlo IMRT dose calculation algorithms for dual target sites

SU‐FF‐T‐199: Heterogeneity Effects in Small‐Field Macular Irradiation

Purpose: To evaluate the effects of bone heterogeneities on the dose distribution to the macula in small‐field macular irradiation. Method and Materials: An elliptical aperture measuring 8 mm × 10 mm on the major axes was constructed to project a circular field onto the macula through each of six fields focused on the macula and angled 28 degrees relative to the central axis of the eye. Using a stereotactic diode, the Tissue‐Maximum‐Ratio was measured for this aperture to a depth beyond the macula. These measurements were repeated using bone‐density material in order to approximate the attenuation through the cartilage of the nose and the supra‐orbital ridge. The beam profile at the depth of the macula was determined by film densitometry and the profiles for strictly soft tissue overlay were compared with the profiles for given thicknesses of bone density phantom added to the soft tissue. Lastly, the profile was determined for bone‐density phantom covering only half the field to simulate a treatment situation where the supra‐orbital ridge might intersect only half the incident field. Results: The excess attenuation of the bone material representing cartilage or supra‐orbital ridge is easily described by a simple exponential. The beam profile at the depth of the macula appears unchanged from the beam profile for soft tissue overlay, while the beam profile as modified by bone density material covering half the field shows a measurable attenuation through the bone. However, there is no widening of the profile in any of these situations. Conclusion: These results suggest that a small‐field pencil beam photon algorithm should be sufficient to calculate the radiation dose distribution for macular irradiation. Pencil beam and Monte Carlo dose calculation algorithms will be compared against these measurements. Conflict of Interest: This work supported by an SBIR grant to DIACOR, Inc.

Monte Carlo evaluation of tissue inhomogeneity effects in the treatment of the head and neck

Purpose: To use Monte Carlo dose calculation to assess the degree to which tissue inhomogeneities in the head and neck affect static field conformal, computed tomography (CT)-based 6-MV photon treatment plans. Methods and Materials: We retrospectively studied the three-dimensional treatment plans that had been used for the treatment of 5 patients with tumors in the nasopharyngeal or paranasal sinus regions. Two patients had large surgical cavities. The plans were designed with a clinical treatment planning system that uses a measurement-based pencil-beam dose-calculation algorithm with an equivalent path-length inhomogeneity correction. Each plan employs conformally-shaped 6-MV photon beams. Patient anatomy and electron densities were obtained from the treatment planning CT images. For each plan, the dose distribution was recalculated with the Monte Carlo method, utilizing the same beam geometry and CT images. The Monte Carlo method accurately accounts for the perturbation effects of local tissue heterogeneities. The Monte Carlo calculated dose distributions were compared with those from the clinical treatment planning system. Results: The degree to which tissue inhomogeneity affects the dose distributions of individual fields varies with the specific anatomic geometry, especially the size and location of air cavities in relation to the beam orientation and field size. Most of the beam apertures completely enclose the air cavities within or adjacent to the gross tumor volume (GTV). Equivalent squares (including blocking) ranged from approximately 5 to 9.5 cm. A common feature observed for individual fields is that the Monte Carlo calculated doses to tissue directly behind and within an air cavity are lower. However, after combining the fields employed in each treatment plan, the overall dose distribution shows only small differences between the two methods. For all 5 patients, the Monte Carlo calculated treatment plans showed a slightly lower dose received by the 95% of target volume (D 95) than the plans calculated with the pencil-beam algorithm. The average difference in the target volume encompassed by the prescription isodose line was less than 2.2%. The difference between the dose-volume histograms (DVHs) of the GTV was generally small. For the brainstem and chiasm, the DVHs of the two plans were similar. For the spinal cord, differences in the details of the DHV and the dose to 1 cc (D 1cc) of the structure were observed, with Monte Carlo calculation generally predicting increased dose indices to the spinal cord. However, these changes are not expected to be clinically significant. Conclusion: For 6-MV photons, the effects of both normal tissue inhomogeneities and surgical air cavities on the target coverage were adequately accounted for by conventional pencil beam methods for all of the cases studied. Although differences in details of the DVHs of the normal structures were observed, depending on whether Monte Carlo or pencil-beam algorithm was used for calculation, these differences are not expected to be clinically significant. In general, the pencil-beam calculation corrected for primary attenuation by the equivalent pathlength is a sufficiently accurate method for head-and-neck treatment planning using 6-MV photons.

Evaluation of a pencil-beam dose calculation technique for charged particle radiotherapy

Purpose : The purpose of this article is to evaluate a pencil-beam dose calculation algorithm for protons and heavier charged particles in complex patient geometries defined by computed tomography (CT) data and to compare isodose distributions calculated with the new technique to those calculated with conventional algorithms in selected patients with skull-base tumors. Methods and Materials : Monte Carlo calculations were performed to evaluate the pencil-beam algorithm in patient geometries for a modulated 150-MeV proton beam. A modified version of a Mont Carlo code described in a previous publication (18) was used for these comparisons. Tissue densities were inferred from patient CT data on a voxel-by-voxel basis, and calculations were peformed with an without tissue compensators. A dose calculation module using the new algorithm was written, and treatment plans using the new algorithm were compared to plans using standard ay-tracing techniques for 10 patients with clival chordoma and three patients with nasopharyngeal carcinoma who were treated with helium ions at Lawrence Berkeley National Laboratory (LBL). Results : Pencil beam calculations agreed well with Monte Carlo calculations in the patient geometries. The pencil-beam algorithm predicted several multiple-scattering effects that are not modeled by conventional ray-tracing calculations. These includes (a) the widening of the penumbra as a function of beam penetration, (b) the degradation in the sharpness of the dose gradient at the end of the particle range in hgihly heterogeneous regions, and (c) the appearance of hot and cold dose regions in the shadow of cmplex heterogeneities. In particular, pencil-beam calculations indicated that the dose distribution within the target was not a homogeneous as expected on the basis of ray-tracing calculations. on average, for the 13 patients considered, only about 72% of the cone down target volume received at least 99% of the prescribed dose, whereas, 93% of the conedwn volume was contained within the 95% isodose surface. This may e significant because in standard charged particle dose calculations, the dose across the spread-Bragg peak is assumed to be uniform and equal to the maximum or prescribed dose. Conclusions : Dose distributions computed with the pencil-beam model are more accurate than ray-tracing calculations, providing additional information to clinicians, which may influence the doses they prescribe. In particular, these calculations indicate that for some patients with skull-base tumors, it may be advantageous to prescribe proton doses to a lower isodose level than is commonly done.