Comparison of IMRT optimization based on a pencil beam and a superposition algorithm

To investigate the role of sophisticated dose calculation methods for treatment planning, we compared conventional pencil beam optimized 6 and 15 MV intensity-modulated treatment plans with optimizations based on the superposition technique. Five lung and five head and neck IMRT cases with spatial resolutions of bixels and dose voxels usually employed in clinical practice were considered for tumor volumes between 15 and 500 cm3. We investigated the systematic error of the pencil beam algorithm and the pencil beam induced error to the optimal solution of bixel weights. For the lung cases, the pencil beam overestimated the mean dose deposited inside the planning target volume (PTV) by about 8%, for small lung tumors even up to 20.6%. In the head and neck cases only a slight overestimation in mean PTV dose of 1.5% was observed. The optimization with the superposition method substantially improved the dose coverage of the considered radiation targets. Additionally, for the head and neck cases, the brainstem was significantly spared by about 4% mean PTV dose through the use of the superposition technique. Our studies showed that, in target regions with intricate tissue inhomogeneities, superposition or Monte Carlo techniques have to be used for the optimization and the final dose calculation of intensity-modulated treatment plans.     

Comparison of dose calculation algorithms with Monte Carlo methods for photon arcs

The objective of this study is to seek an accurate and efficient method to calculate the dose distribution of a photon arc. The algorithms tested include Monte Carlo, pencil beam kernel (PK), and collapsed cone convolution (CCC). For the Monte Carlo dose calculation, EGS4/DOSXYZ was used. The SRCXYZ source code associated with the DOSXYZ was modified so that the gantry angle of a photon beam would be sampled uniformly within the arc range about an isocenter to simulate a photon arc. Specifically, photon beams (6/18 MV, 4 x 4 and 10 x 10 cm2) described by a phase space file generated by BEAM (MCPHS), or by two point sources with different photon energy spectra (MCDIV) were used. These methods were used to calculate three-dimensional (3-D) distributions in a PMMA phantom, a cylindrical water phantom, and a phantom with lung inhomogeneity. A commercial treatment planning system was also used to calculate dose distributions in these phantoms using equivalent tissue air ratio (ETAR), PK and CCC algorithms for inhomogeneity corrections. Dose distributions for a photon arc in these phantoms were measured using a RK ion chamber and radiographic films. For homogeneous phantoms, the measured results agreed well (approximately 2% error) with predictions by the Monte Carlo simulations (MCPHS and MCDIV) and the treatment planning system for the 180 degrees and 360 degrees photon arcs. For the dose distribution in the phantom with lung inhomogeneity with a 90 degrees photon arc, the Monte Carlo calculations agreed with the measurements within 2%, while the treatment planning system using ETAR, PK and CCC underestimated or overestimated the dose inside the lung inhomogeneity from 6% to 12%.     

Accuracy of patient dose calculation for lung IMRT: A comparison of Monte Carlo, convolution/superposition, and pencil beam computations

The accuracy of dose computation within the lungs depends strongly on the performance of the calculation algorithm in regions of electronic disequilibrium that arise near tissue inhomogeneities with large density variations. There is a lack of data evaluating the performance of highly developed analytical dose calculation algorithms compared to Monte Carlo computations in a clinical setting. We compared full Monte Carlo calculations (performed by our Monte Carlo dose engine MCDE) with two different commercial convolution/superposition (CS) implementations (Pinnacle-CS and Helax-TMS's collapsed cone model Helax-CC) and one pencil beam algorithm (Helax-TMS's pencil beam model Helax-PB) for 10 intensity modulated radiation therapy (IMRT) lung cancer patients. Treatment plans were created for two photon beam qualities (6 and 18 MV). For each dose calculation algorithm, patient, and beam quality, the following set of clinically relevant dose-volume values was reported: (i) minimal, median, and maximal dose (Dmin, D50, and Dmax) for the gross tumor and planning target volumes (GTV and PTV); (ii) the volume of the lungs (excluding the GTV) receiving at least 20 and 30 Gy (V20 and V30) and the mean lung dose; (iii) the 33rd percentile dose (D33) and Dmax delivered to the heart and the expanded esophagus; and (iv) Dmax for the expanded spinal cord. Statistical analysis was performed by means of one-way analysis of variance for repeated measurements and Tukey pairwise comparison of means. Pinnacle-CS showed an excellent agreement with MCDE within the target structures, whereas the best correspondence for the organs at risk (OARs) was found between Helax-CC and MCDE. Results from Helax-PB were unsatisfying for both targets and OARs. Additionally, individual patient results were analyzed. Within the target structures, deviations above 5% were found in one patient for the comparison of MCDE and Helax-CC, while all differences between MCDE and Pinnacle-CS were below 5%. For both Pinnacle-CS and Helax-CC, deviations from MCDE above 5% were found within the OARs: within the lungs for two (6 MV) and six (18 MV) patients for Pinnacle-CS, and within other OARs for two patients for Helax-CC (for Dmax of the heart and D33 of the expanded esophagus) but only for 6 MV. For one patient, all four algorithms were used to recompute the dose after replacing all computed tomography voxels within the patient's skin contour by water. This made all differences above 5% between MCDE and the other dose calculation algorithms disappear. Thus, the observed deviations mainly arose from differences in particle transport modeling within the lungs, and the commissioning of the algorithms was adequately performed (or the commissioning was less important for this type of treatment). In conclusion, not one pair of the dose calculation algorithms we investigated could provide results that were consistent within 5% for all 10 patients for the set of clinically relevant dose-volume indices studied. As the results from both CS algorithms differed significantly, care should be taken when evaluating treatment plans as the choice of dose calculation algorithm may influence clinical results. Full Monte Carlo provides a great benchmarking tool for evaluating the performance of other algorithms for patient dose computations.     

The impact of photon dose calculation algorithms on expected dose distributions in lungs under different respiratory phases

A planning study was carried out on a cohort of CT datasets from breast patients scanned during different respiratory phases. The aim of the study was to investigate the influence of different air filling in lungs on the calculation accuracy of photon dose algorithms and to identify potential patterns of failure with clinical implications. Selected respiratory phases were free breathing (FB), representative of typical end expiration, and deep inspiration breath hold (DIBH), a typical condition for clinical treatment with respiratory gating. Algorithms investigated were the pencil beam (PBC), the anisotropic analytical algorithm (AAA) and the collapsed cone (CC) from the Varian Eclipse or Philips Pinnacle planning system. Reference benchmark calculations were performed with the Voxel Monte Carlo (VMC++). An analysis was performed in terms of physical quantities inspecting either dose-volume or dose-mass histograms and in terms of an extension to three dimensions of the gamma index of Low. Results were stratified according to a breathing phase and algorithm. Collectives acquired in FB or DIBH showed well-separated average lung density distributions with mean densities of 0.27 +/- 0.04 and 0.16 +/- 0.02 g cm(-3), respectively, and average peak densities of 0.17 +/- 0.03 and 0.09 +/- 0.02 g cm(-3). Analysis of volume-dose or mass-dose histograms proved the expected deviations on PBC results due to the missing lateral transport of electrons with underestimations in the low dose region and overestimations in the high dose region. From the gamma analysis, it resulted that PBC is systematically defective compared to VMC++ over the entire range of lung densities and dose levels with severe violations in both respiratory phases. The fraction of lung voxels with gamma > 1 for PBC reached 25% in DIBH and about 15% in FB. CC and AAA performed, in contrast, similarly and with fractions of lung voxels with gamma > 1 in average inferior to 2% in FB and 4-5% (AAA) or 6-8% (CC) in DIBH. In summary, PBC proved to be severely defective in calculations involving lungs and particularly for cases where specific respiratory phases (e.g. DIBH) are assumed for treatment. In contrast, CC and AAA manifested a high degree of consistency against the Monte Carlo method and provided stable results over the entire range of clinically relevant densities.     

Underdosage of the upper-airway mucosa for small fields as used in intensity-modulated radiation therapy: a comparison between radiochromic film measurements,Monte Carlo simulations,and collapsed cone convolution calculations

Head-and-neck tumors are often situated at an air-tissue interface what may result in an underdosage of part of the tumor in radiotherapy treatments using megavoltage photons, especially for small fields. In addition to effects of transient electronic disequilibrium, for these small fields, an increased lateral electron range in air will result in an important extra reduction of the central axis dose beyond the cavity. Therefore dose calculation algorithms need to model electron transport accurately. We simulated the trachea by a 2 cm diameter cylindrical air cavity with the rim situated 2 cm beneath the phantom surface. A 6 MV photon beam from an Elekta SLiplus linear accelerator, equipped with the standard multileaf collimator (MLC), was assessed. A 10 x 2 cm2 and a 10 x 1 cm2 field, both widthwise collimated by the MLC, were applied with their long side parallel to the cylinder axis. Central axis dose rebuild-up was studied. Radiochromic film measurements were performed in an in-house manufactured polystyrene phantom with the films oriented either along or perpendicular to the beam axis. Monte Carlo simulations were performed with BEAM and EGSnrc. Calculations were also performed using the pencil beam (PB) algorithm and the collapsed cone convolution (CCC) algorithm of Helax-TMS (MDS Nordion, Kanata, Cahada) version 6.0.2 and using the CCC algorithm of Pinnacle (ADAC Laboratories, Milpitas, CA, USA) version 4.2. A very good agreement between the film measurements and the Monte Carlo simulations was found. The CCC algorithms were not able to predict the interface dose accurately when lateral electronic disequilibrium occurs, but were shown to be a considerable improvement compared to the PB algorithm. The CCC algorithms overestimate the dose in the rebuild-up region. The interface dose was overestimated by a maximum of 31% or 54%, depending on the implementation of the CCC algorithm. At a depth of 1 mm, the maximum dose overestimation was 14% or 24%.     

PBC算法与AAA算法在肺癌调强放疗中的剂量学比较

目的:分析、比较笔形束卷积算法(PBC)和各向异性解析算法(AAA)在非小细胞肺癌(NSCLC)调强放疗计划设计中的剂量学差异。方法:随机选择7例NSCLC患者,采用Eclipse version 7.3.10计划系统提供的PBC算法和AAA算法对每例NSCLC进行IMRT的计划设计,比较靶区及危及器官的剂量分布、DVH等指标。结果:两种算法获得治疗计划的靶区剂量均匀性和适形度均无明显差别,食管、心脏、脊髓等危及器官的受量也基本相同。结论:对于NSCLC,剂量计算应采用受呼吸时相影响更小的AAA算法。     

用蒙特卡罗模拟评估小野条件下肺介质中AAA算法的精度

目的：用蒙特卡罗模拟评估放射治疗剂量计算使用的各向异性分析算法（Anisotropic Analytical Algorithm,AAA）在小野条件下肺介质中的计算精度。材料与方法：建立一包含肺介质的水模体,分别用AAA算法、笔形束卷积算法（Pencil Beam Convolution,PBC算法）（作为对比）和蒙特卡罗（Monte Carlo,MC）模拟计算2cm×2cm到8cm×8cm射野条件下该模体中的深度剂量和离轴比,并以MC模拟为标准比较深度剂量。用一维伽马分析对离轴比进行分析。结果：AAA算法在2cmx2cm射野肺介质区域高估了深度剂量,其它情况均低估了深度剂量,剂量偏差范围为-0．24％-2．66％．PBC算法在肺介质区域高估了深度剂量,剂量偏差的范围为1．18％～14．55％。AAA算法计算的离轴比和MC模拟,在射野剂量平坦区相对内收,在剂量跌落区向两侧发散,但AAA算法略高估了射野边缘的剂量,一维伽马分析（与MC相比）通过率为100％（3mm／3％）。PBC算法在射野剂量平坦区相对发散,而在剂量跌落区向两侧内收。一维伽马分析通过率范围为51％～88％。结论：在肺介质中,AAA剂量计算的结果与MC模拟的一致性较好,与PBC算法相比,剂量计算的精度较高。     

A 3D pencil-beam-based superposition algorithm for photon dose calculation in heterogeneous media

In this work, a novel three-dimensional superposition algorithm for photon dose calculation is presented. The dose calculation is performed as a superposition of pencil beams, which are modified based on tissue electron densities. The pencil beams have been derived from Monte Carlo simulations, and are separated into lateral and depth-directed components. The lateral component is modeled using exponential functions, which allows accurate modeling of lateral scatter in heterogeneous tissues. The depth-directed component represents the total energy deposited on each plane, which is spread out using the lateral scatter functions. Finally, convolution in the depth direction is applied to account for tissue interface effects. The method can be used with the previously introduced multiple-source model for clinical settings. The method was compared against Monte Carlo simulations in several phantoms including lung- and bone-type heterogeneities. Comparisons were made for several field sizes for 6 and 18 MV energies. The deviations were generally within (2%, 2 mm) of the field central axis d(max). Significantly larger deviations (up to 8%) were found only for the smallest field in the lung slab phantom for 18 MV. The presented method was found to be accurate in a wide range of conditions making it suitable for clinical planning purposes.     

Monte carlo evaluation of the AAA treatment planning algorithm in a heterogeneous multilayer phantom and IMRT clinical treatments for an Elekta SL25 linear accelerator

The Anisotropic Analytical Algorithm (AAA) is a new pencil beam convolution/superposition algorithm proposed by Varian for photon dose calculations. The configuration of AAA depends on linear accelerator design and specifications. The purpose of this study was to investigate the accuracy of AAA for an Elekta SL25 linear accelerator for small fields and intensity modulated radiation therapy (IMRT) treatments in inhomogeneous media. The accuracy of AAA was evaluated in two studies. First, AAA was compared both with Monte Carlo (MC) and the measurements in an inhomogeneous phantom simulating lung equivalent tissues and bone ribs. The algorithm was tested under lateral electronic disequilibrium conditions, using small fields (2 x 2 cm(2)). Good agreement was generally achieved for depth dose and profiles, with deviations generally below 3% in lung inhomogeneities and below 5% at interfaces. However, the effects of attenuation and scattering close to the bone ribs were not fully taken into account by AAA, and small inhomogeneities may lead to planning errors. Second, AAA and MC were compared for IMRT plans in clinical conditions, i.e., dose calculations in a computed tomography scan of a patient. One ethmoid tumor, one orophaxynx and two lung tumors are presented in this paper. Small differences were found between the dose volume histograms. For instance, a 1.7% difference for the mean planning target volume dose was obtained for the ethmoid case. Since better agreement was achieved for the same plans but in homogeneous conditions, these differences must be attributed to the handling of inhomogeneities by AAA. Therefore, inherent assumptions of the algorithm, principally the assumption of independent depth and lateral directions in the scaling of the kernels, were slightly influencing AAA's validity in inhomogeneities. However, AAA showed a good accuracy overall and a great ability to handle small fields in inhomogeneous media compared to other pencil beam convolution algorithms.     

各向异性分析算法和光子笔形束卷积算法在食管癌放射治疗中的剂量学比较

目的比较食管癌调强放射治疗各向异性分析算法（AAA）与光子笔形束卷积（PBC）算法的剂量学差异。方法选取9例食管癌患者,其中男性6例,女性3例;年龄54-68岁,平均年龄61岁。用瓦里安Eclipse 8.6治疗计划系统设计5野均分逆向调强计划,分别用AAA和PBC算法模型计算并利用COMPASS进行剂量验证。利用剂量体积直方图比较靶区、肺、心脏和脊髓照射剂量和体积。数据应用SPSS15.0进行配对t检验分析。结果大体肿瘤区（GTV）的均匀性指数（HI）、适合度指数（CI）、Dmean及计划靶区（PTV）的HI,AAA结果均优于PBC算法,差异均有统计学意义（P〈0.05）。AAA双肺各指标差值为-0.02%～-1.87%,即低估了肺2%以内的受量。PBC算法双肺各指标差值为-3.95%～1.05%,低剂量区（V5～15）低估了肺4%以内的受量,高剂量区（V20～30）则稍高估。对于脊髓,AAA和PBC算法分别高估了1.57%、4.49%。两种算法都低估了心脏的受量,但AAA相对准确。结论食管癌放射治疗中采用AAA优于PBC算法。     

The impact of electron transport on the accuracy of computed dose

The aim of this work was to investigate the accuracy of dose predicted by a Batho power law correction, and two models which account for electron range: A superposition/convolution algorithm and a Monte Carlo algorithm. The results of these models were compared in phantoms with cavities and low-density inhomogeneities. An idealized geometry was considered with inhomogeneities represented by regions of air and lung equivalent material. Measurements were performed with a parallel plate ionization chamber, thin TLDs (thermoluminescent dosimeters) and film. Dose calculations were done with a generalized Batho model, the Pinnacle collapsed cone convolution model (CCC), and the Peregrine Monte Carlo dose calculation algorithm. Absolute central axis and off axis dose data at various depths relative to interfaces of inhomogeneities were compared. Our results confirm that for a Batho correction, dose errors in the calculated depth dose arise from the neglect of electron transport. This effect increases as the field size decreases, as the density of the inhomogeneity decreases, and with the energy of incident photons. The CCC calculations were closer to measurements than the Batho model, but significant discrepancies remain. Monte Carlo results agree with measurements within the measurement and computational uncertainties.     

A 3D photon superposition/convolution algorithm and its foundation on results of Monte Carlo calculations

Based on previous publications on a triple Gaussian analytical pencil beam model and on Monte Carlo calculations using Monte Carlo codes GEANT-Fluka, versions 95, 98, 2002, and BEAMnrc/EGSnrc, a three-dimensional (3D) superposition/convolution algorithm for photon beams (6 MV, 18 MV) is presented. Tissue heterogeneity is taken into account by electron density information of CT images. A clinical beam consists of a superposition of divergent pencil beams. A slab-geometry was used as a phantom model to test computed results by measurements. An essential result is the existence of further dose build-up and build-down effects in the domain of density discontinuities. These effects have increasing magnitude for field sizes < or =5.5 cm(2) and densities < or = 0.25 g cm(-3), in particular with regard to field sizes considered in stereotaxy. They could be confirmed by measurements (mean standard deviation 2%). A practical impact is the dose distribution at transitions from bone to soft tissue, lung or cavities.     

The effect of dose calculation accuracy on inverse treatment planning

The effect of dose calculation accuracy during inverse treatment planning for intensity modulated radiotherapy (IMRT) was studied in this work. Three dose calculation methods were compared: Monte Carlo, superposition and pencil beam. These algorithms were used to calculate beamlets. which were subsequently used by a simulated annealing algorithm to determine beamlet weights which comprised the optimal solution to the objective function. Three different cases (lung, prostate and head and neck) were investigated and several different objective functions were tested for their effect on inverse treatment planning. It is shown that the use of inaccurate dose calculation introduces two errors in a treatment plan, a systematic error and a convergence error. The systematic error is present because of the inaccuracy of the dose calculation algorithm. The convergence error appears because the optimal intensity distribution for inaccurate beamlets differs from the optimal solution for the accurate beamlets. While the systematic error for superposition was found to be approximately 1% of Dmax in the tumour and slightly larger outside, the error for the pencil beam method is typically approximately 5% of Dmax and is rather insensitive to the given objectives. On the other hand, the convergence error was found to be very sensitive to the objective function, is only slightly correlated to the systematic error and should be determined for each case individually. Our results suggest that because of the large systematic and convergence errors, inverse treatment planning systems based on pencil beam algorithms alone should be upgraded either to superposition or Monte Carlo based dose calculations.     

On the dosimetric behaviour of photon dose calculation algorithms in the presence of simple geometric heterogeneities: comparison with Monte Carlo calculations

A comparative study was performed to reveal differences and relative figures of merit of seven different calculation algorithms for photon beams when applied to inhomogeneous media. The following algorithms were investigated: Varian Eclipse: the anisotropic analytical algorithm, and the pencil beam with modified Batho correction; Nucletron Helax-TMS: the collapsed cone and the pencil beam with equivalent path length correction; CMS XiO: the multigrid superposition and the fast Fourier transform convolution; Philips Pinnacle: the collapsed cone. Monte Carlo simulations (MC) performed with the EGSnrc codes BEAMnrc and DOSxyznrc from NRCC in Ottawa were used as a benchmark. The study was carried out in simple geometrical water phantoms (rho = 1.00 g cm(-3)) with inserts of different densities simulating light lung tissue (rho = 0.035 g cm(-3)), normal lung (rho = 0.20 g cm(-3)) and cortical bone tissue (rho = 1.80 g cm(-3)). Experiments were performed for low- and high-energy photon beams (6 and 15 MV) and for square (13 x 13 cm2) and elongated rectangular (2.8 x 13 cm2) fields. Analysis was carried out on the basis of depth dose curves and transverse profiles at several depths. Assuming the MC data as reference, gamma index analysis was carried out distinguishing between regions inside the non-water inserts or inside the uniform water. For this study, a distance to agreement was set to 3 mm while the dose difference varied from 2% to 10%. In general all algorithms based on pencil-beam convolutions showed a systematic deficiency in managing the presence of heterogeneous media. In contrast, complicated patterns were observed for the advanced algorithms with significant discrepancies observed between algorithms in the lighter materials (rho = 0.035 g cm(-3)), enhanced for the most energetic beam. For denser, and more clinical, densities a better agreement among the sophisticated algorithms with respect to MC was observed.     

Dose verification of an IMRT treatment planning system with the BEAM EGS4-based Monte Carlo code

Intensity modulated radiation therapy (IMRT) has been increasingly used in radiotherapy departments during the last several years. A major advantage of IMRT in comparison to traditional three-dimensional conformal radiotherapy is the higher capability in providing dose distributions that conform very tightly to the target even for very complex shapes such as, for instance, concave regions. This results in a significant sparing of adjacent normal tissues. Different types of algorithms are employed in the IMRT dose calculation, from the simple pencil beam method, such as the finite-size pencil beam algorithm, to the more sophisticated algorithms, such as the kernel-based convolution/superposition ones. With the latter ones, electronic disequilibrium and inhomogeneities are better dealt with in comparison to the correction-based models like pencil beam. Nevertheless, even these types of algorithms may have some approximations that can potentially affect the dose results, especially considering that in an IMRT plan small segments or beamlets may be present for which electronic disequilibrium and inhomogeneities effects are of paramount importance. The goal of this work was to determine the accuracy in monitor units (MU) and dose distribution calculation of the algorithm implemented in the commercial treatment planning system PINNACLE3 (P3), for two IMRT plans with 6 MV photon beams. This system is based on a convolution/superposition with the Collapsed Cone approximation algorithm. The "BEAM" Monte Carlo (MC) code was employed as a benchmark in comparing the MU calculation and the dose distribution of P3. The model used to calculate the MU, with the separation of collimator scatter from the phantom scatter, valid for broad beams, was verified for narrow and irregular segments. The attention was focused on the way P3 calculates output factors (OF). A difference of 8% compared to MC was found for a particularly narrow segment analyzed. A dependence of the results on field size was found. For the complete plan, the agreement of dose distribution and MU calculation with MC results (affected by a dose uncertainty less than 0.5%) is very good: the dose difference at isocenter is 2.1% (1 standard deviation) for a "Prostate" site and 2.9% (1 standard deviation) for the "Head and Neck" site.     

Fast Monte Carlo dose calculation for photon beams based on the VMC electron algorithm.

A new Monte Carlo algorithm for 3D photon dose calculation in radiation therapy is presented, which is based on the previously developed Voxel Monte Carlo (VMC) for electron beams. The main result is that this new version of VMC (now called XVMC) is more efficient than EGS4/PRESTA photon dose calculation by a factor of 15-20. Therefore, a standard treatment plan for photons can be calculated by Monte Carlo in about 20 min. on a "normal" personal computer. The improvement is caused mainly by the fast electron transport algorithm and ray tracing technique, and an initial ray tracing method to calculate the number of electrons created in each voxel by the primary photon beam. The model was tested in comparison to calculations by EGS4 using several fictive phantoms. In most cases a good coincidence has been found between both codes. Only within lung substitute dose differences have been observed.     

Monte Carlo evaluation of 6 MV intensity modulated radiotherapy plans for head and neck and lung treatments

Intensity modulated radiotherapy (IMRT) beams may have strong fluence variations and are advantageous at disease sites such as lung and head and neck (H&N), where neighboring tissues have very different electron densities. We use Monte Carlo (MC) dose calculations to evaluate the dosimetric effects of these inhomogeneities for 10 clinical IMRT treatment plans for five lung patients and four H&N patients. All beams are 6 MV photons. "Standard plans" were first produced on a clinical treatment planning system which optimizes beam intensity distributions to meet dose and dose-volume constraints and calculates dose using a measurement-based pencil-beam algorithm with an equivalent pathlength inhomogeneity correction. Patient anatomy and electron densities were obtained from patient-specific CT images. The dose distribution of each beam was recalculated with the MC method, using the same CT images, beam geometry, beam weighting and optimized fluence intensity distributions as the corresponding standard plan. For the lung cases, the MC calculated dose distributions are characterized by reduced penetrations and increased penumbra due to larger secondary electron range in the low-density media, which is not accurately accounted for in the pencil beam algorithm. For the lung cases, the PTV was underdosed; except for one dose-volume index, underdose was less than 10%. Individual H&N fields are affected to different degrees by tissue inhomogeneities, depending on specific anatomy, especially the size and location of air cavities in relation to the beam orientation and field size. For four H&N plans, PTV coverage changed by less than 2%; for the fifth, there was less than 10% difference between the standard and the MC plans. Critical normal tissue DVHs (cord, lung, brainstem) are changed by <10% at the high dose end and mean lung doses are changed by <6%.     

Dose perturbation in the presence of metallic implants: treatment planning system versus Monte Carlo simulations

An increasing number of patients receiving radiation therapy have metallic implants such as hip prostheses. Therefore, beams are normally set up to avoid irradiation through the implant; however, this cannot always be accomplished. In such situations, knowledge of the accuracy of the used treatment planning system (TPS) is required. Two algorithms, the pencil beam (PB) and the collapsed cone (CC), are implemented in the studied TPS. Comparisons are made with Monte Carlo simulations for 6 and 18 MV. The studied materials are steel, CoCrMo, Orthinox, TiAlV and Ti. Monte Carlo simulated depth dose curves and dose profiles are compared to CC and PB calculated data. The CC algorithm shows overall a better agreement with Monte Carlo than the PB algorithm. Thus, it is recommended to use the CC algorithm to get the most accurate dose calculation both for the planning target volume and for tissues adjacent to the implants when beams are set up to pass through implants.