AAA and PBC calculation accuracy in the surface build-up region in tangential beam treatments. Phantom and breast case study with the Monte Carlo code penelope

Skin dose during radiotherapy: a summary and general estimation technique

The skin dose associated with radiotherapy may be of interest for clinical evaluation or investigating the risk of late effects. However, skin dose is not intuitive and is difficult to measure. Our objectives were to develop and evaluate a general estimation technique for skin dose based on treatment parameters. The literature on skin dose was supplemented with measurements and Monte Carlo simulations. Using all available data, a general dosimetry system was developed (in the form of a series of equations) to estimate skin dose based on treatment parameters including field size, the presence of a block tray, and obliquity of the treatment field. For out‐of‐field locations, the distance from the field edge was also considered. This dosimetry system was then compared to TLD measurements made on the surface of a phantom. As compared to measurements, the general dosimetry system was able to predict skin dose within, on average, 21% of the local dose (4% of the Dmax dose). Skin dose for patients receiving radiotherapy can be estimated with reasonable accuracy using a set of general rules and equations.

PACS numbers: 87.53.‐j, 87.53.Bn, 87.55.ne

A PENELOPE-based system for the automated Monte Carlo simulation of clinacs and voxelized geometries-application to far-from-axis fields

PURPOSE

Two new codes, PENEASY and PENEASYLINAC, which automate the Monte Carlo simulation of Varian Clinacs of the 600, 1800, 2100, and 2300 series, together with their electron applicators and multileaf collimators, are introduced. The challenging case of a relatively small and far-from-axis field has been studied with these tools.

METHODS

PENEASY is a modular, general-purpose main program for the PENELOPE Monte Carlo system that includes various source models, tallies and variance-reduction techniques (VRT). The code includes a new geometry model that allows the superposition of voxels and objects limited by quadric surfaces. A variant of the VRT known as particle splitting, called fan splitting, is also introduced. PENEASYLINAC, in turn, automatically generates detailed geometry and configuration files to simulate linacs with PENEASY. These tools are applied to the generation of phase-space files, and of the corresponding absorbed dose distributions in water, for two 6 MV photon beams from a Varian Clinac 2100 C∕D: a 40 × 40 cm(2) centered field; and a 3 × 5 cm(2) field centered at (4.5, -11.5) cm from the beam central axis. This latter configuration implies the largest possible over-traveling values of two of the jaws. Simulation results for the depth dose and lateral profiles at various depths are compared, by using the gamma index, with experimental values obtained with a PTW 31002 ionization chamber. The contribution of several VRTs to the computing speed of the more demanding off-axis case is analyzed.

RESULTS

For the 40 × 40 cm(2) field, the percentages γ(1) and γ(1.2) of voxels with gamma indices (using 0.2 cm and 2% criteria) larger than unity and larger than 1.2 are 0.2% and 0%, respectively. For the 3 × 5 cm(2) field, γ(1) = 0%. These figures indicate an excellent agreement between simulation and experiment. The dose distribution for the off-axis case with voxels of 2.5 × 2.5 × 2.5 mm(3) and an average standard statistical uncertainty of 2% (1σ) is computed in 3.1 h on a single core of a 2.8 GHz Intel Core 2 Duo processor. This result is obtained with the optimal combination of the tested VRTs. In particular, fan splitting for the off-axis case accelerates execution by a factor of 240 with respect to standard particle splitting.

CONCLUSIONS

PENEASY and PENEASYLINAC can simulate the considered Varian Clinacs both in an accurate and efficient manner. Fan splitting is crucial to achieve simulation results for the off-axis field in an affordable amount of CPU time. Work to include Elekta linacs and to develop a graphical interface that will facilitate user input is underway.

Impact of the calculation resolution of AAA for small fields and RapidArc treatment plans

PURPOSE

To investigate the impact of the calculation resolution of the anisotropic analytical algorithms (AAA) for a variety of small fields in homogeneous and heterogeneous media and for RapidArc plans.

METHODS

Dose distributions calculated using AAA version 8.6.15 (AAA8) and 10.0.25 (AAA10) were compared to measurements performed with GafChromic EBT film, using phantoms made of polystyrene or a combination of polystyrene and cork. The accuracy of the algorithms calculated using grid resolutions of 2.5 and 1.0 mm was investigated for different field sizes, and for a limited selection of RapidArc plans (head and neck, small meningioma, and lung). Additional plans were optimized to create excessive multileaf collimator modulation and measured on a homogenous phantom. Gamma evaluation criterion of 3% dose difference and 2- or 1-mm distance to agreement (DTA) were applied to evaluate the accuracy of the algorithms.

RESULTS

For fields < or = 3 x 3 cm2, both versions of AAA predicted lower peak doses and broader penumbra widths than the measurements. However, AAA10 and a finer calculation grid improved the agreement. For RapidArc plans with many small multileaf collimator (MLC) segments and relatively high number of monitor units (MU), AAA8 failed to identify small dose peaks within the target. Both versions performed better in polystyrene than in cork. In homogeneous cork layers, AAA8 underestimated the average target dose for a clinical lung plan. This was improved with AAA10 calculated using a 1 mm grid.

CONCLUSIONS

AAA10 improves the accuracy of dose calculations, and calculation grid of 1.0 mm is superior to using 2.5 mm, although calculation times increased by factor of 5. A suitable upper MU constraint should be assigned during optimization to avoid plans with high modulation. For plans with a relative high number of monitor units, calculations using 1 mm grid resolution are recommended. For planning target volume (PTV) which contains relatively large area of low density tissue, users should be aware of possible dose underestimation in the low density region and recalculation with AAA10 grid 1.0 mm is recommended.

Comparison of modulated electron radiotherapy to conventional electron boost irradiation and volumetric modulated photon arc therapy for treatment of tumour bed boost in breast cancer

BACKGROUND AND PURPOSE

To compare few leaf electron collimator (FLEC)-based modulated electron radiotherapy (MERT) to conventional direct electron (DE) and volumetric modulated photon arc therapy (VMAT) for the treatment of tumour bed boost in breast cancer.

MATERIALS AND METHODS

Fourteen patients with breast cancer treated by lumpectomy and requiring post-operative whole breast radiotherapy with tumour bed boost were planned retrospectively using conventional DE, VMAT and FLEC-based MERT. The planning goal was to deliver 10Gy to at least 95% of the tumour bed volume. Dosimetry parameters for all techniques were compared.

RESULTS

Dose evaluation volume (DEV) coverage and homogeneity were best for MERT (D(98)=9.77Gy, D(2)=11.03Gy) followed by VMAT (D(98)=9.56Gy, D(2)=11.07Gy) and DE (D(98)=9.81Gy, D(2)=11.52Gy). Relative to the DE plans, the MERT plans predicted a reduction of 35% in mean breast dose (p<0.05), 54% in mean lung dose (p<0.05) and 46% in mean body dose (p<0.05). Relative to the VMAT plans, the MERT plans predicted a reduction of 24%, 36% and 39% in mean breast dose, heart dose and body dose, respectively (p<0.05).

CONCLUSIONS

MERT plans were a considerable improvement in dosimetry over DE boost plans. There was a dosimetric advantage in using MERT over VMAT for increased DEV conformity and low-dose sparing of healthy tissue including the integral dose; however, the cost is often an increase in the ipsilateral lung high-dose volume.

Superficial doses in breast cancer radiotherapy using conventional and IMRT techniques: a film-based phantom study

BACKGROUND AND PURPOSE

Superficial doses in radiotherapy are affected by the treatment technique. The implications for breast cancer treatments were evaluated.

MATERIAL AND METHODS

Four treatment techniques relevant for breast cancer irradiation were evaluated; tangential standard, tangential IMRT, 7-field IMRT (arc-like field arrangement) and hybrid IMRT (an IMRT plan mixed with non-modulated fields). Only 6MV photons were used. GafChromic EBT film was used for dose measurements at the surface, in the skin (0-5mm depth) and in the superficial parts of CTV (5-10mm depth) of an anthropomorphic thorax phantom.

RESULTS

Only small differences in superficial doses were observed between tangential standard and tangential IMRT. Compared to the tangential standard plan, the surface and skin doses were reduced with the 7-field IMRT plan, on average by 20% and 5%, respectively, while hybrid IMRT reduced the surface and skin doses medially (by 44% and 8%, respectively) and increased the surface and skin dose laterally (by 40% and 15%, respectively). Minimum superficial CTV doses varied between regions, but were mainly between 90% and 95% of the target dose for all plans, only the hybrid IMRT plan resulted in a region with minimum dose below 90%.

CONCLUSIONS

Compared to tangential irradiation, skin sparing was achieved by the 7-field IMRT plan. The minimum dose in the superficial parts of the CTV was below 95% of the target dose for all plans investigated.

The impact of dose calculation algorithms on partial and whole breast radiation treatment plans

Background

This paper compares the calculated dose to target and normal tissues when using pencil beam (PBC), superposition/convolution (AAA) and Monte Carlo (MC) algorithms for whole breast (WBI) and accelerated partial breast irradiation (APBI) treatment plans.

Methods

Plans for 10 patients who met all dosimetry constraints on a prospective APBI protocol when using PBC calculations were recomputed with AAA and MC, keeping the monitor units and beam angles fixed. Similar calculations were performed for WBI plans on the same patients. Doses to target and normal tissue volumes were tested for significance using the paired Student's t-test.

Results

For WBI plans the average dose to target volumes when using PBC calculations was not significantly different than AAA calculations, the average PBC dose to the ipsilateral breast was 10.5% higher than the AAA calculations and the average MC dose to the ipsilateral breast was 11.8% lower than the PBC calculations. For ABPI plans there were no differences in dose to the planning target volume, ipsilateral breast, heart, ipsilateral lung, or contra-lateral lung. Although not significant, the maximum PBC dose to the contra-lateral breast was 1.9% higher than AAA and the PBC dose to the clinical target volume was 2.1% higher than AAA. When WBI technique is switched to APBI, there was significant reduction in dose to the ipsilateral breast when using PBC, a significant reduction in dose to the ipsilateral lung when using AAA, and a significant reduction in dose to the ipsilateral breast and lung and contra-lateral lung when using MC.

Conclusions

There is very good agreement between PBC, AAA and MC for all target and most normal tissues when treating with APBI and WBI and most of the differences in doses to target and normal tissues are not clinically significant. However, a commonly used dosimetry constraint, as recommended by the ASTRO consensus document for APBI, that no point in the contra-lateral breast volume should receive >3% of the prescribed dose needs to be relaxed to >5%.

Dosimetry of oblique tangential photon beams calculated by superposition/convolution algorithms: a Monte Carlo evaluation

Although there are many works on evaluating dose calculations of the anisotropic analytical algorithm (AAA) using various homogeneous and heterogeneous phantoms, related work concerning dosimetry due to tangential photon beam is lacking. In this study, dosimetry predicted by the AAA and collapsed cone convolution (CCC) algorithm was evaluated using the tangential photon beam and phantom geometry. The photon beams of 6 and 15 MV with field sizes of 4 × 4 (or 7 × 7), 10 × 10 and 20 × 20 cm², produced by a Varian 21 EX linear accelerator, were used to test performances of the AAA and CCC using Monte Carlo (MC) simulation (EGSnrc-based code) as a benchmark. Horizontal dose profiles at different depths, phantom skin profiles (i.e., vertical dose profiles at a distance of 2 mm from the phantom lateral surface), gamma dose distributions, and dose-volume histograms (DVHs) of skin slab were determined. For dose profiles at different depths, the CCC agreed better with doses in the air-phantom region, while both the AAA and CCC agreed well with doses in the penumbra region, when compared to the MC. Gamma evaluations between the AAA/CCC and MC showed that deviations of 2D dose distribution occurred in both beam edges in the phantom and air-phantom interface. Moreover, the gamma dose deviation is less significant in the air-phantom interface than the penumbra. DVHs of skin slab showed that both the AAA and CCC underestimated the width of the dose drop-off region for both the 6 and 15 MV photon beams. When the gantry angle was 0°, it was found that both the AAA and CCC overestimated doses in the phantom skin profiles compared to the MC, with various photon beam energies and field sizes. The mean dose differences with doses normalized to the prescription point for the AAA and CCC were respectively: 7.6% ± 2.6% and 2.1% ± 1.3% for a 10 × 10 cm2 field, 6 MV; 16.3%± 2.1% and 6.7% ± 2.1% for a 20 × 20 cm2 field, 6 MV; 5.5% ± 1.2% and 1.7% ± 1.4% for a 10 × 10 cm2, 15 MV; 18.0% ± 1.3% and 8.3% ± 1.8% for a 20 × 20 cm², 15 MV. However, underestimations of doses in the phantom skin profile were found with small fields of 4 × 4 and 7 × 7 cm² for the 6 and 15 MV photon beams, respectively, when the gantry was turned 5° anticlockwise. As surface dose with tangential photon beam geometry is important in some radiation treatment sites such as breast, chest wall and sarcoma, it is found that neither of the treatment planning system algorithms can predict the dose well at depths shallower than 2 mm. The dosimetry data and beam and phantom geometry in this study provide a better knowledge of a dose calculation algorithm in tangential-like irradiation.

Tissue heterogeneity in IMRT dose calculation for lung cancer

The aim of this study was to evaluate the differences in accuracy of dose calculation between 3 commonly used algorithms, the Pencil Beam algorithm (PB), the Anisotropic Analytical Algorithm (AAA), and the Collapsed Cone Convolution Superposition (CCCS) for intensity-modulated radiation therapy (IMRT). The 2D dose distributions obtained with the 3 algorithms were compared on each CT slice pixel by pixel, using the MATLAB code (The MathWorks, Natick, MA) and the agreement was assessed with the γ function. The effect of the differences on dose-volume histograms (DVHs), tumor control, and normal tissue complication probability (TCP and NTCP) were also evaluated, and its significance was quantified by using a nonparametric test. In general PB generates regions of over-dosage both in the lung and in the tumor area. These differences are not always in DVH of the lung, although the Wilcoxon test indicated significant differences in 2 of 4 patients. Disagreement in the lung region was also found when the Γ analysis was performed. The effect on TCP is less important than for NTCP because of the slope of the curve at the level of the dose of interest. The effect of dose calculation inaccuracy is patient-dependent and strongly related to beam geometry and to the localization of the tumor. When multiple intensity-modulated beams are used, the effect of the presence of the heterogeneity on dose distribution may not always be easily predictable.

Boosting the tumor bed from deep-seated tumors in early-stage breast cancer: a planning study between electron, photon, and proton beams

PURPOSE

To assess the potential dosimetric advantages and drawbacks of photon beams (modulated or not), electron beams (EB), and protons as a boost for the tumor bed in deep-seated early-stage breast cancer.

MATERIAL AND METHODS

Planning CTs of 14 women with deep-seated tumors (i.e., > or =4 cm depth) were selected. The clinical target volume (CTV) was defined as the area of architectural distortion surrounded by surgical clips. The planning treatment volume (PTV) was the CTV plus 1cm margin. A dose of 16 Gy in 2 Gy fractions was prescribed. Organs at risk (OARs) were heart, lungs, breasts, and a 5-mm thick skin segment on the breast surface. Dose-volume metrics were defined to quantify the quality of concurrent treatment plans assessing target coverage and sparing of OAR. The following treatment techniques were assessed: photon beams with either static 3D-conformal, dynamic arc (DCA), static gantry intensity-modulated beams (IMRT), or RapidArc (RA); a single conformal EB; and intensity-modulated proton beams (IMPT). The goal for this planning effort was to cover 100% of the CTV with 95% of the prescribed dose and to minimize the volume inside the CTV receiving >107% of the dose.

RESULTS

All techniques but DCA and EB achieved the planning objective for the CTV with an inhomogeneity ranging from 2% to 11%. RA showed the best conformity, EB the worst. Contra-lateral breast and lung were spared by all techniques with mean doses <0.5 Gy (zero for protons). The ipsi-lateral lung received a mean dose <10% of that prescribed with photon beams and <2% with IMPT, increasing to 17% with EB. The heart, in left-sided breast tumors, received also the highest dose with EB. The skin was best protected with RA with a mean dose of 5.4 Gy and V(15Gy)=2.4%.

CONCLUSIONS

Boosting the tumor bed in early-stage breast cancer with optimized photon or proton beams may be preferred to EB especially for deep-seated targets. The marked OAR (i.e., ipsi-lateral breast, lung, heart, and skin surface) dose-sparing effect may allow for a potential long-term toxicity risk reduction and better cosmesis. DCA or RA may also be considered alternative treatment options for patients eligible for accelerated partial breast irradiation trials.

Validation of the Eclipse AAA algorithm at extended SSD

The accuracy of dose calculations at extended SSD is of significant importance in the dosimetric planning of total body irradiation (TBI). In a first step toward the implementation of electronic, multi-leaf collimator compensation for dose inhomogeneities and surface contour in TBI, we have evaluated the ability of the Eclipse AAA to accurately predict dose distributions in water at extended SSD. For this purpose, we use the Eclipse AAA algorithm, commissioned with machine-specific beam data for a 6 MV photon beam, at standard SSD (100 cm). The model was then used for dose distribution calculations at extended SSD (179.5 cm). Two sets of measurements were acquired for a 6 MV beam (from a Varian linear accelerator) in a water tank at extended SSD: i) open beam for 5 x 5, 10 x 10, 20 x 20 and 40 x 40 cm2 field sizes (defined at 179.5 cm SSD), and ii) identical field sizes but with a 1.3 cm thick acrylic spoiler placed 10 cm above the water surface. Dose profiles were acquired at 5 cm, 10 cm and 20 cm depths. Dose distributions for the two setups were calculated using the AAA algorithm in Eclipse. Confidence limits for comparisons between measured and calculated absolute depth dose curves and normalized dose profiles were determined as suggested by Venselaar et al. The confidence limits were within 2% and 2 mm for both setups. Extended SSD calculations were also performed using Eclipse AAA, commissioned with Varian Golden beam data at standard SSD. No significant difference between the custom commissioned and Golden Eclipse AAA was observed. In conclusion, Eclipse AAA commissioned at standard SSD can be used to accurately predict dose distributions in water at extended SSD for 6 MV open beams.