Dosimetric evaluation of a pencil-beam algorithm for electrons employing a two-dimensional heterogeneity correction

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The purpose of this study was to evaluate the accuracy and calculation speed of electron dose distributions calculated by the Eclipse electron Monte Carlo (eMC) algorithm for use with bolus electron conformal therapy (ECT). The recent commercial availability of bolus ECT technology requires further validation of the eMC dose calculation algorithm. eMC‐calculated electron dose distributions for bolus ECT have been compared to previously measured TLD‐dose points throughout patient‐based cylindrical phantoms (retromolar trigone and nose), whose axial cross sections were based on the mid‐PTV (planning treatment volume) CT anatomy. The phantoms consisted of SR4 muscle substitute, SR4 bone substitute, and air. The treatment plans were imported into the Eclipse treatment planning system, and electron dose distributions calculated using 1% and <0.2% statistical uncertainties. The accuracy of the dose calculations using moderate smoothing and no smoothing were evaluated. Dose differences (eMC‐calculated less measured dose) were evaluated in terms of absolute dose difference, where 100% equals the given dose, as well as distance to agreement (DTA). Dose calculations were also evaluated for calculation speed. Results from the eMC for the retromolar trigone phantom using 1% statistical uncertainty without smoothing showed calculated dose at 89% false(41/46false) of the measured TLD‐dose points was within 3% dose difference or 3 mm DTA of the measured value. The average dose difference was −0.21%, and the net standard deviation was 2.32%. Differences as large as 3.7% occurred immediately distal to the mandible bone. Results for the nose phantom, using 1% statistical uncertainty without smoothing, showed calculated dose at 93% false(53/57false) of the measured TLD‐dose points within 3% dose difference or 3 mm DTA. The average dose difference was 1.08%, and the net standard deviation was 3.17%. Differences as large as 10% occurred lateral to the nasal air cavities. Including smoothing had insignificant effects on the accuracy of the retromolar trigone phantom calculations, but reduced the accuracy of the nose phantom calculations in the high‐gradient dose areas. Dose calculation times with 1% statistical uncertainty for the retromolar trigone and nose treatment plans were 30 s and 24 s, respectively, using 16 processors (Intel Xeon E5‐2690, 2.9 GHz) on a framework agent server (FAS). In comparison, the eMC was significantly more accurate than the pencil beam algorithm (PBA). The eMC has comparable accuracy to the pencil beam redefinition algorithm (PBRA) used for bolus ECT planning and has acceptably low dose calculation times. The eMC accuracy decreased when smoothing was used in high‐gradient dose regions. The eMC accuracy was consistent with that previously reported for accuracy of the eMC electron dose algorithm and shows that the algorithm is suitable for clinical implementation of bolus ECT.PACS number(s): 87.55.kd

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Evaluation of the Eclipse eMC algorithm for bolus electron conformal therapy using a standard verification dataset

Carver

Sprunger

Hogstrom

et al. 2016

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“…The high dose‐gradient regions were defined as the penumbra and distal falloff areas with doses between 10% and 90% of the prescription dose. DTA was defined as the minimum distance from a dose point in the MVCT image to a point in the kVCT image with the same dose 10 , 12 . Each point in the MCVT dose matrix was tested against a square region in the kVCT image of size

1.7 \times 1.7 cm

centered on the location of the test point.…”

Section: Methodsmentioning

confidence: 99%

Evaluation of MVCT images with skin collimation for electron beam treatment planning

Beardmore

Rosen

Cheek

et al. 2008

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This study assessed the potential of using megavoltage CT (MVCT) images taken with high density skin collimation in place for electron beam treatment planning. MVCT images were taken using the TomoTherapy Hi‐Art system (TomoTherapy Inc., Madison, WI), and the CT numbers were converted to density by calibrating the Hi‐Art system using an electron density phantom. Doses were computed using MVCT images and kVCT images and compared by calculating dose differences in the uniform dose region false(>90%, excluding buildup region) and calculating distance‐to‐agreement (DTA) in high dose‐gradient regions (penumbra and distal falloff, 90%–10%). For 9 and 16 MeV electron beams of 10×10 cm calculated on a homogeneous CIRS Plastic Water (Computerized Imaging Research Systems Inc., Norfolk, VA) phantom without skin collimation, the maximum dose differences were 2.3% and the maximum DTAs were 2.0 mm for both beams. The same phantom was then MVCT scanned nine times with square skin collimators of Cerrobend on its surface ‐ field sizes of 3×3, 6×6, and 10×10 cm and thicknesses of 6, 8, and 10 mm. Using the Philips Pinnacle 3 treatment planning system (Philips Medical Systems, N.A., Bothwell, WA), a treatment plan was created for combinations of electron energies of 6, 9, 12, and 16 MeV and each field size. The same treatment plans were calculated using kVCT images of the phantom with regions‐of‐interest (ROI) manually drawn to duplicate the sizes, shapes, and density of the skin collimators. With few exceptions, the maximum dose differences exceeded ±5% and the DTAs exceeded 2 mm. We determined that the dose differences were due to small distortions in the MVCT images created by the high density material and manifested as errors in the phantom CT numbers and in the shape of the skin collimator edges. These results suggest that MVCT images without skin collimation have potential for use in patient electron beam treatment planning. However, the small distortion in images with skin collimation makes them unsuitable for clinical use.PACS: 87.53.Tf, 87.59.Fm, 87.53.Fs

“…( 7 , 8 , 11 ) The initial energy spectrum is unique to the PBRA and was calculated using the methods of Boyd et al (2) …”

Section: Methodsmentioning

confidence: 99%

“…The PBA incorporated a formalism whereby rectangular‐field percent depth dose was required as input into the algorithm. ( 7 , 8 ) Rectangular‐field percent depth dose was determined using the square root method from depth doses for two square fields, which were interpolated from a family of measured percent depth dose curves.…”

Section: Introductionmentioning

confidence: 99%

Calculating percent depth dose with the electron pencil‐beam redefinition algorithm

Price

Hogstrom

Antolak

et al. 2007

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In the present work, we investigated the accuracy of the electron pencil‐beam redefinition algorithm (PBRA) in calculating central‐axis percent depth dose in water for rectangular fields. The PBRA energy correction factor C(E) was determined so that PBRA‐calculated percent depth dose best matched the percent depth dose measured in water. The hypothesis tested was that a method can be implemented into the PBRA that will enable the algorithm to calculate central‐axis percent depth dose in water at a 100‐cm source‐to‐surface distance (SSD) with an accuracy of 2% or 1‐mm distance to agreement for rectangular field sizes ≥2×2 cm. Preliminary investigations showed that C(E), determined using a single percent depth dose for a large field (that is, having side‐scatter equilibrium), was insufficient for the PBRA to accurately calculate percent depth dose for all square fields ≥2×2 cm. Therefore, two alternative methods for determining C(E) were investigated. In Method 1, C(E), modeled as a polynomial in energy, was determined by fitting the PBRA calculations to individual rectangular‐field percent depth doses. In Method 2, C(E) for square fields, described by a polynomial in both energy and side of square W [that is, C=C(E,W)], was determined by fitting the PBRA calculations to measured percent depth dose for a small number of square fields. Using the function C(E, W), C(E) for other square fields was determined, and C(E) for rectangular field sizes was determined using the geometric mean of C(E) for the two measured square fields of the dimension of the rectangle (square root method). Using both methods, PBRA calculations were evaluated by comparison with measured square‐field and derived rectangular‐field percent depth doses at 100‐cm SSD for the Siemens Primus radiotherapy accelerator equipped with a 25×25‐cm applicator at 10 MeV and 15 MeV. To improve the fit of C(E) and C(E, W) to the electron component of percent depth dose, it was necessary to modify the PBRA's photon depth dose model to include dose buildup. Results showed that, using both methods, the PBRA was able to predict percent depth dose within criteria for all square and rectangular fields. Results showed that second‐ or third‐order polynomials in energy (Methods 1 and 2) and in field size (Method 2) were typically required. Although the time for dose calculation using Method 1 is approximately twice that using Method 2, we recommend that Method 1 be used for clinical implementation of the PBRA because it is more accurate (most measured depth doses predicted within approximately 1%) and simpler to implement.PACS number: 87.53.Fs