Dosimetric accuracy of the Convolution algorithm for Leksell Gamma Plan radiosurgery treatment planning: Evaluation in the presence of clinically relevant inhomogeneities.

PURPOSE: The Leksell Gamma Plan Convolution algorithm (LGP-Convolution) has not been widely adopted. This mainly stems from the higher calculated beam-on times relative to the standard ray tracing-based LGP-TMR10 dose calculation algorithm. This study aims to evaluate the accuracy of the LGP-Convolution in scenarios where the treated lesions are in the vicinity of or encompassed by bone and/or air inhomogeneities. METHODS: The solid water dosimetry phantom provided by the vendor was modified to include bone and air inhomogeneities. Two treatment planning scenarios were investigated involving a single shot and multiple shots, respectively. Treatment planning and dose prescription were performed using the LGP-Convolution algorithm. Triple channel film dosimetry was performed using GafChromic EBT3 films calibrated in terms of absorbed dose to water in a 60 Co beam. Monte Carlo (MC) simulation dosimetry was also performed in the inhomogeneous experimental geometry using the EGSnrc MC platform and a previously validated sector-based phase-space source model. MC simulations were also employed to determine correction factors required for converting EBT3 measurements at points within the bone and air inhomogeneities from dose-to-water values to the corresponding dose to medium values. RESULTS AND CONCLUSIONS: EBT3 dose to medium correction factors ranged with field size (4, 8, or 16 mm) within 0.941-0.946 for bone and 0.745-0.749 for air inhomogeneities. An excellent agreement was found between the LGP-Convolution calculations with corresponding EBT3 and MC dose to medium results at all measurement points, except those located inside the air inhomogeneity. The latter is of no clinical importance and excluding them yielded gamma index passing rates of nearly 100% for 3% local dose difference and 1 mm distance-to-agreement criteria. The excellent agreement observed between LGP-Convolution calculations and film as well as MC results of dose to medium indicates that the latter is the quantity reported by the LGP-Convolution.

Robustness of single-isocenter multiple-metastasis stereotactic radiosurgery end-to-end testing across institutions.

The accuracy of stereotactic radiosurgery (SRS) to multiple metastases with a single-isocenter using high definition dynamic radiosurgery (HDRS) was evaluated across institutions. An SRS plan was delivered at six HDRS-capable institutions to an anthropomorphic phantom consisting of point, film, and 3D-gel dosimeters. Direct dose comparison and gamma analysis were used to evaluate the accuracy. Point measurements averaged across institutions were within 1.2±0.5%. The average gamma passing rate in the film was 96.6±2.2% (3%/2 mm). For targets within 4 cm of the isocenter, the 3D dosimetric gel gamma passing rate averaged across institutions was >90% (3%/2 mm). The targeting accuracy of high definition dynamic radiosurgery assessed by geometrical offset of the center of dose distributions across multiple institutions in this study was within 1 mm for targets within 4 cm of isocenter. Across variations in clinical practice, comparable dosimetry and localization is possible with this treatment planning and delivery technique.

Validation of PTV margin for Gamma Knife Icon frameless treatment using a PseudoPatient® Prime anthropomorphic phantom.

The Gamma Knife Icon allows the treatment of brain tumors mask-based single-fraction or fractionated treatment schemes. In clinic, uniform axial expansion of 1 mm around the gross tumor volume (GTV) and a 1.5 mm expansion in the superior and inferior directions are used to generate the planning target volume (PTV). The purpose of the study was to validate this margin scheme with two clinical scenarios: (a) the patient's head remaining right below the high-definition motion management (HDMM) threshold, and (b) frequent treatment interruptions followed by plan adaptation induced by large pitch head motion. A remote-controlled head assembly was used to control the motion of a PseudoPatient® Prime head phantom; for dosimetric evaluations, an ionization chamber, EBT3 films, and polymer gels were used. These measurements were compared with those from the Gamma Knife plan. For the absolute dose measurements using an ionization chamber, the percentage differences for both targets were less than 3.0% for all scenarios, which was within the expected tolerance. For the film measurements, the two-dimensional (2D) gamma index with a 2%/2 mm criterion showed the passing rates of ≥87% in all scenarios except the scenario 1. The results of Gel measurements showed that GTV (D100 ) was covered by the prescription dose and PTV (D95 ) was well above the planned dose by up to 5.6% and the largest geometric PTV offset was 0.8 mm for all scenarios. In conclusion, the current margin scheme with HDMM setting is adequate for a typical patient's intrafractional motion.

On the experimental validation of model-based dose calculation algorithms for 192Ir HDR brachytherapy treatment planning.

There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in 192Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1 × 1 × 1 mm3 microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an 192Ir high dose rate (HDR) source. Dose to medium in medium, [Formula: see text], was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k = 1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4-6.4, 2.5-6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between 192Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min-1). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for 192Ir and the reduction of experimental uncertainties.

On source models for (192)Ir HDR brachytherapy dosimetry using model based algorithms.

A source model is a prerequisite of all model based dose calculation algorithms. Besides direct simulation, the use of pre-calculated phase space files (phsp source models) and parameterized phsp source models has been proposed for Monte Carlo (MC) to promote efficiency and ease of implementation in obtaining photon energy, position and direction. In this work, a phsp file for a generic (192)Ir source design (Ballester et al 2015) is obtained from MC simulation. This is used to configure a parameterized phsp source model comprising appropriate probability density functions (PDFs) and a sampling procedure. According to phsp data analysis 15.6% of the generated photons are absorbed within the source, and 90.4% of the emergent photons are primary. The PDFs for sampling photon energy and direction relative to the source long axis, depend on the position of photon emergence. Photons emerge mainly from the cylindrical source surface with a constant probability over ±0.1 cm from the center of the 0.35 cm long source core, and only 1.7% and 0.2% emerge from the source tip and drive wire, respectively. Based on these findings, an analytical parameterized source model is prepared for the calculation of the PDFs from data of source geometry and materials, without the need for a phsp file. The PDFs from the analytical parameterized source model are in close agreement with those employed in the parameterized phsp source model. This agreement prompted the proposal of a purely analytical source model based on isotropic emission of photons generated homogeneously within the source core with energy sampled from the (192)Ir spectrum, and the assignment of a weight according to attenuation within the source. Comparison of single source dosimetry data obtained from detailed MC simulation and the proposed analytical source model show agreement better than 2% except for points lying close to the source longitudinal axis.