The effects of mega-voltage CT scan parameters on offline adaptive radiation therapy.

TomoTherapy involves image-guided radiation therapy (IGRT) using Mega-voltage CT (MVCT) for each treatment session. The acquired MVCT images can be utilized for the retrospective assessment of dose distribution. The TomoTherapy provides 18 distinct imaging conditions that can be selected based on a combination of algorithms, acquisition pitch, and slice interval. We investigated the accuracy of dose calculation and deformable image registration (DIR) depending on MVCT scan parameters and their effects on adaptive radiation therapy (ART). We acquired image values for density calibration tables (IVDTs) under 18 different MVCT conditions and compared them. The planning CT (pCT) was performed using a thoracic phantom, and an esophageal intensity-modulated radiation therapy (IMRT) plan was created. MVCT images of the thoracic phantom were acquired under each of the 18 conditions, and dose recalculation was performed. DIR was performed on the MVCT images acquired under each condition. The accuracy of DIR, depending on the MVCT scan parameters, was compared using the mean distance to agreement (MDA) and Dice similarity coefficient (DSC). The dose distribution calculated on the MVCT images was deformed using deformed vector fields (DVF). No significant differences were observed in the results of the 18 IVDTs. The esophageal IMRT plan also showed a small dose difference. Regarding verifying the DIR accuracy, the MDA increased, and the DSC decreased as the acquisition pitch and slice interval increased. The difference between the dose distributions after dose mapping was comparable to that before DIR. The MVCT scan parameters had little effect on ART.

Impact of transverse magnetic fields on dose response of a nanoDot OSLD in megavoltage photon beams.

PURPOSE: We investigated the impact of transverse magnetic fields on the dose response of a nanoDot optically stimulated luminescence dosimetry (OSLD) in megavoltage photon beams. METHODS: The nanoDot OSLD response was calculated via Monte Carlo (MC) simulations. The responses RQ and RQ,B without and with the transverse magnetic fields of 0.35-3 T were analyzed as a function of depth at a 10 cm × 10 cm field for 4-18 MV photons in a solid water phantom. All responses were determined based on comparisons with the response under the reference conditions (depth of 10 cm and a 10 cm × 10 cm field) for 6 MV without the magnetic field. In addition, the influence of air-gaps on the nanoDot response in the magnetic field was estimated according to Burlin's general cavity theory. RESULTS: The RQ as a function of depth for 4-18 MV ranged from 1.013 to 0.993, excepting the buildup region. The RQ,B increased from 2.8% to 1.5% at 1.5 T and decreased from 3.0% to 1.1% at 3 T in comparison with RQ as the photon energy increased. The depth dependence of RQ,B was less than 1%, excepting the buildup region. The top air-gap and the bottom air- gap were responsible for the response reduction and the response increase, respectively. CONCLUSIONS: The response RQ,B varied depending on the magnetic field intensity, and the variation of RQ,B reduced as the photon beam energy increased. The air-gaps affected the dose deposition in the magnetic fields.

Response of a nanoDot OSLD system in megavoltage photon beams.

PURPOSE: The aim of this study was to investigate the response of a nanoDot optically stimulated luminescence dosimeter (OSLD) system in megavoltage photon beams. METHODS: The nanoDot response was compared with the ionization chamber measurements for 4-18-MV photons in a plastic water phantom. The response was also calculated by the Monte Carlo method. In addition, the perturbation correction factor, PQ, in the nanoDot cavity was calculated according to the Burlin's cavity theory. The angular dependence of the nanoDot was evaluated using a spherical phantom. RESULTS: The calculated and measured nanoDot responses at a 10-cm depth and 10 × 10-cm2 field were in agreement within 1% for 4-18-MV. The response increased by 3% at a 20 × 20-cm2 field for the lower energy of 4 MV; however, it was constant within ±1% for 6-18 MV. The response was in a range from 1.0 to 0.99 for mean photon energy of more than 1.0 MeV but it increased with less than the 1.0 MeV. PQ for the nanoDot cavity was approximately constant at 0.96-0.97 for greater than and equal to 10 MV. The angular dependence decreased by 5% and 3% for 6 and 15 MV, respectively. CONCLUSIONS: The nanoDot was energy-independent in megavoltage photon beams.

Monte Carlo dose verification for a single-isocenter VMAT plan in multiple brain metastases.

The purpose of this study was to verify the accuracy of dose calculation algorithms of a treatment planning system for a single-isocenter volumetric modulated arc therapy (VMAT) plan in multiple brain metastases, by comparing the dose distributions of treatment planning system with those of Monte Carlo (MC) simulations. We used a multitarget phantom containing 9 acrylic balls with a diameter of 15.9 mm inside a Lucy phantom measuring 17 × 17 × 17 cm3. Seven VMAT plans were created using the multitarget phantom: 1 multitarget plan (MTP) and 6 single target plans (STP). Three of the STP plans had a large jaw field setting, almost equivalent to that of the MTP, while the other plans had a jaw field setting fitted to each planning target volume. The isocenter for all VMAT plans was set to the center of the phantom. The VMAT dose distributions were calculated using the analytical anisotropic algorithm (AAA) and were also recalculated through Acuros XB (AXB) and MC simulations under the same irradiation conditions. The AAA and AXB methods tended to overestimate dosage compared with the MC method in the MTP and in STPs with large jaw field settings. The dose distribution in single-isocenter VMAT plans for multiple brain metastases was influenced by jaw field settings. Finally, we concluded that MC-VMAT dose calculations are useful for 3D dose verification of single-isocenter VMAT plans for multiple brain metastases.