Dose Agreement Analysis of Treatment Planning System-Calculated Doses and Markus Chamber-Measured Doses in the Near-Surface Region for Breast Cancer Patients' Conformal Treatment Plans.

Background: Surface/skin dose measurement is one of the most challenging tasks for clinical dosimetry in radiotherapy and comparison with almost all the commercially available treatment planning systems (TPSs) brings a significant variation with the measured dose. Aims and Objectives: In the current study, doses calculated from the TPS in the near-surface region for conformal plans (both three-dimensional conformal radiotherapy [3DCRT] and intensity-modulated radiotherapy [IMRT]) of 35 breast cancer patients were evaluated and compared with the doses measured with Markus chamber. Materials and Methods: The computed tomography (CT) images of a solid water slab phantom with a Markus chamber (at different depths ranging from 1 mm to 5 mm from the surface) were taken and imported into the TPS. All the conformal treatment plans made in TPS were executed on a linear accelerator and dose agreements between TPS calculated and chamber measured doses were analysed. Results: Results showed that this TPS underestimated the calculated doses in the superficial region by up to 26% and 21%, respectively, with respect to mean and maximum dose values obtained within the effective volume of the chamber used. Conclusion: The uncertainty of doses in the superficial region should be kept in mind when evaluating treatment plans for superficial tumours in TPS.

Dose calculation validation of a convolution algorithm in a solid water phantom.

PURPOSE: The dose calculated using a convolution algorithm should be validated in a simple homogeneous water-equivalent phantom before clinical use. The dose calculation accuracy within a solid water phantom was investigated. METHODS: The specific Gamma knife design requires a dose rate calibration within a spherical solid water phantom. The TMR10 algorithm, which approximates the phantom material as liquid water, correctly computes the absolute dose in water. The convolution algorithm, which considers electron density miscalculates the dose in water as the phantom Hounsfield units were converted into higher electron density when the original CT calibration curve was used. To address this issue, the electron density of liquid water was affected by modifying the CT calibration curve. The absolute dose calculated using the convolution algorithm was compared with that computed by the TMR10. The measured depth dose profiles were also compared to those computed by the convolution and TMR10 algorithms. A patient treatment was recalculated in the solid-water phantom and the delivery quality assurance was checked. RESULTS: The convolution algorithm and the TMR10 calculate an absolute dose within 1% when using the modified CT calibration curve. The dose depth profile calculated using the convolution algorithms was superimposed on the TMR10 and measured dose profiles when the modified CT calibration curve was applied. The Gamma index was better than 93%. CONCLUSIONS: Dose calculation algorithms, which consider electron density, require a CT calibration curve adapted to the phantom material to correctly compute the dose in water.

Measurements of the peripheral dose from megavoltage cone-beam CT imaging for head-and-neck region image-guided radiation therapy / 中华放射肿瘤学杂志

Objective To evaluate the peripheral dose (PD) from megavoltage cone-beam CT (MVCBCT) imaging for head-and-neck region image-guided radiation therapy,to determine the correlation of PD with monitor unit (MU),and to investigate the impact of imaging field size on the PD.Methods Measurements of PD from MVCBCT were made with a 0.65 cm3 ionization chamber placed in a specially designed phantom at various depths and distances from the field edges.The PD at reference point inside the phantom was measured with the same ionization chamber to investigate the linearity between MU used for MVCBCT and the PD.The homogeneity of PD in the axial plane of the phantom were measured.Results PD from MVCBCT increased with increasing number of MU used for imaging and with increasing the field size.The measured PD in the phantom decreased exponentially as distance from the field edges increased.PD also decreased as the depth from the phantom surface increased.There was a strong linear relationship between PD and MUs used for MVCBCT.The PD was heterogeneous,with higher dose at the anterior than the posterior.Conclusions The PD from MVCBCT depend much on the MVCBCT delivery MU and the scan field size.In clinic,using the smallest number of MU allowable and reducing MVCBCT scanning field size without compromising acquired image quality is an effective method of reducing the PD.

External Auditing on Absorbed Dose Using a Solid Water Phantom for Domestic Radiotherapy Facilities / 대한방사선종양학회지

PURPOSE: We report the results of an external audit on the absorbed dose of radiotherapy beams independently performed by third parties. For this effort, we developed a method to measure the absorbed dose to water in an easy and convenient setup of solid water phantom. MATERIALS AND METHODS: In 2008, 12 radiotherapy centers voluntarily participated in the external auditing program and 47 beams of X-ray and electron were independently calibrated by the third party's American Association of Physicists in Medicine (AAPM) task group (TG)-51 protocol. Even though the AAPM TG-51 protocol recommended the use of water, water as a phantom has a few disadvantages, especially in a busy clinic. Instead, we used solid water phantom due to its reproducibility and convenience in terms of setup and transport. Dose conversion factors between solid water and water were determined for photon and electron beams of various energies by using a scaling method and experimental measurements. RESULTS: Most of the beams (74%) were within +/-2% of the deviation from the third party's protocol. However, two of 20 X-ray beams and three of 27 electron beams were out of the tolerance (+/-3%), including two beams with a >10% deviation. X-ray beams of higher than 6 MV had no conversion factors, while a 6 MV absorbed dose to a solid water phantom was 0.4% less than the dose to water. The electron dose conversion factors between the solid water phantom and water were determined: The higher the electron energy, the less is the conversion factor. The total uncertainty of the TG-51 protocol measurement using a solid water phantom was determined to be +/-1.5%. CONCLUSION: The developed method was successfully applied for the external auditing program, which could be evolved into a credential program of multi-institutional clinical trials. This dosimetry saved time for measuring doses as well as decreased the uncertainty of measurement possibly resulting from the reference setup in water.

Two-dimensional ion chamber array in dose verification for intensity modulated planning of helical tomotherapy / 中华放射肿瘤学杂志

Objective To investigate the feasibility of dose verification of intensity modulated (IM) planning of helical tomotherapy (HT) using two-dimensional ion chamber array (2DICA),and develop an efficient way to validate the dose delivered under the parameters mirroring those during the treatment. Meth-ods A 2DICA,I'mRT MatriXX and MULTICube equivalent solid water phantom from IBA company were used to verify the dose distribution of 10 IM planning. The combined phantom was set up to measure the dose distributions on coronal and sagittal surface. The precise setup of phantom was guided by HTMVCT images. After the irradiation, the measured dose distributions on the coronal and sngittal plane were compared with those calculated by the IM planning system for verification. The results were evaluated and the feasibility of the different measuring methods was studied. Results The dose distribution measured by the MatriXX 2DICA was well consistent with that calculated by the treatment planning system. The errors between the measured dose and predicted dose in the selected points were within ±3%. In the comparison of the pixel-segmented ionization chamber versus treatment planning system using the 3 mm/3% γ criteria, the passing ratio of pixels with γ parameter ≤1 was 97.76% and 96.83%, respectively. Conclusions MatriXX is a-ble to measure the absolute and relative dose distributions simultaneously,which can be used for dose verifi-cation of IM planning.