Evaluation of positional accuracy of the Varian's exact-arm and retractable-arm support electronic portal imaging device using intensity-modulated radiotherapy graticule phantom.

PURPOSE: The aim of the present study was to compare the positional accuracy of varian's exact-arm (E-arm) and retractable-arm (R-arm) supporting electronic portal imaging device (EPID) systems (amorphous silicon flat-panel detector) using the intensity-modulated radiotherapy (IMRT) graticule phantom. MATERIALS AND METHODS: The known shifts of 0.5, 1.0, and 1.5 cm were introduced to the given phantom in longitudinal, lateral, and vertical directions, respectively, with respect to treatment couch of medical linear accelerator. The experiment was repeated for different gantry angle and varying source to imager distances (SIDs). The images were acquired for each shift at varying SIDs and beam orientations for both EPID supporting systems. The corresponding shifts obtained from treatment planning system (TPS) were recorded and compared. RESULTS: The known (expected) and observed (recorded from TPS) shifts obtained for different beam angles (namely, 0°, 90°, 180°, and 270° for anterior, left lateral, posterior, and right-lateral portal images, respectively) in the longitudinal, lateral, and vertical direction at varying SID were compared. The maximum shift in the observed value from the expected one was 3 and 2 mm, respectively, out of the all beam configuration for R-arm and E-arm. These shifts were randomly observed for all imager position and beam orientation. CONCLUSION: The IMRT graticule phantom is an effective tool to check the mechanical characteristic and consistency of different EPID supporting arms. The effect of EPID sag due to gravity (gantry and treatment couch) was not significant for detection of shift in patient's position. The E-arm support EPID has better mechanical stability and accuracy in detection of patient's position than that of R-arm.

Evaluation of dose calculation accuracy of various algorithms in lung equivalent inhomogeneity: Comparison of calculated data with Gafchromic film measured results.

AIM: To evaluate dose calculation accuracy of various algorithms in lung equivalent inhomogeneity comprising tumor within it and comparison with Gafchromic film data. MATERIALS AND METHODS: Gafchromic film measured central axis absorbed dose in lung insert (-700 Hounsfield unit [HU]), in racemosa wood cylindrical inhomogeneity (-725 HU) and at three surfaces of tumor (-20 HU) created in cylindrical inhomogeneity, put in the cavity of computerized imaging reference systems (CIRS) thorax phantom were compared with convolution (CON), superposition (SP), fast SP (FSP), and X-ray voxel Monte Carlo (XVMC) algorithms calculated dose using 6 MV beams of field size 2 cm × 2 cm, 3 cm × 3 cm, 4 cm × 4 cm, 5 cm × 5 cm, and 8 cm × 8 cm. RESULTS: XVMC was in good agreement with film measured results for all selected field sizes except 3 cm × 3 cm. SP under estimated by 5.7% at the center of the lung insert while deviation up to 6% was found at the cent of wood inhomogeneity in 2 cm × 2 cm. Except CON, increase in dose from proximal to the central surface of the tumor and then dose falloff from central to the distal surface for field size 2 cm × 2 cm to 4 cm × 4 cm was recorded. The change in film measured percentage depth dose from 2 cm × 2 cm to 3 cm × 3 cm field sizes was found -8% however for consecutive field size(s) larger than 3 cm × 3 cm this difference was less. CON and FSP produced overestimated results. CONCLUSION: Out of four algorithms, XVMC found consistent with measured data. The electronic disequilibrium within and at the interface of inhomogeneity make the accurate dose predictions difficult. These limitations results in deviations from the expected results of the treatments.

Evaluation of dose perturbation at the interface of two different density medium using GAFCHROMIC film EBT2 and Monte Carlo code EGSnrc for Co-60 beam.

INTRODUCTION: Accurate dosimetry at the interface of two different density medium (e.g., air cavity in the head and neck cancers and lungs in thoracic region) is a major cause of concern in external beam radiation therapy. It has been observed that there is dose variation in and around air cavities, which occur as a result of the loss of both longitudinal and lateral electronic equilibrium. Heterogeneous structures with spatial differences in functionality and sensitivity for radiation pose challenge to radiation dosimetry. This study is an attempt to evaluate the dose perturbations produced at the interface of two medium for C0-60 gamma radiation. MATERIALS AND METHODS: Low density polyethene foam has been used to mimic air cavity. GAFCHROMIC EBT2 dosimetry film was used for the measurement of dose at different locations. Simulation studies were performed using DOSRZnrc user code that comes with EGSnrc V4 2.4.0. Cylindrical geometry is used for all the simulations. RESULTS AND DISCUSSION: We observed significant variation in dose for smaller fields. There is a dose build down in the backward region and a dose build up in the forward direction. In the region of electronic disequilibrium, dose reduction near interface (proximal end) will have negative impact if target region is embedded there, on the contrary, it would be beneficial if there is normal tissue/critical organ adjacent to it.