Investigation of the effects of a carbon-fiber tabletop on the surface dose and attenuation dose for megavoltage photon beams

Reports of Practical Oncology & Radiotherapy, 2017

The purpose of this study is to determine the central-axis dose in the buildup region and the surface dose delivered by a 6 MV flattened photon beam (6 MV-FB) and a higher energy unflattened (7 MV-FFF) therapeutic photon beam for different-sized square fields with open fields and modifying filters. Materials and methods: The beams are produced by a Siemens Artiste linear accelerator with a NACP-02 ionization chamber and the dose is measured by using GafChromic film and two different, commonly used, dosimeters: a p-type photon semiconductor dosimeter (PFD) and a cylindrical ionization chamber (CC13). Results: The results indicate that the surface dose increases linearly with FS for both open and wedged fields for the 6 MV-FB and 7 MV-FFF beams. The surface dose delivered by the 7 MV-UFB beam is consistent with that delivered by the 6 MV-FB beam for field sizes up to 10 cm × 10 cm, after which the surface dose decreases. The buildup dose for the 7 MV-UFB beam is slightly less than that for the 6 MV-FB beam for field sizes ranging from 5 cm × 5 cm to 15 cm × 15 cm. For both the 6 MV-FB and 7 MV-FFF beams, the measured surface dose clearly increases with increasing field size, regardless of the detector used in the measurement. The surface dose measured with the PFD dosimeter and the NACP-02 and CC13 chambers differ significantly from the results obtained when using GafChromic film. The 7 MV-FFF beam results in a slightly smaller surface dose in the buildup region compared with the 6 MV-FB beam. Conclusions: The surface dose delivered by the higher energy 7 MV-FFF beam is less than that delivered by the energy-unmatched FFF beam in previously published works.

Strahlentherapie und Onkologie, 2000

Since the skin dose becomes the limiting factor while deciding the tumorcidal dose, the detailed analysis of dose distribution in the build-up region is necessary for high-energy photon beams. In this study the beam characteristics affecting the build-up and skin dose for 6-and 18-MV photons are analyzed. Materials and Methods: Measurements were made with 6-and 18-MV photons using a PTW parallel-plate ionization chamber (B 23344-036) and a RDM-1F electrometer. Build-up ionization measurements were made with the chamber fitted into a 25 ϫ 25 ϫ 25 cm polystyrene phantom with a fixed SSD of 100 cm. The entrance and build-up dose measurements were made with a polycarbonate and a mesh type metallic shielding tray and a 45°wedge. Exit dose measurements were carried out for the graphite patient supporting assembly table top, 1.0 cm thick piece of wood and the 1.0 cm thick patient supporting perspex base frame for head and neck treatments. Results: It was observed that the d max decreased slightly with field size as with other accelerators. For both photon energies the surface dose was observed to increase with increase in field size. It was also noticed that the dose in the build-up region increases slightly when the polycarbonate secondary blocking tray is introduced with the increase in surface dose. The data show that the tray perturbation factor (TPF) at surface decreases steadily with tray-surface distance for both photon beams for all field sizes. It was noted that the TPF was more when the polycarbonate tray was introduced at shorter tray-surface distances for both energies. At tray-surface distances above 60 cm the TPF almost remained close to unity for 6-MV photons for all field sizes, whereas the continuous decrease in TPF could be noted for 18-MV photon beams even after the TPF reached unity. The increase in surface dose with field size for both photon energies is due to the electron scattering from the intervening materials. The use of wedge filters absorbs low-energy scattered electrons significantly and hence, the relative surface dose (RSD) is always less than unity. The increase in dose enhancement percentage with graphite compared to perspex supporting assembly indicates that the electron backscatter is proportional to the atomic number of the medium.

Introduction: The purpose of this study was to investigate surface doses of 6 and 10 MV energies of linear accelerator for different clinical setup parameters including (field size, gantry angle, SSD (source skin distance), PW (physical wedge), acrylic block tray and bolus), Comparison between measured surface dose by P.P (parallel plate) and calculated surface dose by TPS (Treatment Planning system). Comparison between surface doses measured by TLD (Thermo luminescence dosimeter) and P.P ion chamber, Comparison between the surface dose of 3DCRT and IMRT techniques.

Physica Medica

We have conducted for the first time a Malaysian postal dosimetry audit of external beam under nonreference conditions by evaluating the output performance while screening for systematic errors within the dosimetry chain. The potential use from the choice of detector were investigated along with the search for other sources of discrepancies. Methods: Ten radiotherapy centres were audited, encompassing 16 megavoltage photon beam arrangements, adopting the IAEA postal dosimetry protocol for non-reference conditions, with a holder modified to accommodate three TLD types: Ge-doped cylindrical silica fibres (CF), Ge-doped flat silica fibres (FF), and TLD-100 powder. Results: Eight of the centres operated within ± 5% of stated dose, one other exceeding tolerance for all measured points, and one did not return any dosimeters for analysis after failing the initial irradiations. Post remedial measures, the mean relative response for CF, FF, and TLD-100 was 1.00, 0.99, and 0.98 respectively, with associated coefficients of variation 6.87%, 6.45%, and 5.06%. Conclusion: High quality radiotherapy clinical practice postal dosimetry audits that are based on sensitive TLDs are seen to be particularly effective in identifying and resolving dose delivery discrepancies.

Advanced Monte Carlo for Radiation Physics, Particle Transport Simulation and Applications, 2001

Progress in Medical Physics, 2016

Generally, it is recommended that the dosimetric effect of carbon fiber couch should be considered especially for an intensity-modulated therapy with a large portion of monitor units from posterior angles. Even a flattening filter free (FFF) beam has been used for stereotactic body radiation therapy (SBRT), the effect of carbon fiber couch for FFF beam is not well known. This work is an effort to evaluate the dosimetric effect of carbon fiber couch for flattened and FFF beam of Elekta linac empirically. The absorbed doses were measured with Farmer type chamber and water-equivalent phantoms with and without couch. And differences of the absorbed doses between with and without couch defined as "couch effect". By comparing calculated dose in treatment planning system (TPS) with measured dose, the optimal density of couch was evaluated. Finally, differences on patient's skin dose and target dose by couch were evaluated in TPS. As a result, the couch effect for 6 and 10 MV flattened beam were −2.71% and −2.32%, respectively. These values were agreed with provided data by vendor within 0.5%. The couch effect for 6 and 10 MV FFF beam were −3.75% and −2.80%, respectively. The patient's skin dose was increased as 18.6% and target dose was decreased as 0.87%, respectively. It was realized that the couch effect of FFF beam was more severe than that of flattened beam. Patient's skin dose and target dose were changed by the couch effect.

2018

In this study we have investigated the scattered photons and electrons contamination effects on dose profiles due beam modifiers and jaw system motion. Firstly, we have calculated the increasing relative dose on beam profile as a function of field size and depth along central beam axis and we have normalized the increasing relative dose on beam profile to the increasing relative dose of standard filed size of 10× 10 cm. Secondly, we have calculated the width of dose profiles as function of field size and depth; then we have investigated the beam symmetry on central beam axis and the jaw motion effects on delivered dose, we have calculated two parameters: the normalized width and the width difference between left and right width of dose profiles as a function of field size and depth. We concluded that scattered photons and electrons contamination effects on dose profiles must be taken in consideration when the physicians treat the tumor because the dose profile peaks have an increasi...

Physica Medica, 2009

The aim of this study was to determine the surface doses using GafChromic EBT films and compare them with plane-parallel ionization chamber measurements for 6 and 18 MV high energy photon beams. The measurements were made in a water equivalent solid phantom in the build-up region of the 6 and 18 MV photon beams at 100 cm SSD for various field sizes. Markus type plane-parallel ion chamber with fixed-separation between collecting electrodes was used to measure the percent depth doses. GafChromic EBT film measurements were performed both on the phantom surface and maximum dose depth at the same geometry with ion chamber measurements. The surface doses found using GafChromic EBT film were 15%, 20%, 29%and 39% AE 2% (1SD) for 6 MV photons, 6%, 11%, 23% and 32% AE 2% (1SD) for 18 MV photons at 5, 10, 20 and 30 cm 2 field sizes, respectively. GafChromic EBT film provides precise measurements for surface dose in the high energy photons. Agreement between film and plane-parallel chamber measurements was found to be within AE3% for 18 MV photon beams. There was 5% overestimate on the surface doses when compared with the plane-parallel chamber measurements for all field sizes in the 6 MV photon beams. ª 2008 Published

International Journal of Radiation Research

Background: Small photon beams are increasingly used in modern radiotherapy modalities. In small photon fields, the dosimetric field size will deviate from the nominal field size. An effective field size (FSeff) for use in small field dosimetry has been defined to overcome this issue. The present study aims to investigate the suitability of two ionization chambers and two semiconductor diodes in the measurement of 6MV photon beam profiles and to analyze the variations of FSeff in smaller fields. Materials and Methods: Measurements were made at 6 MV photon beams of a Siemens Artiste linear accelerator and transverse profiles were acquired for nominal square field sizes of side 1×1 to 10×10 cm 2 via the irradiation of detectors and radiochromic film. Full width at half maximum (FWHM) at the 50% isodose level was used to calculate FSeff. Results: The uncertainty of the FWHM values derived from the in-plane and cross-plane profiles (ΔFWHM%) were below 6% for all the detectors were below 6% except for Semiflex in the 1×1 field size. In small field sizes (less than 3 × 3 cm 2), larger differences occurred between the dosimetric and nominal field sizes in all detectors. No significant differences between nominal and effective field sizes were observed in a field rage of 4×4-10×10 cm 2. Conclusion: In the acquisition of small field profiles, selection of an appropriate detector is influential in accurate measurements. The findings of present study support the argument that both the size and composition of detectors affect the small field profile measurements.

Journal of Radiography and Radiation Sciences

Background: Research findings from percentage depth dose (PDD) are crucial in evaluating patient doses received in radiation therapy. Objective: To compare calculated percentage depth doses at an extended source-to-surface distance (SSD) for 6 MV and 15 MV photon beams of a linear accelerator. Methodology: Measured PDD values of the 100 cm source to surface distance (SSD) and calculated values at extended SSDs for 6 MV and 15 MV photon beams of an Elekta NHA SLi 1998 linear accelerator were analyzed. The PDD data was collected by placing ionization chamber inside water phantom for depths ranging from z = 0 - 30 cm in a water phantom and using a square field sizes of 10 x 10 cm2. Photon energies of 6 MV and 15 MV were used for the measurement, with both gantry and collimator angles fixed at zero degree. PDD was calculated at extended SSD of 110 cm, 120 cm, 130 cm, and 140 cm from the measured PDD values of 100 cm SSD for both 6 MV and 15 MV photon energies using Mayneord factor. Resu...