Cable-induced effects on plane-parallel ionization chamber measurements in large clinical electron beams

Investigation of triaxial cables and microdetectors in small field dosimetry

Background. Small field dosimetry presents unique challenges with source occlusion, lateral charged particle equilibrium and detector size. As detector volume decreases, signal strength declines while noise increases, deteriorating the signal-to-noise ratio (SNR). This issue may be compounded by triaxial cables connecting detectors to electrometers. However, effects of cables, critical for precision dosimetry, are often overlooked. There is a need to evaluate triaxial cable and detector impacts on SNR in small fields. The purpose of this study is to evaluate the influence of triaxial cables and microdetectors on signal-to-noise ratios in small-field dosimetry. This study also aims to establish the importance of cable quality assurance for measurement accuracy.Methods. Six 9.1 m length triaxial cables from different manufacturers were tested with six microdetectors (microDiamond, PinPoint, EDGE, Plastic scintillator, microSilicon, SRS-Diode). A 6 MV photon beam (TrueBeam) was used, with a water phantom at 5 cm depth with 0.5 × 0.5 cm2to 10 × 10 cm2fields at 600 MU min-1. Readings were acquired using cable-detector permutations with a dedicated electrometer (except the scintillator which has its own). Cables had differing connector types, conductor materials, insulation, and diameters. Detectors had various sensitive volumes, materials, typical signals, and bias voltages.Results. Normalized field output correction factors (FOFs) relative differences of 13.4% and 4.6% between the highest and lowest values across triaxial cables for 0.5 × 0.5 cm2and 1 × 1 cm2fields, respectively. The maximum difference in FOF between any cable-detector combinations was 0.2% for the smallest field size. No consistent FOF trend was observed across all detectors when increasing cable diameter. Additionally, the non-normalized FOF differences of 0.9% and 0.3% were observed between cables for 0.5 × 0.5 cm2and 1 × 1 cm2fields, respectively.Conclusions. Regular triaxial cable quality assurance is critical for precision small field dosimetry. A national protocol is needed to standardize cable evaluations/calibrations, particularly for small signals ( Collapse

Key Words

• QA
• SNR
• microdetectors
• small field dosimetry
• triaxial cable

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MESH Headings

• Signal-To-Noise Ratio
• Radiometry/instrumentation
• Radiometry/methods
• Phantoms, Imaging
• Equipment Design
• Photons
• Humans

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Grants Collapse

Affiliation(s)

James J Sohn Department of Radiation and Cellular Oncology, University of Chicago, Chicago, IL, 60637, United States of America
Indra J Das Department of Radiation Oncology, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL, 60611, United States of America

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Dosimetric comparison of 4 MeV and 6 MeV electron beams for total skin irradiation

BACKGROUND

In this study, dosimetric aspects of TSEI consisting of a 4 MeV beam with no spoiler were investigated in comparison to a nominal 6 MeV beam with spoiler, and the potential for clinical applications was evaluated.

METHODS

The TSEI technique is based on the Stanford technique, which utilizes a beam configuration of six-dual fields. MOSFETs were used to measure the optimal gantry angle, profile uniformity, and absolute dose at the calibration point. The depth dose curve of the central axis was measured in the treatment plane using EBT2 film. Photon contamination was measured as the dose at 5 cm depth in a solid water phantom relative to the maximum dose using a parallel plate ion chamber. A MOSFET dosimeter placed on the surface of a humanoid phantom, and EBT2 films inserted into a humanoid phantom were used to verify the TSEI commissioning.

RESULTS

Dosimetric aspects of the 4 MeV TSEI beam, such as profile uniformity (±10%) and relative photon contamination (<0.001%), were comparable to those of a 6 MeV TSEI beam. The relative depth dose of the 4 MeV electrons was 81.4% at the surface and 100% at 0.4 cm. For the 6 MeV electrons, the relative depth dose was 93.4% at the surface and 100% from 0.2 cm to 0.4 cm. The calculated B-factor of the 4 MeV TSEI beam was 1.55, and 1.53 for the 6 MeV TSEI. 80% of the prescribed dose was obtained at 0.22 cm depth for the 4 MeV TSEI beam and 0.53 cm for the 6 MeV TSEI beam in the humanoid phantom measurement.

CONCLUSIONS

The suggested 4 MeV beam for TSEI could be applied to shallow depth skin diseases and to electron boost as second treatment course.

Matching the dosimetry characteristics of a dual-field Stanford technique to a customized single-field Stanford technique for total skin electron therapy

PURPOSE

To compare the dosimetry characteristics of a customized single-field and a matching dual-field electron beam for total skin electron therapy (TSET) within the framework of the Stanford technique. To examine and quantify its impact on patient dosimetry.

METHODS AND MATERIALS

Two characteristically different electron beams were used for TSET employing the Stanford technique: a single-field beam created from a pencil beam of electrons passing through 7 meters of air and a dual-field beam created from two heavily scattered electron beams directed at oblique angles to patients. The dosimetry characteristics of the two beams were measured by using ionization chambers, radiographic films, and thermal luminescent detectors. The impact of beam characteristic on patient dosimetry was quantified on both anthromorphic phantoms and on patients. Treatment protocols aimed at matching the patient dose between the two systems were established on the basis of these and other measurements.

RESULTS

The dual-field beam was matched to the single-field beam, resulting in approximately the same mean energy (approximately 4.0 MeV) and most probable energy (approximately 4.5 MeV) at their respective treatment source-to-patient-surface distance (SSD). The depth dose curves on the beam axis were nearly identical for both beams. X-ray contamination on the beam axis was 0.43% for the dual-field beam, slightly higher than that (0.4%) of the single-field beam. The beam uniformity, however, was quite different: the dual-field beam was more uniform in the vertical direction but was worse in the lateral direction compared to the single-field beam. For a TSET treatment using the Stanford technique, the composite depth dose curves were nearly identically at the level of beam axis: with an effective depth of maximum buildup (d(max)) at approximately 1 mm below the skin surface and the depth to 80% depth dose at around 6 mm. The overall X-ray contamination was approximately 1.0% and 1.2% for the single-field and dual-field system, respectively. Away from the beam axis level, treatment using either beam was able to deliver over 90% of prescription dose to the main body surfaces. For body surfaces tangential to the beam axis (e.g., top of head and shoulders), the dose was low especially when using the dual-field beam. By adding boost radiation to the tangential surfaces and by adjusting the planned shielding for critical structures, the total dose to the patient over a complete course of TSET treatment could be matched closely for the two systems.

CONCLUSIONS

Although the depth doses can be matched at the level of the beam axis, there exist some characteristic differences in the angular distribution of the electrons between the large SSD single-field beam and the short SSD dual-field beam. These differences resulted in lower dose delivered to "tangential" body surfaces and to body structures that extended farther laterally when using the dual-field beam. However, by adjusting the treatment protocol regarding the boost irradiation and planned shielding, the total dose to patients from a complete course of TSET treatment using the dual-field beam can be matched to that given by the single-field beam. Special attention should be paid to the dosimetry at the "tangential" body surfaces when commissioning a dual-field TSET system.

Polarity effects of ionization chambers used in tbi dosimetry due to cable irradiation

This paper presents the results of an investigation on polarity effects in total-body irradiation (TBI) dosimetry. Thimble (NE2571, 0.6 cc) and plane-parallel (Markus NE2534 0.055 cc) chambers were investigated in a 30 x 30 x 30-cm3 acrylic phantom in TBI conditions (6-MV x-rays). The thimble chamber was positioned at the midline and at the entrance and exit Dmax (1.5 cm from the phantom surface) levels. The Markus chamber, which is generally used for skin dose estimations, was positioned at various depths from the entrance surface of the phantom (from 0- to 2-cm depth). The polarity factor (Ppol) was defined as (Q+ + Q-)/2Q-, where Q+ and Q- were the collected charges at positive and negative bias voltage, respectively. The variations of Ppol with many parameters (absorbed dose, dose rate, the presence or absence of a 1-cm acrylic spoiler, irradiated cable length) were investigated. Results show that Ppol is quite small (within 1.002 for on-axis measurements and 1.005 for off-axis measurements) for the NE2571 chamber when the beam spoiler is placed. Ppol was significantly higher without the beam spoiler (within 1.008 for on-axis measurements, up to 1.02 for off-axis measurements). Concerning the Markus chamber, for on-axis skin dose measurements, Ppol was found to be less than unity (around 0.988) or more than unity (around 1.0035), respectively, with and without the beam spoiler. Possible "directional effects" of the currents generated in the cable were investigated for both chambers and found to be insignificant. This shows that the application of Ppol correction has to be considered a reliable procedure in minimizing these effects. When the beam spoiler is placed, the cable has to be drawn to minimize the portion of cable just outside the beam; if this is not the case, Ppol may significantly vary (for the NE2571 chamber values up to 1.0035 were found for on-axis measurements).

On the use of unshielded cables in ionization chamber dosimetry for total-skin electron therapy

The dosimetry of total-skin electron therapy (TSET) usually requires ionization chamber measurements in a large electron beam (up to 120 cm x 200 cm). Exposing the chamber's electric cable, its connector and part of the extension cable to the large electron beam will introduce unwanted electronic signals that may lead to inaccurate dosimetry results. While the best strategy to minimize the cable-induced electronic signal is to shield the cables and its connector from the primary electrons, as has been recommended by the AAPM Task Group Report 23 on TSET, cables without additional shielding are often used in TSET dosimetry measurements for logistic reasons, for example when an automatic scanning dosimetry is used. This paper systematically investigates the consequences and the acceptability of using an unshielded cable in ionization chamber dosimetry in a large TSET electron beam. In this paper, we separate cable-induced signals into two types. The type-I signal includes all charges induced which do not change sign upon switching the chamber polarity, and type II includes all those that do. The type-I signal is easily cancelled by the polarity averaging method. The type-II cable-induced signal is independent of the depth of the chamber in a phantom and its magnitude relative to the true signal determines the acceptability of a cable for use under unshielded conditions. Three different cables were evaluated in two different TSET beams in this investigation. For dosimetry near the depth of maximum buildup, the cable-induced dosimetry error was found to be less than 0.2% when the two-polarity averaging technique was applied. At greater depths, the relative dosimetry error was found to increase at a rate approximately equal to the inverse of the electron depth dose. Since the application of the two-polarity averaging technique requires a constant-irradiation condition, it was demonstrated than an additional error of up to 4% could be introduced if the unshielded cable's spatial configuration were altered during the two-polarity measurements. This suggests that automatic scanning systems with unshielded cables should not be used in TSET ionization chamber dosimetry. However, the data did show that an unshielded cable may be used in TSET ionization chamber dosimetry if the size of cable-induced error in a given TSET beam is pre-evaluated and the measurement is carefully conducted. When such an evaluation has not been performed, additional shielding should be applied to the cable being used, making measurements at multiple points difficult.