Elsevier

Physica Medica

Volume 85, May 2021, Pages 15-23
Physica Medica

Original paper
Delta4 Discover transmission detector: A comprehensive characterization for in-vivo VMAT monitoring

https://doi.org/10.1016/j.ejmp.2021.04.017Get rights and content

Highlights

  • Transmission detector can be a useful tool for in-vivo IMRT monitoring.

  • The study complements published data about Delta4 Discover TRD.

  • Effects on the superficial dose investigated for different open and IMRT fields.

  • Capability to detect different delivery errors investigated in 510 VMAT arcs.

  • Results can serve as a reference for users with Varian TrueBeam platform.

Abstract

Objective

To investigate the dosimetric behaviour, influence on photon beam fluence and error detection capability of Delta4 Discover transmission detector.

Methods

The transmission detector (TRD) was characterized on a TrueBeam linear accelerator with 6 MV beams. Linearity, reproducibility and dose rate dependence were investigated. The effect on photon beam fluence was evaluated in terms of beam profiles, percentage depth dose, transmission factor and surface dose for different open field sizes. The transmission factor of the 10x10 cm2 field was entered in the TPS’s configuration and its correct use in the dose calculation was verified recalculating 17 clinical IMRT/VMAT plans. Surface dose was measured for 20 IMRT fields. The capability to detect different delivery errors was investigated evaluating dose gamma index, MLC gamma index and leaf position of 15 manually modified VMAT plans.

Results

TRD showed a linear dependence on MU. No dose rate dependence was observed. Short-term and long-term reproducibility were within 0.1% and 0.5%. The presence of the TRD did not significantly affect PDDs and profiles. The transmission factor of the 10x10 cm2 field size was 0.985 and 0.983, for FF and FFF beams respectively. The 17 recalculated plans met our clinical gamma-index passing rate, confirming the correct use of the transmission factor by the TPS. The surface dose differences for the open fields increase for shorter SSDs and greater field size. Differences in surface dose for the IMRT beams were less than 2%. Output variation ≥2%, collimator angle variations within 0.3°, gantry angle errors of 1°, jaw tracking and leaf position errors were detected.

Conclusions

Delta4 Discover shows good linearity and reproducibility, is not dependent on dose rate and does not affect beam quality and dose profiles. It is also capable to detect dosimetric and geometric errors and therefore it is suitable for monitoring VMAT delivery.

Introduction

Intensity modulated radiation therapy (IMRT) and Volumetric modulated arc therapy (VMAT) allow to better conform dose to target volumes than traditional 3D conformal radiation therapy while minimizing dose to adjacent normal tissues. Consequently, they have become the predominant radiotherapy techniques for a variety of treatment sites. The steep dose gradients generated by means of variations in multileaf collimator motion, gantry rotation speed and dose rate have increased type and frequency of potential errors. Consequently, extensive quality assurance program (QA) to be appropriate for the treatment technology used are needed. In most of the radiation oncology departments, QA programs consist of two components: AAPM TG-100 [1] and AAPM TG-142 [2] methodology for machine QA, that ensures that each component of the delivery system is working within tolerance, and a patient-specific pre-treatment IMRT QA, that checks the accuracy of IMRT plan dose calculation and detects relevant errors in the radiation delivery. IMRT and VMAT techniques require an extensive quality assurance program for treatment delivery, including, in addition to traditional pre-treatment QA, in-vivo [3], [4], [5] or real time monitoring systems [6], [7], [8].

Along with detector arrays [9], electronic portal imaging devices [10], [11] and delivery log-file analysis [12], [13], transmission detectors (TRD) can be used to perform in-vivo monitoring and verification of the delivery process over the course of treatment. TRDs based on wedge shaped ionization chambers [14], multiwire ionization chambers [15], [16], diodes [17], [18] and scintillating fibers [19] have been developed with the aim to provide an accurate verification of the dose delivered to the patient on a fraction by fraction basis. However, there are some significant concerns about the use of this type of detectors for dosimetric measurements: the potential collision issues during treatment, the perturbation of the beam fluence, the need to account for the beam attenuation in the TPS, the increase in surface dose and finally the cost/benefit ratio.

The aim of the study was to complement previous investigations [20], [21], [22] of the high resolution diode based TRD Delta4 Discover (ScandiDos, Uppsala, Sweden) for its implementation in clinical workflow. This work evaluates the dosimetric behavior in terms of linearity and reproducibility, dependence of the signal from dose rate, influence on photon beam fluence and contribution to surface dose. Additionally, considering the importance to know the error detection capabilities of an IMRT QA system used in clinical environment for in-vivo verification of VMAT delivery, we investigated the ability of the system to detect errors intentionally introduced in arc VMAT plans.

Section snippets

Materials and methods

The Delta4 Discover transmission detector is a fluence measurement device designed to monitor the fidelity of the dose distribution delivered during every treatment [21], [22]. The detector consists of 4040p-type diode detectors each with an active area of 1 mm diameter and separated by 2.5 mm and 5 mm along and perpendicular to the multileaf collimator (MLC) motion direction respectively. The diode array can measure a maximum field size of 25 × 19.5 cm2 when projected to the isocenter level.

Dosimetric quantities and reproducibility

Delta4 Discover TRD signal shows a linear dependence on MU delivered (Pearson correlation coefficient R2 = 1). No dependence from the dose rate was instead observed: the maximum TRD signal difference observed is 0.3% and 0.1% for FF and FFF beams, respectively.

The short-term and long-term reproducibility were within 0.1% and 0.5% respectively, for both test beams.

Transmission factor and surface dose

The presence of the TRD did not significantly affect depth dose curves and dose profiles: depth of maximum dose did not change by

Discussion

In this study, we investigated the performance of the Delta4 Discover transmission detector system: dosimetric characteristics, impact on the radiation dose received by the patient (dose transmission and surface dose contribution) and capability to detect dosimetric and geometric errors.

The transmission detector was found to be very stable in terms of linearity (R2 = 1) and short and long term reproducibility (<1%); the dose rate dependence was less than 0.4% up to typical dose rate values of 6

Conclusion

The aim of this study was to investigate the dosimetric characteristics, the influence on photon beam fluence and the capability to detect Linac output and geometric errors of Delta4 Discover transmission detector, as a propaedeutic study before implementing it in the clinical routine. The results showed that the system has good linearity and reproducibility, is not dependent on dose rate and does not affect beam quality and dose profiles. The Delta4 Discover system is also capable to detect

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