Measurement‐guided volumetric dose reconstruction for helical tomotherapy

Abstract

It was previously demonstrated that dose delivered by a conventional linear accelerator using IMRT or VMAT can be reconstructed — on patient or phantom datasets — using helical diode array measurements and a technique called planned dose perturbation (PDP). This allows meaningful and intuitive analysis of the agreement between the planned and delivered dose, including direct comparison of the dose‐volume histograms. While conceptually similar to modulated arc techniques, helical tomotherapy introduces significant challenges to the PDP formalism, arising primarily from TomoTherapy delivery dynamics. The temporal characteristics of the delivery are of the same order or shorter than the dosimeter's update interval (50 ms). Additionally, the prevalence of often small and complex segments, particularly with the 1 cm Y jaw setting, lead to challenges related to detector spacing. Here, we present and test a novel method of tomotherapy‐PDP (TPDP) designed to meet these challenges. One of the novel techniques introduced for TPDP is organization of the subbeams into larger subunits called sectors, which assures more robust synchronization of the measurement and delivery dynamics. Another important change is the optional application of a correction based on ion chamber (IC) measurements in the phantom. The TPDP method was validated by direct comparisons to the IC and an independent, biplanar diode array dosimeter previously evaluated for tomotherapy delivery quality assurance. Nineteen plans with varying complexity were analyzed for the 2.5 cm tomotherapy jaw setting and 18 for the 1 cm opening. The dose differences between the TPDP and IC were 1.0%±1.1% and 1.1%±1.1%, for 2.5 and 1.0 cm jaw plans, respectively. Gamma analysis agreement rates between TPDP and the independent array were: 99.1%±1.8% (using 3% global normalization/3 mm criteria) and 93.4%±7.1% (using 2% global/2 mm) for the 2.5 cm jaw plans; for 1 cm plans, they were 95.2%±6.7% (3% G/3) and 83.8%±12% (2% G/2). We conclude that TPDP is capable of volumetric dose reconstruction with acceptable accuracy. However, the challenges of fast tomotherapy delivery dynamics make TPDP less precise than the IMRT/VMAT PDP version, particularly for the 1 cm jaw setting.PACS number: 87.55Qr

Clinical implementation of an exit detector‐based dose reconstruction tool for helical tomotherapy delivery quality assurance

“…These systems utilize the measured dose with conventional verification QA tools and reconstruct the dose to the patient to assess the clinical impact. These dose reconstruction algorithms contain significant approximations and their limitations have been demonstrated by recent studies …”

“…These dose reconstruction algorithms contain significant approximations and their limitations have been demonstrated by recent studies. 20 HT is a dedicated IMRT system with an on-board megavoltage computed tomography (MVCT) imaging system. The on board detector (OBD) consists of an arc-shaped xenon detector array positioned opposite the linear accelerator on a rotating slip-ring gantry.…”

Novel methodologies for dosimetry audits: Adapting to advanced radiotherapy techniques

“…Similarly to the previously mentioned audits for advanced treatment techniques, 3D dose reconstruction has not yet been applied in Tomotherapy auditing. CT data sets for planning and phantom-based 3D dosimetry have been investigated and validated [72] . The volumetric dose reconstruction based on helical diode array measurements is generally similar to the approach in rotational IMRT [67] , but the calculation algorithm is different due to the conceptual differences between helical Tomotherapy and VMAT.…”

Filmless methods for quality assurance of Tomotherapy using ArcCHECK

“…15,16 Devices with 2D arrays also have successful applications in helical Tomotherapy QA 17,18 despite the possible limitation due to its directional dependency. 19,20 The ArcCHECK (Sun Nuclear, Melbourne, FL, Australia), a cylindrical acrylic phantom with an array of spirally arranged diode detectors, has been found to be well suited for QA of VMAT, IMRT, and Tomotherapy, [21][22][23][24][25][26][27] since the cylindrical design of Arc-CHECK removes rotational dependence of the detectors.…”