Mark Geurts

Longitudinal study using a diode phantom for helical tomotherapy IMRT QA

PURPOSE The Caribbean Radiation Oncology Center acquired a DELTA4 diode phantom for helical tomotherapy IMRT QA and presents the results of their first 264 clinical cases. METHODS The validation consisted of several case studies comparing existing ionization chamber and Gafchromic film IMRT QA results to diode phantom results, along with a longitudinal study analyzing the IMRT QA results against other machine QA procedures for a complete sample of IMRT patients. RESULTS The case studies resulted in a maximum observed difference of 0.7% between the diode phantom and the ionization chamber measurements in low dose-gradient regions. Over the longitudinal study, every IMRT QA plan passed a gamma specification of a 3%/3 mm and 98% of the diodes yielded a value of less than 1. In addition, the mean 90% isodose absolute difference for all plans was 0.05% with a (lsigma) standard deviation of 1.19%. CONCLUSIONS The phantom measurements closely match the planned dose distributions in high and low dose-gradient regions. In addition, a significant positive statistical correlation was determined between the IMRT QA, daily QA, and rotational variation output measurements. Together, these results signify high degree of accuracy of both the DELTA4 phantom as well as the TomoTherapy Hi-Art system.

Monte Carlo simulations of patient dose perturbations in rotational-type radiotherapy due to a transverse magnetic field: A tomotherapy investigation

PURPOSE Several groups are exploring the integration of magnetic resonance (MR) image guidance with radiotherapy to reduce tumor position uncertainty during photon radiotherapy. The therapeutic gain from reducing tumor position uncertainty using intrafraction MR imaging during radiotherapy could be partially offset if the negative effects of magnetic field-induced dose perturbations are not appreciated or accounted for. The authors hypothesize that a more rotationally symmetric modality such as helical tomotherapy will permit a systematic mediation of these dose perturbations. This investigation offers a unique look at the dose perturbations due to homogeneous transverse magnetic field during the delivery of Tomotherapy(®) Treatment System plans under varying degrees of rotational beamlet symmetry. METHODS The authors accurately reproduced treatment plan beamlet and patient configurations using the Monte Carlo code geant4. This code has a thoroughly benchmarked electromagnetic particle transport physics package well-suited for the radiotherapy energy regime. The three approved clinical treatment plans for this study were for a prostate, head and neck, and lung treatment. The dose heterogeneity index metric was used to quantify the effect of the dose perturbations to the target volumes. RESULTS The authors demonstrate the ability to reproduce the clinical dose-volume histograms (DVH) to within 4% dose agreement at each DVH point for the target volumes and most planning structures, and therefore, are able to confidently examine the effects of transverse magnetic fields on the plans. The authors investigated field strengths of 0.35, 0.7, 1, 1.5, and 3 T. Changes to the dose heterogeneity index of 0.1% were seen in the prostate and head and neck case, reflecting negligible dose perturbations to the target volumes, a change from 5.5% to 20.1% was observed with the lung case. CONCLUSIONS This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality.

Gadoxetate for direct tumor therapy and tracking with real-time MRI-guided stereotactic body radiation therapy of the liver.

SBRT is increasingly utilized in liver tumor treatment. MRI-guided RT allows for real-time MRI tracking during therapy. Liver tumors are often poorly visualized and most contrast agents are transient. Gadoxetate may allow for sustained tumor visualization. Here, we report on the first use of gadoxetate during real-time MRI-guided SBRT.

Measurement-guided volumetric dose reconstruction for helical tomotherapy

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 dosimeters 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

Dosimetric Comparison of Real-Time MRI-Guided Tri-Cobalt-60 Versus Linear Accelerator-Based Stereotactic Body Radiation Therapy Lung Cancer Plans

Purpose: Magnetic resonance imaging–guided radiation therapy has entered clinical practice at several major treatment centers. Treatment of early-stage non-small cell lung cancer with stereotactic body radiation therapy is one potential application of this modality, as some form of respiratory motion management is important to address. We hypothesize that magnetic resonance imaging–guided tri-cobalt-60 radiation therapy can be used to generate clinically acceptable stereotactic body radiation therapy treatment plans. Here, we report on a dosimetric comparison between magnetic resonance imaging–guided radiation therapy plans and internal target volume–based plans utilizing volumetric-modulated arc therapy. Materials and Methods: Ten patients with early-stage non-small cell lung cancer who underwent radiation therapy planning and treatment were studied. Following 4-dimensional computed tomography, patient images were used to generate clinically deliverable plans. For volumetric-modulated arc therapy plans, the planning tumor volume was defined as an internal target volume + 0.5 cm. For magnetic resonance imaging–guided plans, a single mid-inspiratory cycle was used to define a gross tumor volume, then expanded 0.3 cm to the planning tumor volume. Treatment plan parameters were compared. Results: Planning tumor volumes trended larger for volumetric-modulated arc therapy–based plans, with a mean planning tumor volume of 47.4 mL versus 24.8 mL for magnetic resonance imaging–guided plans (P = .08). Clinically acceptable plans were achievable via both methods, with bilateral lung V20, 3.9% versus 4.8% (P = .62). The volume of chest wall receiving greater than 30 Gy was also similar, 22.1 versus 19.8 mL (P = .78), as were all other parameters commonly used for lung stereotactic body radiation therapy. The ratio of the 50% isodose volume to planning tumor volume was lower in volumetric-modulated arc therapy plans, 4.19 versus 10.0 (P < .001). Heterogeneity index was comparable between plans, 1.25 versus 1.25 (P = .98). Conclusion: Magnetic resonance imaging–guided tri-cobalt-60 radiation therapy is capable of delivering lung high-quality stereotactic body radiation therapy plans that are clinically acceptable as compared to volumetric-modulated arc therapy–based plans. Real-time magnetic resonance imaging provides the unique capacity to directly observe tumor motion during treatment for purposes of motion management.

Clinical significance of treatment delivery errors for helical TomoTherapy nasopharyngeal plans - A dosimetric simulation study.

PURPOSE Develop a framework to characterize helical TomoTherapy (HT) machine delivery errors and their clinical significance. METHOD AND MATERIALS Ten nasopharynx HT plans were edited to introduce errors in Jaw width (JW), couch speed (CS), gantry period (GP), gantry start position (GSP), multi leaf collimator leaf open times (MLC LOT). In case of MLC LOT only, both systematic and random delivery errors were investigated. Each error type was simulated independently for a range of magnitudes. Dose distributions for the clinical reference plans and the error simulated plans were compared to establish the magnitude for each error type which resulted in a change in clinical tolerance, defined as 5% variation in D95 of PTV70, D0.1cc of spinal cord, D0.1cc of brainstem and the smallest value of either a 10% or 3.6Gy dose variation in mean parotid dose. RESULTS Dose variation from systematic delivery errors in JW ±0.5mm, CS ranges between -1% to 1.5%, GP ±1s, GSP ranges between -20 to 2.50 and MLC LOT random error up to 2% from the planned value relative to the clinical reference plan was within the set tolerance values for all the patient cohorts. GSP errors and the random MLC LOT errors with up to 10% standard deviation were found to be relatively insensitive compared to other delivery errors. CONCLUSION This work has established a framework to characterize HT machine delivery errors. This framework could be applied to any patient dataset to determine clinically relevant HT QA tolerances.

Sensitivity evaluation of two commercial dosimeters in detecting Helical TomoTherapy treatment delivery errors

PURPOSE To assess the sensitivity of two commercial dosimetry systems in detecting Helical TomoTherapy (HT) delivery errors. METHOD Two commercial dosimeters i) MatriXXEvolution and ii) ArcCHECK® were considered. Ten retrospective nasopharynx HT patients were analysed. For each patient, error plans were created by independently introducing systematic offsets in: a) Jaw width error ±1, ±1.5 and ±2mm, b) Couch speed error ±2%, ±2.5, ±3% and ±4%, and c) MLC Leaf Open Time (MLCLOT) errors (3 separate MLC errors: either leaf 32 open or leaf 42 remains open during delivery, and 4% random reductions in MLCLOT). All error plans, along with the no error plan for each patient, were measured using both dosimeters in the same session. Gamma evaluation (3%/3mm) was applied to quantitatively compare the measured dose from each dosimeter to the treatment planning system. The error sensitivity was quantified as the rate of decrease in gamma pass rate. RESULTS The gamma pass rate decreases with increase in error magnitude for both dosimeters. ArcCHECK was insensitive for couch speed error up to 2.5% and jaw width error up to -1.5mm while MatriXXEvolution was found to be insensitive to couch speed error up to 2% and couch speed up to -1mm. Both of the detectors show similar sensitivity to all the MLCLOT errors that were clinically relevant. CONCLUSION No statistically significant (p>0.05) differences exist in detecting the simulated delivery errors between MatriXXEvolution and ArcCHECK dosimeter systems for HT plans. Both dosimeters were able to pick up clinically relevant delivery errors.

A New Era of Image Guidance with Magnetic Resonance-guided Radiation Therapy for Abdominal and Thoracic Malignancies

Magnetic resonance-guided radiation therapy (MRgRT) offers advantages for image guidance for radiotherapy treatments as compared to conventional computed tomography (CT)-based modalities. The superior soft tissue contrast of magnetic resonance (MR) enables an improved visualization of the gross tumor and adjacent normal tissues in the treatment of abdominal and thoracic malignancies. Online adaptive capabilities, coupled with advanced motion management of real-time tracking of the tumor, directly allow for high-precision inter-/intrafraction localization. The primary aim of this case series is to describe MR-based interventions for localizing targets not well-visualized with conventional image-guided technologies. The abdominal and thoracic sites of the lung, kidney, liver, and gastric targets are described to illustrate the technological advancement of MR-guidance in radiotherapy.

TU-H-BRC-04: Feasibility of Using TomoDirect for Pulsed Reduced Dose-Rate Radiotherapy.

PURPOSE Pulsed reduced dose-rate radiotherapy (PRDR) is a technique used for treatment of recurrent disease where a planned treatment is split into 0.2 Gy pulses, each separated by three-minute intervals, to achieve a time averaged dose rate of 0.0667 Gy/min. This study investigated the feasibility of using TomoTherapys fixed angle delivery feature (TomoDirect) to create high quality IMRT plans that meet the radiobiological constraints of PRDR. METHODS Two plans (one craniospinal irradiation and a complex shaped temporal lobe re-irradiation) were created using a conventional technique and then TomoDirect. Plan quality was compared between techniques. Next, the dose rate to each voxel in the target was computed for each TomoDirect beam to verify that the time-averaged dose rate is less than 0.0667 Gy/min. RESULTS The TomoDirect CSI plan provided improved target coverage and organ at risk sparing with a per-beam treatment time ranging from 21.6 to 43.5 seconds and mean time-averaged dose rate of 0.032 Gy/min. The TomoDirect temporal lobe plan showed equivalent plan quality to a non-PRDR TomoHelical IMRT plan with per-beam treatment times ranging from 43.7 to 44.9 seconds and a mean time-averaged dose rate of 0.052 Gy/min. CONCLUSION Our initial investigation suggests that TomoDirect PRDR is not only feasible but ideal for delivering superior conformality and target homogeneity to complex and large fields.

TH-EF-BRB-06: Intensity Modulated Imaging?: Clinical Workflow for Fluence Field Modulated CT On a TomoTherapy System

Purpose: This work presents a new approach that uses the multi leaf collimator present on TomoTherapy devices to modulate the imaging beam. This allows for targeted imaging in which only regions of clinical importance for IGRT (patient positioning confirmation) receive a high level of image quality. This allows the total imaging dose to decrease, especially for healthy tissue. Methods: A clinical TomoTherapy machine was programmed to deliver imaging dose such that small pre-defined volumes of interest (VOI) received relatively higher levels of image quality with respect to surrounding regions. Four different size ROIs were placed at varying distances from isocenter. The noise and mean CT number were compared between VOI and un-modulated scans. Dose distributions were generated using a treatment planning dose calculator. A clinically feasible workflow was developed to implement this technique that used treatment planning contours to define VOI positions. Results: The VOI-FFMCT technique produced an image noise within 5% of an unmodulated scan for all size VOIs. The VOI technique required a total imaging dose of 0.61, 0.69, 0.60, and 0.50 times the “full dose” acquisition dose for VOI sizes/locations of 10/13/10/6 cm in diameter and located 0/2/5/6 cm from isocenter respectively. Mean CT numbers for the VOI scans were within 1% of the unmodulated case. Conclusion: This approach, which combines state-of-the-art radiation therapy fluence control with a novel imaging approach has been implemented on a clinical system with no hardware or software changes. A clinically feasible method for implementing this technique was developed which requires no additional user input relative to the current procedures in our clinic. The method allows physicians to choose between: (1) better image quality at no dose penalty or (2) equal image quality while reducing dose levels relative to today’s standard of care. Research grants from GE HealthCare and TomoTherapy.