Evaluation of the ability of three commercially available dosimeters to detect systematic delivery errors in step-and-shoot IMRT plans

Background

There is limited data on error detectability for step-and-shoot intensity modulated radiotherapy (sIMRT) plans, despite significant work on dynamic methods. However, sIMRT treatments have an ongoing role in clinical practice. This study aimed to evaluate variations in the sensitivity of three patient-specific quality assurance (QA) devices to systematic delivery errors in sIMRT plans.

Materials and methods

Four clinical sIMRT plans (prostate and head and neck) were edited to introduce errors in: Multi-Leaf Collimator (MLC) position (increasing field size, leaf pairs offset (1-3 mm) in opposite directions; and field shift, all leaves offset (1-3 mm) in one direction); collimator rotation (1-3 degrees) and gantry rotation (0.5-2 degrees). The total dose for each plan was measured using an ArcCHECK diode array. Each field, excluding those with gantry offsets, was also measured using an Electronic Portal Imager and a MatriXX Evolution 2D ionisation chamber array. 132 plans (858 fields) were delivered, producing 572 measured dose distributions. Measured doses were compared to calculated doses for the no-error plan using Gamma analysis with 3%/3 mm, 3%/2 mm, and 2%/2 mm criteria (1716 analyses).

Results

Generally, pass rates decreased with increasing errors and/or stricter gamma criteria. Pass rate variations with detector and plan type were also observed. For a 3%/3 mm gamma criteria, none of the devices could reliably detect 1 mm MLC position errors or 1 degree collimator rotation errors.

Conclusions

This work has highlighted the need to adapt QA based on treatment plan type and the need for detector specific assessment criteria to detect clinically significant errors.

Quality management in radiation therapy: A 15 year review of incident reporting in two integrated cancer centres

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Fifteen years of reported incidents were reviewed.

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Overall reduction in incident severity overtime identified.

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New technology associated to reduced incident severity.

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Reporting culture associated to reporting rates.

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Taxonomy changes required to improve ISL and incident classification.

Fifteen years of reported incidents were reviewed to provide insight into the effectiveness of an Incident Learning System (ISL). The actual error rate over the 15 years was 1.3 reported errors per 1000 treatment attendances. Incidents were reviewed using a regression model. The average number of incidents per year and the number of incidents per thousand attendances declined over time. Two seven-year periods were considered for analysis and the average for the first period (2005–2011) was 6 reported incidents per 1000 attendances compared to 2 incidents for the later period (2012–2018), p < 0.05. SAC 1 and SAC 2 errors have reduced over time and the reduction could be attributed to the quality assurance aspect of IGRT where the incident is identified prior to treatment delivery rather than after, reducing the severity of any potential incidents. The reasoning behind overall reduction in incident reporting over time is unclear but may be associated to quality and technology initiatives, issues with the ISL itself or a change in the staff reporting culture.

Lung organ-at-risk volumes: A survey of practice and the need for a consistent definition in the 4DCT era

INTRODUCTION

In the 4DCT era, the definition of lung organ-at-risk (OAR) volumes for dose-volume histogram (DVH) calculation is unclear, introducing potential for variability in practice. We aimed to identify definitions used clinically and evaluate the magnitude of DVH differences between these.

METHODS

We surveyed Australian & New Zealand departments about lung radiotherapy protocols including lung OAR volume definition. We used these definitions to calculate lung DVHs on 10 patients prescribed lung IMRT (60-66 Gy/30-33 fractions). We calculated mean lung dose (MLD), V20 and V30 for 'Lungs - PTV', 'Lungs - CTV', 'Lungs - iGTV' (internal GTV in all respiratory phases) and 'Lungs - GTV_EX' (expiratory phase).

RESULTS

The response rate was 39% (34/88). 14% and 29% of departments did not have a departmental protocol for OAR and tumour volume delineation, respectively. All permutations for lung OAR volumes were used with no clear preference. For conventional radiotherapy (n = 27), this included Lungs alone (n = 1), Lungs - PTV (n = 6), Lungs - CTV (n = 2), Lungs - iGTV (n = 6), Lungs - GTV in single phase (n = 5) and individual clinician preference (n = 7). The different lung OAR volumes resulted in MLD difference ranging from 0.9 to 4.15 Gy, V20 from 1.5% to 6.6% and V30 from 1.34% to 7.11%. The largest differences between subtraction of GTV_EX and iGTV were 0.32 Gy, 0.43% and 0.46% for MLD, V20 and V30, respectively.

CONCLUSION

A significant number of departments lacked lung cancer radiotherapy contouring protocols. Lung OAR volume definition was variable between and within departments. Potentially clinically significant differences in lung DVH parameters were seen according to the volume used.

Remote dosimetric auditing of clinical trials: The need for vendor specific models to convert images to dose

INTRODUCTION

A previous pilot study has demonstrated the feasibility of a novel image-based approach for remote dosimetric auditing of clinical trials. The approach uses a model to convert in-air acquired intensity modulated radiotherapy (IMRT) images to delivered dose inside a virtual phantom. The model was developed using images from an electronic portal imaging device (EPID) on a Varian linear accelerator. It was tuned using beam profiles and field size factors (FSFs) of a series of square fields measured in water tank. This work investigates the need for vendor specific conversion models for image-based auditing. The EPID measured profile and FSF data for Varian (vendor 1) and Elekta (vendor 2) systems are compared along with the performance of the existing Varian model (VM) and a new Elekta model (EM) for a series of audit IMRT fields measured on vendor 2 systems.

MATERIALS AND METHODS

The EPID measured beam profile and FSF data were studied for the two vendors to quantify and understand their relevant dosimetric differences. Then, an EM was developed converting EPID to dose in the virtual water phantom using a vendor 2 water tank data and images from corresponding EPID. The VM and EM were compared for predicting vendor 2 measured dose in water tank. Then, the performance of the new EM was compared to the VM for auditing of 54 IMRT fields from four vendor 2 facilities. Statistical significance of using vendor specific models was determined.

RESULTS

Observed dosimetry differences between the two vendors suggested developing an EM would be beneficial. The EM performed better than VM for vendor 2 square and IMRT fields. The IMRT audit gamma pass rates were (99.8 ± 0.5)%, (98.6 ± 2.3)% and (97.0 ± 3.0)% at respectively 3%/3 mm, 3%/2 mm and 2%/2 mm with improvements at most fields compared with using the VM. For the pilot audit, the difference between gamma results of the two vendors was reduced when using vendor specific models (VM: P < 0.0001, vendor specific models: P = 0.0025).

CONCLUSION

A new model was derived to convert images from vendor 2 EPIDs to dose for remote auditing vendor 2 deliveries. Using vendor specific models is recommended to remotely audit systems from different vendors, however, the improvements found were not major.

A remote EPID-based dosimetric TPS-planned audit of centers for clinical trials: outcomes and analysis of contributing factors

Background

A novel remote method for external dosimetric TPS-planned auditing of intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) for clinical trials using electronic portal imaging device (EPID) has been developed. The audit has been applied to multiple centers across Australia and New Zealand. This work aims to assess the audit outcomes and explores the variables that contributed to the audit results.

Methods

Thirty audits were performed of 21 radiotherapy facilities, 17 facilities underwent IMRT audits and 13 underwent VMAT audits. The assessment was based on comparisons between the delivered doses derived from images acquired with EPIDs and planned doses from the local treatment planning systems (TPS). Gamma pass-rate (GPR) and gamma mean value (GMV) were calculated for each IMRT field and VMAT arc (total 268 comparisons). A multiple variable linear model was applied to the GMV results (3%/3 mm criteria) to assess the influence and significance of explanatory variables. The explanatory variables were Linac-TPS combination, TPS grid resolution, IMRT/VMAT delivery, age of EPID, treatment site, record and verification system (R&V) type and dose-rate. Finally, the audit results were compared with other recent audits by calculating the incidence ratio (IR) as a ratio of the observed mean/median GPRs for the remote audit to the other audits.

Results

The average (± 1 SD) of the centers’ GPRs were: 99.3 ± 1.9%, 98.6 ± 2.7% & 96.2 ± 5.5% at 3%, 3 mm, 3%, 2 mm and 2%, 2 mm criteria respectively. The most determinative variables on the GMVs were Linac-TPS combination, TPS grid resolution and IMRT/VMAT delivery type. The IR values were 1 for seven comparisons, indicating similar GPRs of the remote audit with the reference audits and > 1 for four comparisons, indicating higher GPRs of the remote audit than the reference audits.

Conclusion

The remote dosimetry audit method for clinical trials demonstrated high GPRs and provided results comparable to established more resource-intensive audit methods. Several factors were found to influence the results including some effect of Linac-TPS combination.

Electronic supplementary material

The online version of this article (10.1186/s13014-018-1125-8) contains supplementary material, which is available to authorized users.

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

PURPOSE

The aim of this study was to validate the accuracy of an exit detector-based dose reconstruction tool for helical tomotherapy (HT) delivery quality assurance (DQA).

METHODS AND MATERIAL

Exit detector-based DQA tool was developed for patient-specific HT treatment verification. The tool performs a dose reconstruction on the planning image using the sinogram measured by the HT exit detector with no objects in the beam (i.e., static couch), and compares the reconstructed dose to the planned dose. Vendor supplied (three "TomoPhant") plans with a cylindrical solid water ("cheese") phantom were used for validation. Each "TomoPhant" plan was modified with intentional multileaf collimator leaf open time (MLC LOT) errors to assess the sensitivity and robustness of this tool. Four scenarios were tested; leaf 32 was "stuck open," leaf 42 was "stuck open," random leaf LOT was closed first by mean values of 2% and then 4%. A static couch DQA procedure was then run five times (once with the unmodified sinogram and four times with modified sinograms) for each of the three "TomoPhant" treatment plans. First, the original optimized delivery plan was compared with the original machine agnostic delivery plan, then the original optimized plans with a known modification applied (intentional MLC LOT error) were compared to the corresponding error plan exit detector measurements. An absolute dose comparison between calculated and ion chamber (A1SL, Standard Imaging, Inc., WI, USA) measured dose was performed for the unmodified "TomoPhant" plans. A 3D gamma evaluation (2%/2 mm global) was performed by comparing the planned dose ("original planned dose" for unmodified plans and "adjusted planned dose" for each intentional error) to exit detector-reconstructed dose for all three "Tomophant" plans. Finally, DQA for 119 clinical (treatment length <25 cm) and three cranio-spinal irradiation (CSI) plans were measured with both the ArcCHECK phantom (Sun Nuclear Corp., Melbourne, FL, USA) and the exit detector DQA tool to assess the time required for DQA and similarity between two methods.

RESULTS

The measured ion chamber dose agreed to within 1.5% of the reconstructed dose computed by the exit detector DQA tool on a cheese phantom for all unmodified "Tomophant" plans. Excellent agreement in gamma pass rate (>95%) was observed between the planned and reconstructed dose for all "Tomophant" plans considered using the tool. The gamma pass rate from 119 clinical plan DQA measurements was 94.9% ± 1.5% and 91.9% ± 4.37% for the exit detector DQA tool and ArcCHECK phantom measurements (P = 0.81), respectively. For the clinical plans (treatment length <25 cm), the average time required to perform DQA was 24.7 ± 3.5 and 39.5 ± 4.5 min using the exit detector QA tool and ArcCHECK phantom, respectively, whereas the average time required for the 3 CSI treatments was 35 ± 3.5 and 90 ± 5.2 min, respectively.

CONCLUSION

The exit detector tool has been demonstrated to be faster for performing the DQA with equivalent sensitivity for detecting MLC LOT errors relative to a conventional phantom-based QA method. In addition, comprehensive MLC performance evaluation and features of reconstructed dose provide additional insight into understanding DQA failures and the clinical relevance of DQA results.

Technical Note: Experimental results from a prototype high-field inline MRI-linac

PURPOSE

The pursuit of real-time image guided radiotherapy using optimal tissue contrast has seen the development of several hybrid magnetic resonance imaging (MRI)-treatment systems, high field and low field, and inline and perpendicular configurations. As part of a new MRI-linac program, an MRI scanner was integrated with a linear accelerator to enable investigations of a coupled inline MRI-linac system. This work describes results from a prototype experimental system to demonstrate the feasibility of a high field inline MR-linac.

METHODS

The magnet is a 1.5 T MRI system (Sonata, Siemens Healthcare) was located in a purpose built radiofrequency (RF) cage enabling shielding from and close proximity to a linear accelerator with inline (and future perpendicular) orientation. A portable linear accelerator (Linatron, Varian) was installed together with a multileaf collimator (Millennium, Varian) to provide dynamic field collimation and the whole assembly built onto a stainless-steel rail system. A series of MRI-linac experiments was performed to investigate (1) image quality with beam on measured using a macropodine (kangaroo) ex vivo phantom; (2) the noise as a function of beam state measured using a 6-channel surface coil array; and (3) electron contamination effects measured using Gafchromic film and an electronic portal imaging device (EPID).

RESULTS

(1) Image quality was unaffected by the radiation beam with the macropodine phantom image with the beam on being almost identical to the image with the beam off. (2) Noise measured with a surface RF coil produced a 25% elevation of background intensity when the radiation beam was on. (3) Film and EPID measurements demonstrated electron focusing occurring along the centerline of the magnet axis.

CONCLUSIONS

A proof-of-concept high-field MRI-linac has been built and experimentally characterized. This system has allowed us to establish the efficacy of a high field inline MRI-linac and study a number of the technical challenges and solutions.

Virtual EPID standard phantom audit (VESPA) for remote IMRT and VMAT credentialing

A virtual EPID standard phantom audit (VESPA) has been implemented for remote auditing in support of facility credentialing for clinical trials using IMRT and VMAT. VESPA is based on published methods and a clinically established IMRT QA procedure, here extended to multi-vendor equipment. Facilities are provided with comprehensive instructions and CT datasets to create treatment plans. They deliver the treatment directly to their EPID without any phantom or couch in the beam. In addition, they deliver a set of simple calibration fields per instructions. Collected EPID images are uploaded electronically. In the analysis, the dose is projected back into a virtual cylindrical phantom. 3D gamma analysis is performed. 2D dose planes and linear dose profiles are provided and can be considered when needed for clarification. In addition, using a virtual flat-phantom, 2D field-by-field or arc-by-arc gamma analyses are performed. Pilot facilities covering a range of planning and delivery systems have performed data acquisition and upload successfully. Advantages of VESPA are (1) fast turnaround mainly driven by the facility's capability of providing the requested EPID images, (2) the possibility for facilities performing the audit in parallel, as there is no need to wait for a phantom, (3) simple and efficient credentialing for international facilities, (4) a large set of data points, and (5) a reduced impact on resources and environment as there is no need to transport heavy phantoms or audit staff. Limitations of the current implementation of VESPA for trials credentialing are that it does not provide absolute dosimetry, therefore a Level I audit is still required, and that it relies on correctly delivered open calibration fields, which are used for system calibration. The implemented EPID based IMRT and VMAT audit system promises to dramatically improve credentialing efficiency for clinical trials and wider applications.

Superior target volume and organ stability with the use of endorectal balloons in post-prostatectomy radiotherapy

INTRODUCTION

We investigated the endorectal balloon (ERB) as a method to improve post-prostatectomy clinical target volume (CTV) stability.

METHODS

Seventy cone-beam CT (CBCT) obtained during radiotherapy treatment from seven patients treated with an ERB and 68 CBCT from seven patients treated without an ERB were contoured according to published guidelines. CTV was subdivided into superior and inferior CTV; whole rectal volume was subdivided into superior and inferior rectum and anal volume. Concordance index (CI) of CBCT treatment volumes compared with planning volumes was calculated and displacements were measured.

RESULTS

Whole rectal, superior and inferior rectum and anal CI were significantly improved with the ERB by 21%, 17%, 26% and 17% respectively (P < 0.0001). Overall CTV and inferior CTV CI was improved by 4% with the ERB (overall CTV P = 0.021; Inferior CTV P < 0.0001). In the ERB cohort, average displacement for superior CTV was 0.37 cm anterior-posterior (AP) and 0.10 cm left-right (LR). Average standard deviation was 0.27 cm AP and 0.11 cm LR. Inferior CTV average displacement was 0.11 cm AP and 0.02 cm LR. Average standard deviation was 0.11 cm AP and 0.02 cm LR. In the non-ERB cohort, average displacement for superior CTV was 0.43 cm AP and 0.10 mm left-right (LR). Average standard deviation was 0.45 cm AP and 0.13 cm LR. Inferior CTV average displacement was 0.16 cm AP and 0.01 cm LR. Average standard deviation was 0.17 cm AP and 0.03 cm LR. There was no statistically significant impact of bladder filling on CTV CI in ERB patients (P = 0.551) as opposed to non-ERB patients (P = 0.0421).

CONCLUSION

ERBs in the post-prostatectomy setting resulted in increased rectal and CTV stability while negating the effects of bladder filling on CTV stability.

Dose calibration of EPIDs for segmented IMRT dosimetry

The purpose of this study was to investigate the dose response of amorphous silicon (a‐Si) electronic portal imaging devices (EPIDs) under different acquisition settings for both open jaw defined fields and segmented intensity‐modulated radiation therapy (IMRT) fields. Four different EPIDs were used. Two Siemens and one Elekta plus a standalone Perkin Elmer research EPID. Each was operated with different acquisition systems and settings. Dose response linearity was measured for open static jaw defined fields and ‘simple’ segmented IMRT fields for a range of equipment and system settings. Six ‘simple’ segmented IMRT fields were used. The segments of each IMRT field were fixed at 10×10cm2 field size with equal MU per segment, each field having a total of 20 MU. Simultaneous measurements with an ionization chamber array (ICA) and EPID were performed to separate beam and detector response characteristics. Three different pixel calibration methods were demonstrated and compared for an example ‘clinical IMRT field’. The dose response with the Elekta EPID for ‘simple’ segmented IMRT fields versus static fields agreed to within 2.5% for monitor unit (MU)≥2. The dose response for the Siemens systems was difficult to interpret due to the poor reproducibility for segmented delivery, at MU≤5, which was not observed with the standalone research EPID nor ICA on the same machine. The dose response measured under different acquisition settings and different linac/EPID combinations matched closely (≤1%), except for the Siemens EPID. Clinical IMRT EPID dosimetry implemented with the different pixel‐to‐dose calibration methods indicated that calibration at 20 MU provides equivalent results to implementing a ghosting correction model. The nonlinear dose response was consistent across both clinical EPIDs and the standalone research EPID, with the exception of the poor reproducibility seen with Siemens EPID images of IMRT fields. The nonlinear dose response was relatively insensitive to acquisition settings and appears to be primarily due to gain ghosting effects. No additional ghosting correction factor is necessary when the pixel‐to‐dose calibration factor at small MU calibration method is used.

PACS numbers: 87.53.Bn, 87.55.Qr, 87.56.Fc, 87.57.uq

Endorectal balloons in the post prostatectomy setting: do gains in stability lead to more predictable dosimetry?

PURPOSE

To perform a comparative study assessing potential benefits of endorectal-balloons (ERB) in post-prostatectomy patients.

METHOD AND MATERIALS

Ten retrospective post-prostatectomy patients treated without ERB and ten prospective patients treated with the ERB in situ were recruited. All patients received IMRT and IGRT using kilovoltage cone-beam computed tomography (kVCBCT). kVCBCT datasets were registered to the planning dataset, recontoured and the original plan recalculated on the kVCBCTs to recreate anatomical conditions during treatment. The imaging, structure and dose data were imported into in-house software for the assessment of geometric variation and cumulative equivalent uniform dose (EUD) in the two groups.

RESULTS

The difference in location (ΔCOV) for the bladder between planning and each CBCT was similar for each group. The range of mean ΔCOV for the rectum was 0.15-0.58 cm and 0.15-0.59 cm for the non-ERB and ERB groups. For superior-CTV and inferior-CTV the difference between planned and delivered D95% (mean ± SD) for the non-ERB group was 2.1 ± 6.0 Gy and -0.04 ± 0.20 Gy. While for the ERB group the difference in D95% was 8.7 ± 12.6 Gy and 0.003 ± 0.104 Gy.

CONCLUSIONS

The use of ERBs in the post-prostatectomy setting did improve geometric reproducibility of the target and surrounding normal tissues, however no improvement in dosimetric stability was observed for the margins employed.

How important is dosimetrist experience for intensity modulated radiation therapy? A comparative analysis of a head and neck case

PURPOSE

Treatment planning for IMRT is a complex process that requires additional training and expertise. The aim of this study was to compare and analyze IMRT plans generated by dosimetrists with varying levels of IMRT planning experience.

METHODS AND MATERIALS

The computed tomography (CT) data of a patient previously treated with IMRT for left tonsillar carcinoma were used. The patient's preexisting planning target volumes (PTVs) and all organs at risk were provided with the CT data set. Six dosimetrists with variable IMRT planning experience generated IMRT plans according to the department's protocol. Plan analysis included visual inspection and comparison of dose-volume histogram, conformity indices, treatment delivery efficiency, and dose delivery accuracy.

RESULTS

Visual review of the dose distribution showed that the 6 plans were comparable. However, only the 2 most experienced dosimetrists were able to meet the strict PTV aims and critical structure constraints. The least experienced dosimetrist had the worst planning outcome. Comparison of delivery efficiency showed that the number of segments, total monitor units, and treatment time increased as the IMRT planning experience decreased.

CONCLUSIONS

Dosimetrists with higher levels of IMRT planning experience produced a better quality head and neck IMRT plan. Different planning experience may need to be considered when organizing appropriate departmental resources.

Transit dosimetry in IMRT with an a-Si EPID in direct detection configuration

In this study an amorphous silicon electronic portal imaging device (a-Si EPID) converted to direct detection configuration was investigated as a transit dosimeter for intensity modulated radiation therapy (IMRT). After calibration to dose and correction for a background offset signal, the EPID-measured absolute IMRT transit doses for 29 fields were compared to a MatriXX two-dimensional array of ionization chambers (as reference) using Gamma evaluation (3%, 3 mm). The MatriXX was first evaluated as reference for transit dosimetry. The accuracy of EPID measurements was also investigated by comparison of point dose measurements by an ionization chamber on the central axis with slab and anthropomorphic phantoms in a range of simple to complex fields. The uncertainty in ionization chamber measurements in IMRT fields was also investigated by its displacement from the central axis and comparison with the central axis measurements. Comparison of the absolute doses measured by the EPID and MatriXX with slab phantoms in IMRT fields showed that on average 96.4% and 97.5% of points had a Gamma index<1 in head and neck and prostate fields, respectively. For absolute dose comparisons with anthropomorphic phantoms, the values changed to an average of 93.6%, 93.7% and 94.4% of points with Gamma index<1 in head and neck, brain and prostate fields, respectively. Point doses measured by the EPID and ionization chamber were within 3% difference for all conditions. The deviations introduced in the response of the ionization chamber in IMRT fields were<1%. The direct EPID performance for transit dosimetry showed that it has the potential to perform accurate, efficient and comprehensive in vivo dosimetry for IMRT.