Andrea McNiven

A new metric for assessing IMRT modulation complexity and plan deliverability

PURPOSE To evaluate the utility of a new complexity metric, the modulation complexity score (MCS), in the treatment planning and quality assurance processes and to evaluate the relationship of the metric with deliverability. METHODS A multisite (breast, rectum, prostate, prostate bed, lung, and head and neck) and site-specific (lung) dosimetric evaluation has been completed. The MCS was calculated for each beam and the overall treatment plan. A 2D diode array (MapCHECK, Sun Nuclear, Melbourne, FL) was used to acquire measurements for each beam. The measured and planned dose (PINNACLE3, Phillips, Madison, WI) was evaluated using different percent differences and distance to agreement (DTA) criteria (3%/ 3 mm and 2%/ 1 mm) and the relationship between the dosimetric results and complexity (as measured by the MCS or simple beam parameters) assessed. RESULTS For the multisite analysis (243 plans total), the mean MCS scores for each treatment site were breast (0.92), rectum (0.858), prostate (0.837), prostate bed (0.652), lung (0.631), and head and neck (0.356). The MCS allowed for compilation of treatment site-specific statistics, which is useful for comparing different techniques, as well as for comparison of individual treatment plans with the typical complexity levels. For the six plans selected for dosimetry, the average diode percent pass rate was 98.7% (minimum of 96%) for 3%/3 mm evaluation criteria. The average difference in absolute dose measurement between the planned and measured dose was 1.7 cGy. The detailed lung analysis also showed excellent agreement between the measured and planned dose, as all beams had a diode percentage pass rate for 3%/3 mm criteria of greater than 95.9%, with an average pass rate of 99.0%. The average absolute maximum dose difference for the lung plans was 0.7 cGy. There was no direct correlation between the MCS and simple beam parameters which could be used as a surrogate for complexity level (i.e., number of segments or MU). An evaluation criterion of 2%/ 1 mm reliably allowed for the identification of beams that are dosimetrically robust. In this study we defined a robust beam or plan as one that maintained a diode percentage pass rate greater than 90% at 2%/ 1 mm, indicating delivery that was deemed accurate when compared to the planned dose, even under stricter evaluation criterion. MCS and MU threshold criteria were determined by defining a required specificity of 1.0. A MCS threshold of 0.8 allowed for identification of robust deliverability with a sensitivity of 0.36. In contrast, MU had a lower sensitivity of 0.23 for a threshold of 50 MU. CONCLUSIONS The MCS allows for a quantitative assessment of plan complexity, on a fixed scale, that can be applied to all treatment sites and can provide more information related to dose delivery than simple beam parameters. This could prove useful throughout the entire treatment planning and QA process.

Three-dimensional dosimetry of small megavoltage radiation fields using radiochromic gels and optical CT scanning.

The dosimetry of small fields as used in stereotactic radiotherapy, radiosurgery and intensity-modulated radiation therapy can be challenging and inaccurate due to partial volume averaging effects and possible disruption of charged particle equilibrium. Consequently, there exists a need for an integrating, tissue equivalent dosimeter with high spatial resolution to avoid perturbing the radiation beam and artificially broadening the measured beam penumbra. In this work, radiochromic ferrous xylenol-orange (FX) and leuco crystal violet (LCV) micelle gels were used to measure relative dose factors (RDFs), percent depth dose profiles and relative lateral beam profiles of 6 MV x-ray pencil beams of diameter 28.1, 9.8 and 4.9 mm. The pencil beams were produced via stereotactic collimators mounted on a Varian 2100 EX linear accelerator. The gels were read using optical computed tomography (CT). Data sets were compared quantitatively with dosimetric measurements made with radiographic (Kodak EDR2) and radiochromic (GAFChromic EBT) film, respectively. Using a fast cone-beam optical CT scanner (Vista), corrections for diffusion in the FX gel data yielded RDFs that were comparable to those obtained by minimally diffusing LCV gels. Considering EBT film-measured RDF data as reference, cone-beam CT-scanned LCV gel data, corrected for scattered stray light, were found to be in agreement within 0.5% and -0.6% for the 9.8 and 4.9 mm diameter fields, respectively. The validity of the scattered stray light correction was confirmed by general agreement with RDF data obtained from the same LCV gel read out with a laser CT scanner that is less prone to the acceptance of scattered stray light. Percent depth dose profiles and lateral beam profiles were found to agree within experimental error for the FX gel (corrected for diffusion), LCV gel (corrected for scattered stray light), and EBT and EDR2 films. The results from this study reveal that a three-dimensional dosimetry method utilizing optical CT-scanned radiochromic gels allows for the acquisition of a self-consistent volumetric data set in a single exposure, with sufficient spatial resolution to accurately characterize small fields.

Clinical outcomes in patients with T4 laryngeal cancer treated with primary radiotherapy versus primary laryngectomy.

The purpose of this study was to determine the clinical outcomes of T4 laryngeal cancers.

Fully automated treatment planning for head and neck radiotherapy using a voxel-based dose prediction and dose mimicking method

Recent works in automated radiotherapy treatment planning have used machine learning based on historical treatment plans to infer the spatial dose distribution for a novel patient directly from the planning image. We present a probabilistic, atlas-based approach which predicts the dose for novel patients using a set of automatically selected most similar patients (atlases). The output is a spatial dose objective, which specifies the desired dose-per-voxel, and therefore replaces the need to specify and tune dose-volume objectives. Voxel-based dose mimicking optimization then converts the predicted dose distribution to a complete treatment plan with dose calculation using a collapsed cone convolution dose engine. In this study, we investigated automated planning for right-sided oropharaynx head and neck patients treated with IMRT and VMAT. We compare four versions of our dose prediction pipeline using a database of 54 training and 12 independent testing patients by evaluating 14 clinical dose evaluation criteria. Our preliminary results are promising and demonstrate that automated methods can generate comparable dose distributions to clinical. Overall, automated plans achieved an average of 0.6% higher dose for target coverage evaluation criteria, and 2.4% lower dose at the organs at risk criteria levels evaluated compared with clinical. There was no statistically significant difference detected in high-dose conformity between automated and clinical plans as measured by the conformation number. Automated plans achieved nine more unique criteria than clinical across the 12 patients tested and automated plans scored a significantly higher dose at the evaluation limit for two high-risk target coverage criteria and a significantly lower dose in one critical organ maximum dose. The novel dose prediction method with dose mimicking can generate complete treatment plans in 12-13 min without user interaction. It is a promising approach for fully automated treatment planning and can be readily applied to different treatment sites and modalities.

Outcomes after reirradiation for recurrent nasopharyngeal carcinoma: North American experience

The purpose of this article was to report outcomes of reirradiation for locoregionally recurrent nasopharyngeal carcinoma (NPC).

Multicenter Collaborative Quality Assurance Program for the Province of Ontario, Canada: First-Year Results

PURPOSE The objective of this work was to develop a collaborative quality assurance (CQA) program to assess the performance of intensity modulated radiation therapy (IMRT) planning and delivery across the province of Ontario, Canada. METHODS AND MATERIALS The CQA program was designed to be a comprehensive end-to-end test that can be completed on multiple planning and delivery platforms. The first year of the program included a head-and-neck (H&N) planning exercise and on-site visit to acquire dosimetric measurements to assess planning and delivery performance. A single dosimeter was used at each institution, and the planned to measured dose agreement was evaluated for both the H&N plan and a standard plan (linear-accelerator specific) that was created to enable a direct comparison between centers with similar infrastructure. RESULTS CQA program feasibility was demonstrated through participation of all 13 radiation therapy centers in the province. Planning and delivery was completed on a variety of infrastructure (treatment planning systems and linear accelerators). The planning exercise was completed using both static gantry and rotational IMRT, and planned-to-delivered dose agreement (pass rates) for 3%/3-mm gamma evaluation were greater than 90% (92.6%-99.6%). CONCLUSIONS All centers had acceptable results, but variation in planned to delivered dose agreement for the same planning and delivery platform was noted. The upper end of the range will provide an achievable target for other centers through continued quality improvement, aided by feedback provided by the program through the use of standard plans and simple test fields.

Multileaf collimator performance monitoring and improvement using semiautomated quality control testing and statistical process control

PURPOSE High-quality radiation therapy using highly conformal dose distributions and image-guided techniques requires optimum machine delivery performance. In this work, a monitoring system for multileaf collimator (MLC) performance, integrating semiautomated MLC quality control (QC) tests and statistical process control tools, was developed. The MLC performance monitoring system was used for almost a year on two commercially available MLC models. Control charts were used to establish MLC performance and assess test frequency required to achieve a given level of performance. MLC-related interlocks and servicing events were recorded during the monitoring period and were investigated as indicators of MLC performance variations. METHODS The QC test developed as part of the MLC performance monitoring system uses 2D megavoltage images (acquired using an electronic portal imaging device) of 23 fields to determine the location of the leaves with respect to the radiation isocenter. The precision of the MLC performance monitoring QC test and the MLC itself was assessed by detecting the MLC leaf positions on 127 megavoltage images of a static field. After initial calibration, the MLC performance monitoring QC test was performed 3-4 times/week over a period of 10-11 months to monitor positional accuracy of individual leaves for two different MLC models. Analysis of test results was performed using individuals control charts per leaf with control limits computed based on the measurements as well as two sets of specifications of ± 0.5 and ± 1 mm. Out-of-specification and out-of-control leaves were automatically flagged by the monitoring system and reviewed monthly by physicists. MLC-related interlocks reported by the linear accelerator and servicing events were recorded to help identify potential causes of nonrandom MLC leaf positioning variations. RESULTS The precision of the MLC performance monitoring QC test and the MLC itself was within ± 0.22 mm for most MLC leaves and the majority of the apparent leaf motion was attributed to beam spot displacements between irradiations. The MLC QC test was performed 193 and 162 times over the monitoring period for the studied units and recalibration had to be repeated up to three times on one of these units. For both units, rate of MLC interlocks was moderately associated with MLC servicing events. The strongest association with the MLC performance was observed between the MLC servicing events and the total number of out-of-control leaves. The average elapsed time for which the number of out-of-specification or out-of-control leaves was within a given performance threshold was computed and used to assess adequacy of MLC test frequency. CONCLUSIONS A MLC performance monitoring system has been developed and implemented to acquire high-quality QC data at high frequency. This is enabled by the relatively short acquisition time for the images and automatic image analysis. The monitoring system was also used to record and track the rate of MLC-related interlocks and servicing events. MLC performances for two commercially available MLC models have been assessed and the results support monthly test frequency for widely accepted ± 1 mm specifications. Higher QC test frequency is however required to maintain tighter specification and in-control behavior.

Knowledge-Based Automated Planning for Oropharyngeal Cancer

Purpose The purpose of this study was to automatically generate radiation therapy plans for oropharynx patients by combining knowledge‐based planning (KBP) predictions with an inverse optimization (IO) pipeline. Methods We developed two KBP approaches, the bagging query (BQ) method and the generalized principal component analysis‐based (gPCA) method, to predict achievable dose–volume histograms (DVHs). These approaches generalize existing methods by predicting physically feasible organ‐at‐risk (OAR) and target DVHs in sites with multiple targets. Using leave‐one‐out cross validation, we applied both models to a large dataset of 217 oropharynx patients. The predicted DVHs were input into an IO pipeline that generated treatment plans (BQ and gPCA plans) via an intermediate step that estimated objective function weights for an inverse planning model. The KBP predictions were compared to the clinical DVHs for benchmarking. To assess the complete pipeline, we compared the BQ and gPCA plans to both the predictions and clinical plans. To isolate the effect of the KBP predictions, we put clinical DVHs through the IO pipeline to produce clinical inverse optimized (CIO) plans. This approach also allowed us to estimate the complexity of the clinical plans. The BQ and gPCA plans were benchmarked against the CIO plans using DVH differences and clinical planning criteria. Iso‐complexity plans (relative to CIO) were also generated and evaluated. Results The BQ method tended to predict that less dose is delivered than what was observed in the clinical plans while the gPCA predictions were more similar to clinical DVHs. Both populations of KBP predictions were reproduced with inverse plans to within a median DVH difference of 3 Gy. Clinical planning criteria for OARs were satisfied most frequently by the BQ plans (74.4%), by 6.3% points more than the clinical plans. Meanwhile, target criteria were satisfied most frequently by the gPCA plans (90.2%), and by 21.2% points more than clinical plans. However, once the complexity of the plans was constrained to that of the CIO plans, the performance of the BQ plans degraded significantly. In contrast, the gPCA plans still satisfied more clinical criteria than both the clinical and CIO plans, with the most notable improvement being in target criteria. Conclusion Our automated pipeline can successfully use DVH predictions to generate high‐quality plans without human intervention. Between the two KBP methods, gPCA plans tend to achieve comparable performance as clinical plans, even when controlling for plan complexity, whereas BQ plans tended to underperform.

Postgraduate Global Health Competency Profile for Radiation Oncology

AIMS To identify core competencies for postgraduate radiation oncology trainees in global health and cancer that may inform revisions across radiation oncology residency specialty training curricula. MATERIALS AND METHODS A review of the literature was conducted to identify all potential global health competency items. An international two-phase Delphi process was conducted with experts in oncology. In phase 1, all experts scored, on a nine-point Likert scale, the degree to which they agreed an item should be included in the competency profile. Items with a mean score ≥7 were included, those scoring 4-6 were reviewed in phase 2, and items scored ≤3 were excluded. In phase 2, items were discussed and subsequently ranked for inclusion or exclusion in the competency profile. Items with >75% voting for inclusion were included in the final competency profile. RESULTS In total, 142 potential global health competency items were identified. Sixty-one items were removed as redundant or irrelevant, leaving 81 items for the Delphi process. Eighteen specialists were invited to participate, with 10 (56%) agreeing to participate in phase 1 of the Delphi process. Participants represented 10 centres in seven countries. Of the 81 items ranked in phase 1, 72 items (89%) had a mean score ≥7 and were automatically included in the final competency profile. The remaining nine items received a score of 4-6 and were discussed in phase 2 of the Delphi process, of which three received <75% agreement for inclusion and were excluded. The result was a final list of 78 enabling competency items. CONCLUSIONS The radiation oncology global health competency profile represents an international consensus on the items that can inform radiation oncology training requirements.

Patterns of failure in anaplastic and differentiated thyroid carcinoma treated with intensity-modulated radiotherapy

BACKGROUND The radiotherapy (rt) volumes in anaplastic (atc) and differentiated thyroid carcinoma (dtc) are controversial. METHODS We retrospectively examined the patterns of failure after postoperative intensity-modulated rt for atc and dtc. Computed tomography images were rigidly registered with the original rt plans. Recurrences were considered in-field if more than 95% of the recurrence volume received 95% of the prescribed dose, out-of-field if less than 20% received 95% of the dose, and marginal otherwise. RESULTS Of 30 dtc patients, 4 developed regional recurrence: 1 being in-field (level iii), and 3 being out-of-field (all level ii). Of 5 atc patients, all 5 recurred at 7 sites: 2 recurrences being local, and 5 being regional [2 marginal (intramuscular to the digastric and sternocleidomastoid), 3 out-of-field (retropharyngeal, soft tissues near the manubrium, and lateral to the sternocleidomastoid)]. CONCLUSIONS In dtc, locoregional recurrence is unusual after rt. Out-of-field dtc recurrences infrequently occurred in level ii. Enlarged treatment volumes to level ii must be balanced against a potentially greater risk of toxicity.