Pre-treatment verification of lung SBRT VMAT plans with delivery errors: Toward a better understanding of the gamma index analysis.

PURPOSE: To study the sensitivity of the ArcCHECK in detecting delivery errors for lung stereotactic body radiotherapy (SBRT) using the Volumetric Modulated Arc Therapy (VMAT) technique and to evaluate the sensitivity of eight global and local gamma tolerances with different cut-off percentages. METHODS: Baseline VMAT plans were generated for 15 lung SBRT patients. We delivered the smallest errors(gantry, collimator, and multileaf collimator MLC) which had ≥ ±2% dose difference in the modified treatment plans compared to the baseline plan (the clinical significance of those errors were assessed in our previous study. A total of 100 plan in which 15 baseline plans were measured using the ArcCheck detector along with ion chamber measurements. The sensitivity of the global and local gamma-index method using criteria of 1%/1 mm, 2%/1 mm, 2%/2 mm, and 3%/3 mm was investigated. RESULTS: The gamma (γ) pass rates for these plans exhibited considerable spread. The majority of simulated errors were not detected. Broadly similar detection levels were achieved with the different gamma criteria and cut-offs. Combining ion chamber measurements with ArcCHECK did not improve error detection. CONCLUSIONS: Commonly adopted gamma criteria are not sensitive enough to validate lung SBRT VMAT plans at the 2% dose difference level. The error detection levels are fairly consistent despite changes in gamma criteria and cut-offs. The choice of gamma criteria was not significant and there was no clear benefit in tightening the gamma criteria.

The impact of imaging modality (CT vs MRI) and patient position (supine vs prone) on tangential whole breast radiation therapy planning.

PURPOSE: The purpose of this study was to evaluate the impact of magnetic resonance imaging (MRI) versus computed tomography (CT)-derived planning target volumes (PTVs), in both supine and prone positions, for whole breast (WB) radiation therapy. METHODS AND MATERIALS: Four WB radiation therapy plans were generated for 28 patients in which PTVs were generated based on CT or MRI data alone in both supine and prone positions. A 6-MV tangential intensity modulated radiation therapy technique was used, with plans designated as ideal, acceptable, or noncompliant. Dose metrics for PTVs and organs at risk were compared to analyze any differences based on imaging modality (CT vs MRI) or patient position (supine vs prone). RESULTS: With respect to imaging modality 2/11 whole breast planning target volume (WB_PTV) dose metrics (percentage of PTV receiving 90% and 110% of prescribed dose) displayed statistically significant differences; however, these differences did not alter the average plan compliance rank. With respect to patient positioning, the odds of having an ideal plan versus a noncompliant plan were higher for the supine position compared with the prone position (P = .026). The minimum distance between the seroma cavity planning target volume (SC_PTV) and the chest wall was increased with prone positioning (P < .001, supine and prone values 1.1 mm and 8.7 mm, respectively). Heart volume was greater in the supine position (P = .005). Heart doses were lower in the supine position than prone (P < .01, mean doses 3.4 ± 1.55 Gy vs 4.4 ± 1.13 Gy for supine vs prone, respectively). Mean lung doses met ideal dose constraints in both positions, but were best spared in the prone position. The contralateral breast maximum dose to 1cc (D1cc) showed significantly lower doses in the supine position (P < .001, 4.64 Gy vs 9.51 Gy). CONCLUSIONS: Planning with PTVs generated from MRI data showed no clinically significant differences from planning with PTVs generated from CT with respect to PTV and doses to organs at risk. Prone positioning within this study reduced mean lung dose and whole heart volumes but increased mean heart and contralateral breast doses compared with supine.

Multi-institutional comparison of simulated treatment delivery errors in ssIMRT, manually planned VMAT and autoplan-VMAT plans for nasopharyngeal radiotherapy.

PURPOSE: To quantify the impact of simulated errors for nasopharynx radiotherapy across multiple institutions and planning techniques (auto-plan generated Volumetric Modulated Arc Therapy (ap-VMAT), manually planned VMAT (mp-VMAT) and manually planned step and shoot Intensity Modulated Radiation Therapy (mp-ssIMRT)). METHODS: Ten patients were retrospectively planned with VMAT according to three institution's protocols. Within one institution two further treatment plans were generated using differing treatment planning techniques. This resulted in mp-ssIMRT, mp-VMAT, and ap-VMAT plans. Introduced treatment errors included Multi Leaf Collimator (MLC) shifts, MLC field size (MLCfs), gantry and collimator errors. A change of more than 5% in most selected dose metrics was considered to have potential clinical impact. The original patient plan total Monitor Units (MUs) were correlated to the total number of dose metrics exceeded. RESULTS: The impact of different errors was consistent, with ap-VMAT plans (two institutions) showing larger dose deviations than mp-VMAT created plans (one institution). Across all institutions' VMAT plans the significant errors included; ±5° for the collimator angle, ±5mm for the MLC shift and +1, ±2 and ±5mm for the MLC field size. The total number of dose metrics exceeding tolerance was positively correlated to the VMAT total plan MUs (r=0.51, p<0.001), across all institutions and techniques. CONCLUSIONS: Differences in VMAT robustness to simulated errors across institutions occurred due to planning method differences. Whilst ap-VMAT was most sensitive to MLC errors, it also produced the best quality treatment plans. Mp-ssIMRT was most robust to errors. Higher VMAT treatment plan complexity led to less robust plans.

Comparison of Magnetic Resonance Imaging and Computed Tomography for Breast Target Volume Delineation in Prone and Supine Positions.

PURPOSE: To determine whether T2-weighted MRI improves seroma cavity (SC) and whole breast (WB) interobserver conformity for radiation therapy purposes, compared with the gold standard of CT, both in the prone and supine positions. METHODS AND MATERIALS: Eleven observers (2 radiologists and 9 radiation oncologists) delineated SC and WB clinical target volumes (CTVs) on T2-weighted MRI and CT supine and prone scans (4 scans per patient) for 33 patient datasets. Individual observer's volumes were compared using the Dice similarity coefficient, volume overlap index, center of mass shift, and Hausdorff distances. An average cavity visualization score was also determined. RESULTS: Imaging modality did not affect interobserver variation for WB CTVs. Prone WB CTVs were larger in volume and more conformal than supine CTVs (on both MRI and CT). Seroma cavity volumes were larger on CT than on MRI. Seroma cavity volumes proved to be comparable in interobserver conformity in both modalities (volume overlap index of 0.57 (95% Confidence Interval (CI) 0.54-0.60) for CT supine and 0.52 (95% CI 0.48-0.56) for MRI supine, 0.56 (95% CI 0.53-0.59) for CT prone and 0.55 (95% CI 0.51-0.59) for MRI prone); however, after registering modalities together the intermodality variation (Dice similarity coefficient of 0.41 (95% CI 0.36-0.46) for supine and 0.38 (0.34-0.42) for prone) was larger than the interobserver variability for SC, despite the location typically remaining constant. CONCLUSIONS: Magnetic resonance imaging interobserver variation was comparable to CT for the WB CTV and SC delineation, in both prone and supine positions. Although the cavity visualization score and interobserver concordance was not significantly higher for MRI than for CT, the SCs were smaller on MRI, potentially owing to clearer SC definition, especially on T2-weighted MR images.

A phantom assessment of achievable contouring concordance across multiple treatment planning systems.

In this paper, the highest level of inter- and intra-observer conformity achievable with different treatment planning systems (TPSs), contouring tools, shapes, and sites have been established for metrics including the Dice similarity coefficient (DICE) and Hausdorff Distance. High conformity values, e.g. DICE(Breast_Shape)=0.99±0.01, were achieved. Decreasing image resolution decreased contouring conformity.

Australian survey on current practices for breast radiotherapy.

INTRODUCTION: Detailed, published surveys specific to Australian breast radiotherapy practice were last conducted in 2002. More recent international surveys specific to breast radiotherapy practice include a European survey conducted in 2008/2009 and a Spanish survey conducted in 2009. Radiotherapy techniques continue to evolve, and the utilisation of new techniques, such as intensity-modulated radiation therapy (IMRT), is increasing. This survey aimed to determine current breast radiotherapy practices across Australia. METHOD: An online survey was completed by 50 of the 69 Australian radiation therapy treatment centres. RESULTS: Supine tangential beam whole breast irradiation remains the standard of care for breast radiotherapy in Australia. A growing number of institutions are exploring prone positioning and IMRT utilisation. This survey demonstrated a wide variation in the benchmarks used to limit and report organ at risk doses, prescribed dose regimen, and post-mastectomy bolus practices. This survey also indicated, when compared with international literature, that there may be less interest in or uptake of external beam partial breast irradiation, prone positioning, simultaneous integrated boost and breath hold techniques. These are areas where further review and research may be warranted to ensure Australian patients are receiving the best care possible based on the best evidence available. CONCLUSION: This survey provides insight into the current radiotherapy practice for breast cancer in Australia.

Spatial dispersion in three-dimensional drawn magnetic metamaterials.

We characterize spatial dispersion in longitudinally invariant drawn metamaterials with a magnetic response at terahertz frequencies, whereby a change in the angle of the incident field produces a shift in the resonant frequency. We present a simple analytical model to predict this shift. We also demonstrate that the spatial dispersion is eliminated by breaking the longitudinal invariance using laser ablation. The experimental results are in agreement with numerical simulations.

Stacked-and-drawn metamaterials with magnetic resonances in the terahertz range.

We present a novel method for producing drawn metamaterials containing slotted metallic cylinder resonators, possessing strong magnetic resonances in the terahertz range. The resulting structures are either spooled to produce a 2-dimensional metamaterial monolayer, or stacked to produce three-dimensional multi-layered metamaterials. We experimentally investigate the effects of the resonator size and number of metamaterial layers on transmittance, observing magnetic resonances between 0.1 and 0.4 THz, in good agreement with simulations. Such fibers promise future applications in mass-produced stacked or woven metamaterials.