Evaluation of stereotactic radiosurgery conformity indices for 170 target volumes in patients with brain metastases.

A database of clinically approved stereotactic radiosurgery treatment plans was created. One hundred and seventy targets in the database were then retrospectively evaluated using conformity indices suggested by RTOG, SALT-Lomax and Paddick. Relationships between the three alternative conformity indices were determined. The Paddick index combines the information provided by the RTOG and SALT-Lomax indices into a single index. The variation in the geometric overlap ratio, which is related to the SALT-Lomax index, was found to be not clinically relevant for our cohort of patients, and thus the Paddick and RTOG indices can be directly related. It was found that access to a dose volume histogram or dose distribution for a treatment plan renders the RTOG conformity index sufficient for plan quality evaluation.

Technical Note: Empirical altitude correction factors for well chamber measurements of permanent prostate and breast seed implant sources.

PURPOSE: Previous studies in the literature have measured an altitude effect for low-energy brachytherapy seeds; a correction factor applied in addition to PTP to account for the breakdown of Bragg-Gray cavity theory at low energies in well-type ionization chambers. In clinical practice, many centers use altitude correction factors that are not seed-model-specific. The purpose of this work is to present altitude correction factors for several seed models without documented factors in the literature. METHODS: An in-house constructed pressure vessel was used with a well-type ionization chamber to measure the air-kerma strength of the IsoAid Advantage (Pd-103), Theragenics AgX100 (I-125), and Nucletron selectSeed (I-125) at a pressure range representative of those encountered worldwide. The TheraSeed 200 (Pd-103) was also measured for comparison to the originally published correction factor for validation of the experimental process. When correction factors derived in this work were within experimental uncertainties of those published, no new correction factors were proposed. RESULTS: The three seed models measured herein all demonstrated a similar response to change in pressure as previously documented in the literature with the HDR 1000 Plus well-type ionization chamber. Correction factors of the functional form PA=k1(P[torr])k2, consistent with those previously published, were found to be appropriate for these seed models. A new correction factor is proposed for the Theragenics AgX100 and Nucletron selectSeed (k1  = 0.0417, k2  = 0.479). The IsoAid Advantage, however, agreed to within uncertainty with the published altitude correction factor for the TheraSeed 200; thus the application of the same correction factor is appropriate (k1  = 0.0241, k2  = 0.562). CONCLUSIONS: This work presents altitude correction factors for three permanent implant brachytherapy seed models in clinical use. This will allow clinics to utilize model-specific factors, reducing systematic errors in their air-kerma strength verifications.

Functional CT in lung with a conventional scanner: simulations and sampling considerations.

Due to rapid transit times, motion artefacts from breathing and the low signal intensity, functional computed tomography (f-CT) studies in lung tissue remain challenging with conventional CT scanners. The purpose of this study is to examine the accuracy of parameter estimates when performing deconvolution analysis with signals from lung tissue. The effects of partial volume averaging in lung tissue, differing transit times, variable vascular and capillary responses, expected noise levels, differing sampling rate and durations were simulated on a computer. Deconvolution using singular-value decomposition (SVD) analysis was performed for realistic lung signals using published and measured values of the arterial input and noise levels. The accuracy, bias and variance of the estimated residue functions and their associated parameter estimates were evaluated. We find that f-CT signals may be measured and analysed using SVD and other deconvolution approaches. Functional CT signals in the lung may be analysed provided that the rise and fall of the tissue and input curves are well sampled (regardless of sampling rate) and noise levels in the lung ROI tissue are approximately 20 HU or less, even for regions of interest that are mostly occupied by air. Estimates of the mean tissue transit time (MTT) are insensitive to air volume. Other decovolution methods such as fast Fourier transform methods provide more accurate estimates of PBF, whereas SVD approaches provide more accurate estimates of pulmonary blood volume and MTT. F-CT of the lung with a conventional scanner should be possible, when the extra dose is not a consideration.

The effect of contouring variability on dosimetric parameters for brain metastases treated with stereotactic radiosurgery.

PURPOSE: To quantify the effect of contouring variation on stereotactic radiosurgery plan quality metrics for brain metastases. METHODS AND MATERIALS: Fourteen metastases, each contoured by 8 physicians, formed the basis of this study. A template-based dynamic conformal 5-arc dose distribution was developed for each of the 112 contours, and each dose distribution was applied to the 7 other contours in each patient set. Radiation Therapy Oncology Group (RTOG) plan quality metrics and the Paddick conformity index were calculated for each of the 896 combinations of dose distributions and contours. RESULTS: The ratio of largest to smallest contour volume for each metastasis varied from 1.25 to 4.47, with a median value of 1.68 (n=8). The median absolute difference in RTOG conformity index between the value for the reference contour and the values for the alternative contours was 0.35. The variation of the range of conformity index for all contours for a given tumor varied with the tumor size. CONCLUSIONS: The high degree of interobserver contouring variation strongly suggests that peer review or consultation should be adopted to standardize tumor volume prescription. Observer confidence was not reflected in contouring consistency. The impact of contouring variability on plan quality metrics, used as criteria for clinical trial protocol compliance, was such that the category of compliance was robust to interobserver effects only 70% of the time.

Image-Guided Treatment of Fractionated Stereotactic Radiotherapy Patients: A Quantitative Analysis of Pre- and Post-Treatment Orthogonal kV Images of Patients Immobilized with Thermoplastic Masks.

OBJECTIVES: To measure setup and intrafraction variability for intracranial targets during treatment of patients immobilized with a Brainlab, Inc. thermoplastic head mask using ExacTrac© imaging on the treatment unit. PATIENTS AND METHODS: Between November 2007 and June 2008, 12 patients were treated with cranial fractionated stereotactic radiation therapy for 25-28 fractions. Patient offset was calculated by the ExacTrac© algorithm from the planning computed tomography and orthogonal kV images taken at the time of treatment. Shifts of the patient position were done when the initial offset was greater than 1.5 mm. If a shift was made, post-shift images were acquired before treatment. Images were also acquired post-treatment to determine intrafraction movement. RESULTS: A total of 849 pairs of images were obtained. The averaged results of all 12 patients were: setup offset was 1.98 mm, treatment offset was 0.57 mm, and post-treatment offset was 0.66 mm. Of 303 fractions treated, 30 fractions had a post-treatment offset greater than 1.5 mm. CONCLUSION: Quantitative analysis of the data set including variability in patient setup and intrafraction motion shows patient position can be reproduced within 1.0 mm using Brainlab thermoplastic head mask systems using the ExacTrac© system.