David P. Spencer

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. PACS number: 87.53.Ly

Quality assurance using a photodiode array

Improved treatment techniques in radiation therapy provide incentive to reduce treatment margins, thereby increasing the necessity for more accurate geometrical setup of the linear accelerator and accompanying components. In the present paper, we describe the development of a novel device that enables precise and automated measurement of geometric parameters for the purpose of improving initial setup accuracy, and for standardizing repeated quality control activities. The device consists of a silicon photodiode array, an evaluation board, a data acquisition card, and a laptop. Measurements that demonstrate the utility of the device are also presented. Using the device, we show that the radiation light field congruence for both 6 and 15 MV beams is within 1.3 mm. The maximum measured disagreement between radiation field edges and light field edges was 1.290±0.002u2009mm, while the smallest disagreement between the light field and radiation field edge was 0.016±0.003u2009mm. Because measurements are automated, ambiguities resulting from interobserver variability are removed, greatly improving the reproducibility of measurements across observers. We expect the device to find use in consistency measurements on linear accelerators used for stereotactic radiosurgery, during the commissioning of new linear accelerators, or as an alternative to film or other commercially available devices for performing monthly or annual quality control checks. PACS numbers: 87.55.Qr, 87.56.Fc, 87.57.N‐, 87.15mn, 87.15mq

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.

A framework for clinical commissioning of 3D‐printed patient support or immobilization devices in photon radiotherapy

Abstract Purpose The objective of this work is to outline a framework for dosimetric characterization that will comprehensively detail the clinical commissioning steps for 3D‐printed materials applied as patient support or immobilization devices in photon radiotherapy. The complex nature of 3D‐printed materials with application to patient‐specific configurations requires careful consideration. The framework presented is generalizable to any 3D‐printed object where the infill and shell combinations are unknown. Methods A representative cylinder and wedge were used as test objects to characterize devices that may be printed of unknown, patient‐specific dimensions. A case study of a 3D‐printed CSI immobilization board was presented as an example of an object of known, but adaptable dimensions and proprietary material composition. A series of measurements were performed to characterize the materials kV radiologic properties, MV attenuation measurements and calculations, energy spectrum water equivalency, and surface dose measurements. These measurements complement the recommendations of the AAPMs TG176 to characterize the additional complexity of 3D‐printed materials for use in a clinical radiotherapy environment. Results The dosimetric characterization of 3D‐printed test objects and a case study device informed the development of a step‐by‐step template that can easily be followed by clinicians to accurately and safely utilize 3D‐printed materials as patient‐specific support or immobilization devices. Conclusions A series of steps is outlined to provide a formulaic approach to clinically commission 3D‐printed materials that may possess varying material composition, infill patterns, and patient‐specific dimensions.

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 Symbol, 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). Symbol. No Caption available. 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.

SU‐E‐T‐548: Retrospective Evaluation of Stereotactic Radiosurgery Plans at the Tom Baker Cancer Centre

Purpose: This study summarizes the stereotactic radiosurgery(SRS) plans created over a period of five years. Additionally, factors that influenced the Radiation TherapyOncology Group (RTOG) conformity indices of the plans were determined. Methods: A database of 170 SRS plans for brain metastases patients treated at the Tom Baker Cancer Centre using dynamic conformal arcs between November 2004 and December 2009 was constructed. This database was then used to retrospectively evaluate SRS plans and to determine target parameters that affect RTOG conformity index which is defined by CI= VRI/TV where VRI is the volume that receives the reference isodose and TV is the target volume. Results: Factors such as maximum dose to the target and minimum dose to the target did not appear to influence RTOG conformity index. However, target volume did relate to RTOG conformity index: conformity index increased as target volume decreased. For the 101 plans with target volumes larger than 1cm3 the average conformity index value was 1.68. For plans with target volumes smaller than 1cm3 the average conformity index value was 2.41. Additionally, a decrease in RTOG conformity index was correlated with a decrease in the volume of normal tissue receiving the prescription isodose. For plans where the volume of normal tissue receiving the prescription isodose was less than 1 cm3 the average RTOG conformity index was 1.32 while for plans where the volume of normal tissue receiving the prescription isodose was greater than 1 cm3 the average RTOG conformity index was 2.36 Conclusions: 170 SRS plans for brain metastases were evaluated and it was found that RTOG conformity index was influenced by target size and could be linked to the volume of non target tissue receiving the prescription isodose

Poster — Thur Eve — 07: Measurement of Radiation‐Light Congruence Using a Photodiode Array

Many factors affect treatment delivery, including setup errors during the simulation process, dose calculation uncertainty, patient setup errors during treatment, and errors that result from incorrect calibration and geometric setup of the linear accelerator. Among these, the radiation‐light congruence is especially important because the light field is used to simulate the radiation beam. The light field is an integral part of patient setup and jaw calibration, thus radiation‐light congruence is essential for accurate treatment delivery. We have developed a novel device that enables precise and automated measurement of radiation‐light congruence. Using the device, we show that the radiation‐light field congruence for both 6 and 15 MV beams is within standard tolerance limits. However, our results indicate that radiation‐light congruence is dependent on collimator angle and energy. The maximum measured disagreement between radiation field edge and light field edge was 1.290 ± 0.004 mm at collimator angle 270 (X1, 15 MV) and 0.932 ± 0.003 mm at collimator angle 90 (Y2, 15MV). The minimum disagreement was 0.016 ± 0.003 mm for 270 (X2, 6 MV) and 0.102 ± 0.004 mm for 90 (X2, 6MV). This detector and measurement method will give us a better understanding of the radiation‐light congruence dependence on collimator angle and energy. It could also be used to determine location of the x‐ray source within the linear accelerator.

SU-EE-A4-01: Regional Change in Brain Perfusion, in Irradiated Normal Tissue: Correlation Study Between Perfusion MRI and Spatial Distribution of Radiation Dose Delivered

Purpose: When irradiating the normal brain, one of the principal causes of complications is damage to the cerebral vasculature, particularly the micro‐vasculature. This is of particular concern when treating diseases which are slow‐growing or are not malignant. While there is some data relating loss of perfusion to radiation dose in other tissues such as lung, there is very little data for the brain. We intend to determine the relationship between the change due to irradiation in hemodynamic measures, such as relative regional Mean Transit Time (rrMTT) and relative regional Cerebral Blood Volume (rrCBV), and the radiation dose delivered to the normal brain tissue. Method and Materials: We acquired data in the form of perfusion weighted images (PWI) for two patients. We used a 3.0 T magnet at the Seaman Family MRI Centre at the Foothills Medical Centre and a single‐shot echo‐planar imaging (EPI) sequence following the injection of a paramagnetic contrast agent (Gd‐DTPA‐Magnevist; Berlex, Wayne, NJ). These images have been processed to yield rrCBV and rrMTT. The patients had previously been treated with surgery, but had received no chemotherapy. Results: Our preliminary results show that with a follow‐up time of 4 months after receiving approximately 5000 cGy/25 fractions, in normal brain tissue the rrMTT is reduced by approximately 2% (1.8 ± 0.5) and the rrCBV is reduced by almost 12% (11.8 ± 2.2). The tumor showed reductions in both rrMTT (1.2 ± 1.5)% and rrCBV (19.9 ± 3.5)%. It is expected that these changes will increase with longer term follow‐up. Conclusion: Perfusion Weighted MR Imaging can be used to assess the change in hemodynamic measures in the normal brain tissue after radiotherapy.

Sci‐YIS Fri ‐ 08: Regional change in brain perfusion after fractionated stereotactic radiotherapy (FSRT) at 4 months and 3 years follow‐up

Purpose: The change in hemodynamic parameters, such as mean transit time and cerebral blood volume, reflect the damage to vasculature. A relationship between the change in hemodynamic parameters and radiationdose delivered would help predict the degree and nature of damage, and would be most beneficial for patients with a long life‐expectancy who are at risk of long‐term radiation‐induced injury. Method and Materials: We applied the relative perfusion weighted MRI technique, currently used in strokeimaging, to calculate the relative regional mean transit time (rrMTT) and relative regional cerebral blood volume (rrCBV). We acquired data for one patient. We used a 3.0 T magnet at the Seaman Family MRI Centre in Calgary and a single‐shot echo‐planar imaging (EPI) sequence following the injection of a paramagnetic contrast agent (Gd‐DTPA‐Magnevist; Berlex, Wayne, NJ). These images have been processed to yield rrMTT and rrCBV. The patient had previously been treated with surgery, but had received no chemotherapy. The percentage change in rrMTT and rrCBV was correlated to the spatial distribution of radiationdose delivered using the Pinnacle® radiation treatment planning system. Results: Our preliminary results show that with a follow‐up time of 4 months and 3 years after receiving approximately 5000 cGy/25 fractions, rrMTT and rrCBV change significantly in normal tissue and tumour. The most important normal tissue changes occur in the near‐target area. Conclusion: The change in rrMTT and rrCBV indicates response to treatment. Perfusion weighted MRI can be used to assess the change in hemodynamic measures after radiotherapy.