COMP Report: An updated algorithm to estimate medical physics staffing levels for radiation oncology.

PURPOSE: Medical physics staffing models require periodic review due to the rapid evolution of technology and clinical techniques in radiation oncology. We present an update to a grid-based physics staffing algorithm for radiation oncology (originally published in 2012) that has been widely used in Canada over the last decade. MATERIALS AND METHODS: The physics staffing algorithm structure was modified to improve the clarity and consistency of input data. We collected information on clinical procedures, equipment inventory, and teaching activities from 15 radiation treatment centers in the province of Ontario from April 1, 2018, to March 31, 2019. Using these data sets, the algorithm's weighting parameters were adjusted to align the prediction of full-time equivalent (FTE) personnel with actual staffing levels in Ontario. The algorithm computes FTE estimates for medical physicists, physics assistants, engineering (electrical and mechanical), and information technology (IT) support. The performance of the algorithm was also tested in eight Canadian cancer centers outside of Ontario. RESULTS: The mean difference between the algorithm and actual staffing for the 23 Canadian cancer centers did not exceed 0.5 FTE for any staffing group. The results were slightly better in Ontario than in other provinces, as expected since the algorithm was optimized using Ontario data. There was a linear correlation between the algorithm predictions and the number of annual-treated cases for physicists, and physicists plus physics assistants. For other staff categories, the algorithm weighting parameters were not significantly altered, except for a reduction in mechanical engineering staff. Comparison with other published models suggests that the updated algorithm should be considered as a minimum recommended staffing level for the clinical support of radiation oncology programs. CONCLUSIONS: We support the use of grid-based physics staffing algorithms that account for clinical workload with flexibility to adapt to local conditions with variable academic and research demands.

Hypofractionated accelerated radiotherapy using concomitant intensity-modulated radiotherapy boost technique for localized high-risk prostate cancer: acute toxicity results.

PURPOSE: To evaluate the acute toxicities of hypofractionated accelerated radiotherapy (RT) using a concomitant intensity-modulated RT boost in conjunction with elective pelvic nodal irradiation for high-risk prostate cancer. METHODS AND MATERIALS: This report focused on 66 patients entered into this prospective Phase I study. The eligible patients had clinically localized prostate cancer with at least one of the following high-risk features (Stage T3, Gleason score >or=8, or prostate-specific antigen level >20 ng/mL). Patients were treated with 45 Gy in 25 fractions to the pelvic lymph nodes using a conventional four-field technique. A concomitant intensity-modulated radiotherapy boost of 22.5 Gy in 25 fractions was delivered to the prostate. Thus, the prostate received 67.5 Gy in 25 fractions within 5 weeks. Next, the patients underwent 3 years of adjuvant androgen ablative therapy. Acute toxicities were assessed using the Common Terminology Criteria for Adverse Events, version 3.0, weekly during treatment and at 3 months after RT. RESULTS: The median patient age was 71 years. The median pretreatment prostate-specific antigen level and Gleason score was 18.7 ng/L and 8, respectively. Grade 1-2 genitourinary and gastrointestinal toxicities were common during RT but most had settled at 3 months after treatment. Only 5 patients had acute Grade 3 genitourinary toxicity, in the form of urinary incontinence (n = 1), urinary frequency/urgency (n = 3), and urinary retention (n = 1). None of the patients developed Grade 3 or greater gastrointestinal or Grade 4 or greater genitourinary toxicity. CONCLUSION: The results of the present study have indicated that hypofractionated accelerated RT with a concomitant intensity-modulated RT boost and pelvic nodal irradiation is feasible with acceptable acute toxicity.

A study of tumor motion management in the conformal radiotherapy of lung cancer.

PURPOSE: To assess the benefit derived from the reduction of planning target volumes (PTVs) afforded by tumor motion management in treatment planning for lung cancer. METHODS: We use a simple formula that combines measurements of tumor motion and set-up error for 7 patients to determine PTVs based on the following scenarios: standard uniform 15 mm margin, individualized PTVs (no gating), spirometry-based gating, and active breath-control (ABC). We compare the percent volumes of lung receiving at least 20 Gy (V20) for a standard prescription, and the maximum tolerated doses (MTDs) at fixed V20. In anticipation of improvements in set-up accuracy, we repeat the analysis assuming a reduced set-up margin of 3mm. RESULTS: Relative to the standard, the average percent reductions in V20 (+/- 1 standard deviation) for the ungated and gated scenarios are 17+/-5 and 21+/-8; the percent gains in MTD are 25+/-12 and 33+/-11, respectively. For the 3mm set-up margin, the corresponding results for V20 are 28+/-7 and 36+/-7, and for MTD are 57+/-23 and 79+/-31. CONCLUSIONS: Any form of motion management provides a benefit over the use of a standard margin. The benefit derived from gating compared to the use of ungated individualized PTVs increases with tumor mobility but is generally modest. While motion management may benefit patients with highly mobile tumors, we expect efforts to reduce set-up error to be of greater overall significance. The practical limit for lung PTV margins is likely around 4-5mm, provided set-up error can be reduced sufficiently.

Prediction of lung tumour position based on spirometry and on abdominal displacement: accuracy and reproducibility.

BACKGROUND AND PURPOSE: A simulation investigating the accuracy and reproducibility of a tumour motion prediction model over clinical time frames is presented. The model is formed from surrogate and tumour motion measurements, and used to predict the future position of the tumour from surrogate measurements alone. PATIENTS AND METHODS: Data were acquired from five non-small cell lung cancer patients, on 3 days. Measurements of respiratory volume by spirometry and abdominal displacement by a real-time position tracking system were acquired simultaneously with X-ray fluoroscopy measurements of superior-inferior tumour displacement. A model of tumour motion was established and used to predict future tumour position, based on surrogate input data. The calculated position was compared against true tumour motion as seen on fluoroscopy. Three different imaging strategies, pre-treatment, pre-fraction and intrafractional imaging, were employed in establishing the fitting parameters of the prediction model. The impact of each imaging strategy upon accuracy and reproducibility was quantified. RESULTS: When establishing the predictive model using pre-treatment imaging, four of five patients exhibited poor interfractional reproducibility for either surrogate in subsequent sessions. Simulating the formulation of the predictive model prior to each fraction resulted in improved interfractional reproducibility. The accuracy of the prediction model was only improved in one of five patients when intrafractional imaging was used. CONCLUSIONS: Employing a prediction model established from measurements acquired at planning resulted in localization errors. Pre-fractional imaging improved the accuracy and reproducibility of the prediction model. Intrafractional imaging was of less value, suggesting that the accuracy limit of a surrogate-based prediction model is reached with once-daily imaging.

Correlation of lung tumor motion with external surrogate indicators of respiration.

PURPOSE: To assess the correlation of respiratory volume and abdominal displacement with tumor motion as seen with X-ray fluoroscopy. Measurements throughout the patient's treatment course allowed an assessment of the interfractional reproducibility of this correlation. METHODS AND MATERIALS: Data were acquired from 11 patients; 5 were studied over multiple days. Measurements of respiratory volume by spirometry and abdominal displacement by a real-time position tracking system were correlated to simultaneously acquired X-ray fluoroscopy measurements of superior-inferior tumor displacement. The linear correlation coefficient was computed for each data acquisition. The phase relationship between the surrogate and tumor signals was estimated through cross-correlation delay analysis. RESULTS: Correlation coefficients ranged from very high to very low (0.99-0.39, p < 0.0001). The correlation between tumor displacement and respiratory volume was higher and more reproducible from day to day than between tumor displacement and abdominal displacement. A nonzero phase relationship was observed in nearly all patients (-0.65 to +0.50 s). This relationship was observed to vary over inter- and intrafractional time scales. Only 1 of 5 patients studied over multiple days had a consistent relationship between tumor motion and either surrogate. CONCLUSIONS: Respiratory volume has a more reproducible correlation with tumor motion than does abdominal displacement. If forming a tumor-surrogate prediction model from a limited series of observations, the use of surrogates to guide treatment might result in geographic miss.

Reproducibility of lung tumor position and reduction of lung mass within the planning target volume using active breathing control (ABC).

PURPOSE: The active breathing control (ABC) device allows for temporary immobilization of respiratory motion by implementing a breath hold at a predefined relative lung volume and air flow direction. The purpose of this study was to quantitatively evaluate the ability of the ABC device to immobilize peripheral lung tumors at a reproducible position, increase total lung volume, and thereby reduce lung mass within the planning target volume (PTV). MATERIALS AND METHODS: Ten patients with peripheral non-small-cell lung cancer tumors undergoing radiotherapy had CT scans of their thorax with and without ABC inspiration breath hold during the first 5 days of treatment. Total lung volumes were determined from the CT data sets. Each peripheral lung tumor was contoured by one physician on all CT scans to generate gross tumor volumes (GTVs). The lung density and mass contained within a 1.5-cm PTV margin around each peripheral tumor was calculated using CT numbers. Using the center of the GTV from the Day 1 ABC scan as the reference, the displacement of subsequent GTV centers on Days 2 to 5 for each patient with ABC applied was calculated in three dimensions. RESULTS: With the use of ABC inspiration breath hold, total lung volumes increased by an average of 42%. This resulted in an average decrease in lung mass of 18% within a standard 1.5-cm PTV margin around the GTV. The average (+/- standard deviation) displacement of GTV centers with ABC breath hold applied was 0.3 mm (+/- 1.8 mm), 1.2 mm (+/- 2.3 mm), and 1.1 mm (+/- 3.5 mm) in the lateral direction, anterior-posterior direction, and superior-inferior direction, respectively. CONCLUSIONS: Results from this study indicate that there remains some inter-breath hold variability in peripheral lung tumor position with the use of ABC inspiration breath hold, which prevents significant PTV margin reduction. However, lung volumes can significantly increase, thereby decreasing the mass of lung within a standard PTV.

Digital fluoroscopy to quantify lung tumor motion: potential for patient-specific planning target volumes.

PURPOSE: To apply digital fluoroscopy integrated with CT simulation to measure lung tumor motion and aid in the quantification of individualized planning target volumes. METHODS AND MATERIALS: A flat panel digital fluoroscopy unit was modified and integrated with a CT simulator. The stored fluoroscopy images were overlaid with digitally reconstructed radiographs, allowing measurement of the observed lung tumor motion in relation to the corresponding contours on the static digitally reconstructed radiographs. CT simulation and digital fluoroscopy was performed on 10 patients with non-small-cell lung cancer. Actual tumor motion was measured in three dimensions using the overlaid images. RESULTS: Combining the dynamic data with digitally reconstructed radiographs allowed the tumor shadow from the fluoroscopy to be tracked in relation to the CT lung tumor contour. For all patients, the extent of tumor motion in three dimensions was unique. The motion was greatest in the superoinferior direction and minimal in the AP and lateral directions. CONCLUSION: We have developed a tool that allows CT simulation to be combined with digital fluoroscopy. Quantitative evaluation of the tumor motion in relation to the CT plan allows for customization of the planning target volume. The variability observed clearly demonstrates the need to generate patient-specific internal motion margins.

Integration of digital fluoroscopy with CT-based radiation therapy planning of lung tumors.

Radiation dose escalation may be a means to increase the local control rate of inoperable lung tumors. Treatment plans involve the creation of a uniform planning target volume (PTV) to ensure proper coverage despite patient breathing and setup error. This may lead to unnecessary radiation of normal tissue in shallow breathers or target underdosing for patients with excess internal motion. Therefore, the nature of tumor motion for each patient should be measured in 3D, something that cannot be done with CT alone. We have developed a method that acquires 2D real-time fluoroscopic images (loops) and coregisters them with 2D digitally reconstructed radiographs (DRR) formed from the CT scan. The limitations of CT to encompass motion can be overcome by merging the two modalities together. The accuracy of the coregistration method is tested with a stationary grid of radio-opaque markers at various spatial positions. The in-plane (at-depth) displacement between markers on the fluoroscopic image versus the DRR varies with position across the image due to slight misalignments between the x-ray source used in fluoroscopy and the virtual source used for the DRR relative to the test object. At clinically relevant positions, the maximum, measured in-plane displacement, is 1.1 mm. The method is applied to the thorax of an anthropomorphic phantom and a good fit is observed between the appearances of the bony anatomical structures on the coregistered image. Finally, a series of motion measurements are carried out on two oscillating cylindrical objects. The degree of motion as measured by fluoroscopy is accurate to within 1.0 mm, whereas the DRR is inconsistent in predicting motion. The coregistration of fluoroscopic loops with the DRR shows at what point within the oscillation the DRR fails to encompass motion. For any treatment site involving target motion, this real-time imaging is a useful asset in the planning stage.