Sarah Quirk

External respiratory motion analysis and statistics for patients and volunteers

We analyzed a large patient and volunteer study of external respiratory motion in order to develop a population database of respiratory information. We analyzed 120 lung, liver, and abdominal patients and 25 volunteers without lung disease to determine the extent of motion using the Varian Real‐Time Position Management system. The volunteer respiratory motion was measured for both abdominal and thoracic placement of the RPM box. Evaluation of a subset of 55 patients demonstrates inter‐ and intrafraction variation over treatment. We also calculated baseline drift and duty cycle for patients and volunteers. The mean peak‐to‐peak amplitude (SD) for the patients was 1.0 (0.5) cm, and for the volunteers it was abdomen 0.8 (0.3) cm and thoracic 0.2 (0.2) cm. The mean period (SD) was 3.6 (1.0) s, 4.2 (1.1) s, and 4.1 (0.8) s, and the mean end exhale position (SD) was 60% (6), 58% (7), and 56% (7) for patient, volunteer abdomen, and volunteer thoracic, respectively. Baseline drift was greater than 0.5 cm for 40% of patients. We found statistically significant differences between the patient and volunteer groups. Peak‐to‐peak amplitude was significantly larger for patients than the volunteer abdominal measurement and the volunteer abdominal measurement is significantly larger than the volunteer thoracic measurement. The patient group also exhibited significantly larger baseline drift than the volunteer group. We also found that peak‐to‐peak amplitude was the most variable parameter for both intra‐ and interfraction motion. This database compilation can be used as a resource for expected motion when using external surrogates in radiotherapy applications. PACS number: 87.19.Wx, 87.55.Km

Using cone-beam CT projection images to estimate the average and complete trajectory of a fiducial marker moving with respiration

Stereotactic body radiotherapy of lung cancer often makes use of a static cone-beam CT (CBCT) image to localize a tumor that moves during the respiratory cycle. In this work, we developed an algorithm to estimate the average and complete trajectory of an implanted fiducial marker from the raw CBCT projection data. After labeling the CBCT projection images based on the breathing phase of the fiducial marker, the average trajectory was determined by backprojecting the fiducial position from images of similar phase. To approximate the complete trajectory, a 3D fiducial position is estimated from its position in each CBCT project image as the point on the source-image ray closest to the average position at the same phase. The algorithm was tested with computer simulations as well as phantom experiments using a gold seed implanted in a programmable phantom capable of variable motion. Simulation testing was done on 120 realistic breathing patterns, half of which contained hysteresis. The average trajectory was reconstructed with an average root mean square (rms) error of less than 0.1 mm in all three directions, and a maximum error of 0.5 mm. The complete trajectory reconstruction had a mean rms error of less than 0.2 mm, with a maximum error of 4.07 mm. The phantom study was conducted using five different respiratory patterns with the amplitudes of 1.3 and 2.6 cm programmed into the motion phantom. These complete trajectories were reconstructed with an average rms error of 0.4 mm. There is motion information present in the raw CBCT dataset that can be exploited with the use of an implanted fiducial marker to sub-millimeter accuracy. This algorithm could ultimately supply the internal motion of a lung tumor at the treatment unit from the same dataset currently used for patient setup.

External respiratory motion: Shape analysis and custom realistic respiratory trace generation

PURPOSE The authors developed a realistic respiratory trace generating (RTG) tool for use with phantom and simulation studies. METHODS The authors analyzed the extent of abdominal wall motion from a real-time position management system database comprised of 125 lung, liver, and abdominal patients to determine the shape and extent of motion. Using Akaikes information criterion (AIC), the authors compared different model types to find the optimal realistic model of respiratory motion. RESULTS The authors compared a family of sigmoid curves and determined a four parameter sigmoid fit was optimal for over 98% patient inhale and exhale traces. This fit was also better than sin (2)(x) for 98% of patient exhale and 70% of patient inhale traces and better than sin (x) for 100% of both patient inhale and exhale traces. This analysis also shows that sin (2)(x) is better than sin (x) for over 95% of patient inhale and exhale traces. With results from shape and extent of motion analysis, we developed a realistic respiratory trace generating (RTG) software tool. The software can be run in two modes: population and user defined. In population mode, the RTG draws entirely from the population data including inter- and intra fraction amplitude and period variability and baseline drift. In user-defined mode, the user customizes the respiratory parameters by inputting the peak-to-peak amplitude, period, end exhale position, as well as controls variability in these parameters and baseline drift. CONCLUSIONS This work provides a method of generating custom respiratory data that can be used for initial implementation and testing of new technologies.

Evaluation of target and cardiac position during visually monitored deep inspiration breath-hold for breast radiotherapy

A low-resource visually monitored deep inspiration breath-hold (VM-DIBH) technique was successfully implemented in our clinic to reduce cardiac dose in left-sided breast radiotherapy. In this study, we retrospectively characterized the chest wall and heart positioning accuracy of VM-DIBH using cine portal images from 42 patients. Central chest wall position from field edge and in-field maximum heart distance (MHD) were manually measured on cine images and compared to the planned positions based on the digitally reconstructed radiographs (DRRs). An in-house program was designed to measure left anterior descending artery (LAD) and chest wall separation on the planning DIBH CT scan with respect to breath-hold level (BHL) during simulation to determine a minimum BHL for VM-DIBH eligibility. Systematic and random setup uncertainties of 3.0 mm and 2.6 mm, respectively, were found for VM-DIBH treatment from the chest wall measurements. Intrabeam breath-hold stability was found to be good, with over 96% of delivered fields within 3 mm. Average treatment MHD was significantly larger for those patients where some of the heart was planned in the field compared to patients whose heart was completely shielded in the plan (p < 0.001). No evidence for a minimum BHL was found, suggesting that all patients who can tolerate DIBH may yield a benefit from it. PACS number(s): 87.53.Jw, 87.53.Kn, 87.55.D.A low‐resource visually monitored deep inspiration breath‐hold (VM‐DIBH) technique was successfully implemented in our clinic to reduce cardiac dose in left‐sided breast radiotherapy. In this study, we retrospectively characterized the chest wall and heart positioning accuracy of VM‐DIBH using cine portal images from 42 patients. Central chest wall position from field edge and in‐field maximum heart distance (MHD) were manually measured on cine images and compared to the planned positions based on the digitally reconstructed radiographs (DRRs). An in‐house program was designed to measure left anterior descending artery (LAD) and chest wall separation on the planning DIBH CT scan with respect to breath‐hold level (BHL) during simulation to determine a minimum BHL for VM‐DIBH eligibility. Systematic and random setup uncertainties of 3.0 mm and 2.6 mm, respectively, were found for VM‐DIBH treatment from the chest wall measurements. Intrabeam breath‐hold stability was found to be good, with over 96% of delivered fields within 3 mm. Average treatment MHD was significantly larger for those patients where some of the heart was planned in the field compared to patients whose heart was completely shielded in the plan (p < 0.001). No evidence for a minimum BHL was found, suggesting that all patients who can tolerate DIBH may yield a benefit from it. PACS number(s): 87.53.Jw, 87.53.Kn, 87.55.D‐

Accounting for respiratory motion in partial breast intensity modulated radiotherapy during treatment planning: A new patient selection metric

PURPOSE External beam partial breast irradiation intensity modulated radiotherapy (PBI IMRT) plans experience degradation in coverage and dose homogeneity when delivered during respiration. We examine which characteristics of the breast and seroma result in unacceptable plan degradation due to respiration. METHODS Thirty-six patient datasets were planned with inverse-optimised PBI IMRT. Population respiratory data were used to create a probability density function. This probability density function (PDF) was convolved with the static plan fluences to calculate the delivered dose with respiration. To quantify the difference between static and respiratory plan quality, we analysed the mean dose shift of the target dose volume histogram (DVH), the dose shift at 95% of the volume and the dose shift at the hotspot to 2 cm(3)of the volume. We explore which patient characteristics indicate a clinically significant degradation in delivered plan quality due to respiration. RESULTS Dose homogeneity constraints, rather than dosimetric coverage, were the limiting factors for all patient plans. We propose the dose evaluation volume-to-planning target volume (DEV-to-PTV) ratio as a delineating metric for identifying patient plans that will be more degraded by respiratory motion. The DEV-to-PTV ratio may be a more robust metric than ipsilateral breast volume because the seroma volume is contoured more consistently between physicians and clinics. CONCLUSIONS For patients with a DEV-to-PTV ratio less than 55% we recommend either not using PBI IMRT or employing motion management. Small DEV-to-PTV ratios occur when the seroma is close to inhomogeneities (i.e. air/lung), which exacerbates the dosimetric effect of respiratory motion. For small breast sizes it is unlikely that the DEV-to-PTV ratio will meet these criteria.

Clinical consequences of changing the sliding window IMRT dose rate

Changing pulse repetition frequency or dose rate used for IMRT treatments can alter the number of monitor units (MUs) and the time required to deliver a plan. This work was done to develop a practical picture of the magnitude of these changes. We used Varians Eclipse Treatment Planning System to calculate the number of MUs and beam‐on times for a total of 40 different treatment plans across an array of common IMRT sites including prostate/pelvis, prostate bed, head and neck, and central nervous system cancers using dose rates of 300, 400 and 600 MU/min. In general, we observed a 4%–7% increase in the number of MUs delivered and a 10–40 second decrease in the beam‐on time for each 100 MU/min of dose rate increase. The increase in the number of MUs resulted in a reduction of the “beam‐on time saved”. The exact magnitude of the changes depended on treatment site and planning target volume. These changes can lead to minor, but not negligible, concerns with respect to radiation protection and treatment planning. Although the number of MUs increased more rapidly for more complex treatment plans, the absolute beam‐on time savings was greater for these plans because of the higher total number of MUs required to deliver them. We estimate that increasing the IMRT dose rate from 300 to 600 MU/min has the potential to add up to two treatment slots per day for each IMRT linear accelerator. These results will be of value to anyone considering general changes to IMRT dose rates within their clinic. PACS number: 87.55.N

When is respiratory management necessary for partial breast intensity modulated radiotherapy: A respiratory amplitude escalation treatment planning study

PURPOSE The impact of typical respiratory motion amplitudes (∼2 mm) on partial breast irradiation (PBI) is minimal; however, some patients have larger respiratory amplitudes that may negatively affect dose homogeneity. Here we determine at what amplitude respiratory management may be required to maintain plan quality. METHODS AND MATERIALS Ten patients were planned with PBI IMRT. Respiratory motion (2-20 mm amplitude) probability density functions were convolved with static plan fluence to estimate the delivered dose. Evaluation metrics included target coverage, ipsilateral breast hotspot, homogeneity, and uniformity indices. RESULTS Degradation of dose homogeneity was the limiting factor in reduction of plan quality due to respiratory motion, not loss of coverage. Hotspot increases were observed even at typical motion amplitudes. At 2 and 5 mm, 2/10 plans had a hotspot greater than 107% and at 10 mm this increased to 5/10 plans. Target coverage was only compromised at larger amplitudes: 5/10 plans did not meet coverage criteria at 15 mm amplitude and no plans met minimum coverage at 20 mm. CONCLUSIONS We recommend that if respiratory amplitude is greater than 10 mm, respiratory management or alternative radiotherapy should be considered due to an increase in the hotspot in the ipsilateral breast and a decrease in dose homogeneity.

Measurement of time delays in gated radiotherapy for realistic respiratory motions

PURPOSE Gated radiotherapy is used to reduce internal motion margins, escalate target dose, and limit normal tissue dose; however, its temporal accuracy is limited. Beam-on and beam-off time delays can lead to treatment inefficiencies and/or geographic misses; therefore, AAPM Task Group 142 recommends verifying the temporal accuracy of gating systems. Many groups use sinusoidal phantom motion for this, under the tacit assumption that use of sinusoidal motion for determining time delays produces negligible error. The authors test this assumption by measuring gating time delays for several realistic motion shapes with increasing degrees of irregularity. METHODS Time delays were measured on a linear accelerator with a real-time position management system (Varian TrueBeam with RPM system version 1.7.5) for seven motion shapes: regular sinusoidal; regular realistic-shape; large (40%) and small (10%) variations in amplitude; large (40%) variations in period; small (10%) variations in both amplitude and period; and baseline drift (30%). Film streaks of radiation exposure were generated for each motion shape using a programmable motion phantom. Beam-on and beam-off time delays were determined from the difference between the expected and observed streak length. RESULTS For the system investigated, all sine, regular realistic-shape, and slightly irregular amplitude variation motions had beam-off and beam-on time delays within the AAPM recommended limit of less than 100 ms. In phase-based gating, even small variations in period resulted in some time delays greater than 100 ms. Considerable time delays over 1 s were observed with highly irregular motion. CONCLUSIONS Sinusoidal motion shapes can be considered a reasonable approximation to the more complex and slightly irregular shapes of realistic motion. When using phase-based gating with predictive filters even small variations in period can result in time delays over 100 ms. Clinical use of these systems for patients with highly irregular patterns of motion is not advised due to large beam-on and beam-off time delays.

Realistic respiratory motion margins for external beam partial breast irradiation.

PURPOSE Respiratory margins for partial breast irradiation (PBI) have been largely based on geometric observations, which may overestimate the margin required for dosimetric coverage. In this study, dosimetric population-based respiratory margins and margin formulas for external beam partial breast irradiation are determined. METHODS Volunteer respiratory data and anterior-posterior (AP) dose profiles from clinical treatment plans of 28 3D conformal radiotherapy (3DCRT) PBI patient plans were used to determine population-based respiratory margins. The peak-to-peak amplitudes (A) of realistic respiratory motion data from healthy volunteers were scaled from A = 1 to 10 mm to create respiratory motion probability density functions. Dose profiles were convolved with the respiratory probability density functions to produce blurred dose profiles accounting for respiratory motion. The required margins were found by measuring the distance between the simulated treatment and original dose profiles at the 95% isodose level. RESULTS The symmetric dosimetric respiratory margins to cover 90%, 95%, and 100% of the simulated treatment population were 1.5, 2, and 4 mm, respectively. With patient set up at end exhale, the required margins were larger in the anterior direction than the posterior. For respiratory amplitudes less than 5 mm, the population-based margins can be expressed as a fraction of the extent of respiratory motion. The derived formulas in the anterior/posterior directions for 90%, 95%, and 100% simulated population coverage were 0.45A/0.25A, 0.50A/0.30A, and 0.70A/0.40A. The differences in formulas for different population coverage criteria demonstrate that respiratory trace shape and baseline drift characteristics affect individual respiratory margins even for the same average peak-to-peak amplitude. CONCLUSIONS A methodology for determining population-based respiratory margins using real respiratory motion patterns and dose profiles in the AP direction was described. It was found that the currently used respiratory margin of 5 mm in partial breast irradiation may be overly conservative for many 3DCRT PBI patients. Amplitude alone was found to be insufficient to determine patient-specific margins: individual respiratory trace shape and baseline drift both contributed to the dosimetric target coverage. With respiratory coaching, individualized respiratory margins smaller than the full extent of motion could reduce planning target volumes while ensuring adequate coverage under respiratory motion.

Retrospective evaluation of visually monitored deep inspiration breath hold for breast cancer patients using edge detection

Purpose: Deep inspiration breath hold (DIBH) can reduce cardiac dose during left-sided breast cancer radiotherapy. This study uses cine imaging with edge detection to evaluate a visually-monitored DIBH technique (VM-DIBH)