Leigh Conroy

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.

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.

Deep inspiration breath-hold produces a clinically meaningful reduction in ipsilateral lung dose during locoregional radiation therapy for some women with right-sided breast cancer

PURPOSE The goal of the work described here was to determine whether deep inspiration breath-hold (DIBH) produces a clinically meaningful reduction in pulmonary dose compared with free breathing (FB) during locoregional radiation for right-sided breast cancer. METHODS AND MATERIALS Four-field, modified-wide tangent plans with full nodal coverage were developed for 30 consecutive patients on paired DIBH and FB CT scans. Nodes were contoured according to European Society for Radiotherapy and Oncology guidelines. Plan metrics were compared using Wilcoxon signed-rank testing. RESULTS In 21 patients (70%), there was a ≥5% reduction in ipsilateral lung V20Gy with DIBH compared with FB. The mean decrease in ipsilateral lung V20Gy was 7.8% (0%-20%, P < .001). The mean lung dose decreased on average by 3.4 Gy with DIBH (-0.2 to 9.1, P < .001). The mean reduction in liver volume receiving 50% of the prescribed dose was 42.3 cm3 (0-178.9 cm3, P < .001). CONCLUSIONS DIBH reduced ipsilateral lung V20Gy by ≥5% in the majority of patients. For some patients, the volume of liver receiving a potentially toxic dose decreased with DIBH. DIBH should be available as a treatment strategy to reduce ipsilateral lung V20Gy prior to compromising internal mammary chain nodal coverage for patients with right-sided breast cancer during locoregional radiation therapy if the V20Gy on FB exceeds 30%.

Technical Note: Issues related to external marker block placement for deep inspiration breath hold breast radiotherapy

Purpose: It has been suggested that the Real‐time Position Management (RPM) marker block should be placed directly on the breast or sternum to verify deep inspiration breath hold (DIBH) level for breast radiotherapy. We explore three potential issues with this practice: (a) surface dose effect of placing the marker block in the primary beam; (b) effect of marker block tilt on the accuracy of the RPM system; and (c) correlation between marker block positions on the patient surface and internal chest wall position. Methods: (a) The surface dose under the two‐, four‐, and six‐dot marker blocks was measured at incident angles of 0° and 30°; (b) the motion amplitude detected when using the two‐ and six‐dot marker blocks was recorded for block tilts from 0° to 60° about the RPM camera line of sight; (c) the correlation between median displacement of the chest wall and median displacement of the surface contour between breath holds was investigated for superior, middle, and inferior block positions using contours extracted from portal images of eight left‐sided breast cancer patients. Results: (a) The marker blocks increased the surface dose for a 6 MV direct field by 48.2–52.2% of Dmax; (b) at lateral tilts greater than 10°, the two‐dot marker block overestimated the motion amplitude; however, the six‐dot marker block amplitude remained accurate up to 60°; (c) the whole, superior, and middle surface positions were strongly correlated with chest wall displacement (R2 = 0.83; R2 = 0.90; R2 = 0.83), whereas the inferior position was moderately correlated (R2 = 0.36). Conclusions: The RPM marker block can be placed on the breast for DIBH treatments; however, caution should be used regarding surface dose effects. The two‐dot marker block should not be used for block tilts beyond 20°. Marker block placement at a middle or superior position on the breast results in the strongest correlation with chest wall position.

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.

Is there a ‘Leaky Pipeline’ for Women in Clinical Medical Physics in Canada?

We examined the role of women in academic medical physics departments in Canada through various stages of a representative career path.

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)

Clinical implementation of AXB from AAA for breast: Plan quality and subvolume analysis

Abstract Purpose Two dose calculation algorithms are available in Varian Eclipse software: Anisotropic Analytical Algorithm (AAA) and Acuros External Beam (AXB). Many Varian Eclipse‐based centers have access to AXB; however, a thorough understanding of how it will affect plan characteristics and, subsequently, clinical practice is necessary prior to implementation. We characterized the difference in breast plan quality between AXB and AAA for dissemination to clinicians during implementation. Methods Locoregional irradiation plans were created with AAA for 30 breast cancer patients with a prescription dose of 50 Gy to the breast and 45 Gy to the regional node, in 25 fractions. The internal mammary chain (IMCCTV) nodes were covered by 80% of the breast dose. AXB, both dose‐to‐water and dose‐to‐medium reporting, was used to recalculate plans while maintaining constant monitor units. Target coverage and organ‐at‐risk doses were compared between the two algorithms using dose–volume parameters. An analysis to assess location‐specific changes was performed by dividing the breast into nine subvolumes in the superior–inferior and left–right directions. Results There were minimal differences found between the AXB and AAA calculated plans. The median difference between AXB and AAA for breastCTV V 95%, was <2.5%. For IMCCTV, the median differences V 95%, and V 80% were <5% and 0%, respectively; indicating IMCCTV coverage only decreased when marginally covered. Mean superficial dose increased by a median of 3.2 Gy. In the subvolume analysis, the medial subvolumes were “hotter” when recalculated with AXB and the lateral subvolumes “cooler” with AXB; however, all differences were within 2 Gy. Conclusion We observed minimal difference in magnitude and spatial distribution of dose when comparing the two algorithms. The largest observable differences occurred in superficial dose regions. Therefore, clinical implementation of AXB from AAA for breast radiotherapy is not expected to result in changes in clinical practice for prescribing or planning breast radiotherapy.