Tangential Volumetric Modulated Arc Therapy for Locally Advanced Breast Cancer.

Cardiovascular toxicity from breast radiation therapy (RT) is a concern to patients and providers. Herein, we present a cardiac-sparing strategy using tangential volumetric modulated arc therapy (tVMAT) in comparison with standard 3-dimensional conformal RT. Ten patients with left-sided breast cancer previously treated with adjuvant RT covering the breast, as well as the axillary and supraclavicular nodal regions, were selected for the study. For each patient, 2 plans were created: a dual-isocenter 3-field 3-dimensional conformal RT plan and a monoisocentric tVMAT plan. The prescription for both techniques was 50 Gy in 25 fractions to the breast and nodal target volumes. Compared with 3-dimensional conformal RT, tVMAT provided more uniform coverage to the breast and regional lymph nodes (mean conformity index: 1.42 for tVMAT vs 2.42 for 3-dimensional conformal RT; P < .01), and the maximum point dose for tVMAT was lower on average (112.8% for tVMAT vs 121.5% for 3-dimensional conformal RT; P < .001). Coverage to the lymph nodes was superior for tVMAT (average minimum coverage to 95% of entire nodal target volumes: 99.5% of prescribed dose for tVMAT vs 94.9% for 3-dimensional conformal RT; P < .001). Organ-at-risk sparing was improved with tVMAT, with a lower average V20Gy for the left lung (15.0% for tVMAT vs 24.6% for 3-dimensional conformal RT; P < .01) and lower mean heart dose (156 cGy for tVMAT vs 200 cGy for 3-dimensional conformal RT; P < .01). Tangential volumetric modulated arc therapy is a promising technique for the treatment of intact breast and regional lymphatics, and it may improve target coverage and organ-at-risk avoidance compared with 3-dimensional conformal techniques.

Stereotactic radiosurgery commissioning and QA test cases-A TG-119 approach for Stereotactic radiosurgery.

PURPOSE: To develop a standardized set of representative clinical treatment cases that pose a range of optimization problems for evaluating the plan quality and dosimetric accuracy within the commissioning process for linac-based stereotactic radiosurgery (SRS). METHODS: Five test cases with increasing complexity were created to validate delivery accuracy in SRS commissioning similar to the approach used by AAPM TG-119 in developing a test suite for IMRT commissioning. Standardized structure sets, planning goals, and delivery requirements were specified for each case including a small sphere target, irregular target, irregular target placed off-axis, multi-target, and abutting organs-at-risk (OARs). Various VMAT field arrangements including a single arc, two coplanar arcs, full arc and vertex half arc, and four noncoplanar arcs were tested to generate clinically appropriate treatment plans. RESULTS: The small spherical target was 1.0 cm in diameter. The irregular target was a clinical cavity (2.3 × 2.2 × 1.4 cm³) and was shifted 4.5 cm for the irregular target off-axis case. The multi-target case used the irregular target and four spherical targets representing metastases ranging 0.9 to 1.6 cm in diameter, placed up to 7.5 cm off-axis. The abutting OARs case included an acoustic neuroma and target placed near the optic nerve. All spherical targets received 24 Gy and the cavity received 18 Gy. The abutting OAR cases included a 3.74 cc lesion adjacent to the brainstem receiving 13 Gy and a 1.11 cc lesion adjacent to the optic nerve receiving 12 Gy. All plans used a single-isocenter placed at the target center or geometric center of multiple targets. Planning goals for all cases included constraints for the target and brain minus PTV, along with brainstem and optic nerve where applicable. Deliverability was assessed through ion chamber measurements, in addition to composite and per-field planar measurements on Gafchromic film and small-field diode array. A mean and SD for measured versus planned doses of 101.0% ± 2.9% was observed over the 14 ion chamber measurements. Mean and SD for gamma pass rates were 98.5% ± 2.2% and 97.1% ± 4.9% for film and diode array, respectively, for gamma criteria of 2% and 1 mm. CONCLUSION: These cases could provide the preliminary groundwork for a novel benchmark for institutions to evaluate linac-based SRS commissioning and delivery accuracy prior to clinical implementation. The rapid widespread implementation of linac-based SRS, the complexity associated with dosimetry and delivery, and high-profile treatment deviations that have already resulted from its use, highlight the importance of such a benchmark test suite. Comprehensive dosimetric measurements from this standardized set of SRS optimization problems were used to fine-tune and understand the limitations of our SRS planning and delivery system and establish a set of baseline data for comparison with other delivery platforms.

MVCT versus kV-CBCT for targets subject to respiratory motion: A phantom study.

The use of kilovoltage cone-beam computed tomography (kV-CBCT) or megavoltage computed tomography (MVCT) for image guidance prior to lung stereotactic body radiation therapy (SBRT) is common clinical practice. We demonstrate that under equivalent respiratory conditions, image guidance using both kV-CBCT and MVCT may result in the inadequate estimation of the range of target motion under free-breathing (FB) conditions when standard low-density window and levels are used. Two spherical targets within a respiratory motion phantom were imaged using both long-exhale (LE) and sinusoidal respiratory traces. MVCT and kV-CBCT images were acquired and evaluated for peak-to-peak amplitudes of 10 or 20 mm in the cranial-caudal direction, and with 2, 4 or 5 s periods. All images were visually inspected for artifacts and conformity to the ITV for each amplitude, period, trace-type, and target size. All LE respiratory traces required a lower threshold HU window for MVCT and kV-CBCT compared to sinusoidal traces to obtain 100% volume conformity compared with the theoretical ITV (ITVT ). Excess volume was less than 2% for all kV-CBCT contours regardless of trace-type, breathing period, or amplitude, while the maximum excess volume for MVCT was 48%. Adjusting window and level to maximize conformity with the ITVT is necessary to reduce registration uncertainty to less than 5 mm. To fully capture target motion with either MVCT or kV-CBCT, substantial changes in HU levels up to -600 HU are required which may not be feasible clinically depending on the target's location and surrounding tissue contrast. This registration method, utilizing a substantially decreased window and level compared to standard low-density settings, was retrospectively compared to the automated registration algorithm for five lung SBRT patients exposed to pre-treatment kV-CBCT image guidance. Differences in registrations in the super-inferior (SI) direction greater than the commonly used ITV to PTV margin of 5 mm were encountered for several cases. In conclusion, pre-treatment image guidance for lung SBRT targets using MVCT or kV-CBCT is unlikely to capture the full extent of target motion as defined by the ITVT and additional caution is warranted to avoid registration errors for small targets and patients with LE respiratory traces.