Verification of representative data for output factors of SRS cones utilizing IAEA TRS 483 recommendations.

Measurements of small fields continue to be a clinical challenge despite the recent work done to identify their characteristics. Due to this challenge, many physicists use representative data supplied by their vendors to verify their own measurements for small field output factors. However, with recent guidelines being released in IAEA TRS 483, the question remains if this representative data provides an accurate representation for small field dosimetry. A Sun Nuclear EDGE detector, PTW 60012 stereotactic diode, and GafChromic EBT3 films were used to measure the output factor for a set of Varian SRS cones (4 mm-17.5 mm diameters) on a TrueBeam linear accelerator. The measured output factors were then compared to the Varian provided SRS representative data. IAEA TRS 483 recommendations for measuring small field output factors were applied and the impact of those recommendations were examined. The EDGE detector showed good agreement with the representative data when correction factors were not applied (0.01%-1.64% difference) but the PTW 60012 diode showed larger deviation (0.61%-3.35% difference). The EBT3 film showed the largest difference with the representative data (0.66%-9.19%). After application of IAEA TRS 483 detector specific correction factors the output factors measured by the diodes showed good agreement with the EBT3 film for 6MV (<1.8% difference) but showed a large deviation with the representative data (up to 9% difference). The 6FFF energy output factors agreed between the EDGE, the PTW 60012, and EBT3 Film. This work shows that the use of uncorrected representative data on the Truebeam can lead to a significant over estimation of the SRS cone output factors.

A New Technique Using 4DCT Scans to Create Custom Cardiac Blocks for Left-Sided Breast Cancer Treatments.

PURPOSE: For patients who receive treatment for left breast cancer, cardiac toxicity increases with increased radiation dose to the heart. A new method was developed using 4-dimensional computed tomography (CT) scans to improve custom cardiac blocks to minimize heart dose. METHODS AND MATERIALS: Four-dimensional CT scans were acquired, and a maximum intensity projection (MIP) heart was contoured for 20 patients. Custom heart blocks were created to fully block the MIP heart volume. A heart block based on the standard free-breathing (FB) CT image was retrospectively created. Differences in heart block sizes were compared. Differences in heart block areas and dose statistics were analyzed. RESULTS: In all 20 cases, the heart block created using the 4-dimensional scan had a larger area than the corresponding FB block. The mean increase in multileaf collimator coverage was 3.9 mm (range, 0.5-20.1 mm). The mean increase in the area of the heart block was 2.58 cm2 (range, 0.22-6.65 cm2). The dose-volume histogram showed that the MIP heart received 17.8 cGy (range, 0.02-70.3 cGy) more than the FB heart on average. Replanning with an FB heart block showed that the mean dose to the MIP heart increased by 16.2 cGy (range, 1.8-44.8 cGy) compared with the FB heart dose. CONCLUSIONS: Four-dimensional heart blocks accounting for intrinsic respiratory motion and cardiac motion were greater in size than FB heart blocks. These larger blocks resulted from the larger contour created from the MIP image and blocked more dose to the heart. This technique for improving custom heart blocks can be easily implemented with 4DCT machine capabilities that are currently available in clinical practice.