(Saturday, 4/7/2018)
Room: Foyer
Purpose: For patients receiving treatment for left breast cancer, cardiac toxicity increases with increased radiation dose to the heart. Further, the accurate heart dose is unknown when its intrinsic (breathing and cardiac cycle) motion is unaccounted for. Current clinical techniques (e.g. deep inspiration breath hold and prone treatment) are not feasible for all patients. A new method was developed utilizing 4DCT scans to improve custom cardiac blocks to minimize heart dose.
Methods: 4DCT scans were acquired and a maximum intensity pixel (MIP) heart was contoured for twenty patients. Custom heart blocks were created to fully block the MIP heart volume. A heart block based on the standard 3DCT image was retrospectively created. The difference in heart block leaf parameters were compared. Differences in MLC leaf positions, heart block area, and dose statistics were analyzed.
Results: In all twenty cases, the heart block created using the 4D scan had a larger area than the corresponding 3D block. The mean increase in MLC leaf coverage was 3.9mm (0.5mm-20.1mm). The mean increase in area of the heart block was 2.6cm² (0.27cm²-6.65cm²). The increase in MIP heart volume from the dose volume histogram (DVH) from the patient plans showed that the average of the mean dose for MIP heart versus 3D heart volumes was larger by 17.8cGy (0.02cGy-70.3cGy).
Conclusion: 4D heart volume and blocks accounting for intrinsic respiratory motion and cardiac motion are greater in size than 3D heart volume and blocks. These larger blocks result from the larger contour created from the MIP image. This technique for improving custom heart blocks requires no special patient techniques and can be implemented easily with 4DCT capabilities. The larger mean doses found for the MIP heart account for a more accurate heart dose representation and would be even greater if adequate blocking was not used.
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