SU-D-202-05: Evaluation of Four-Dimensional Dose Reconstruction Under Breathing Irregularity.

PURPOSE: A method of four dimensional dose reconstruction (4D-DR) using a cine mode of an EPID to determine the delivered 4D dose distribution has been suggested. This method, however, has not been tested under irregular breathing. This study investigated their effects on 4D-DR.

METHODS: The 4D-DR attempts to find the patient's breathing phase associated with each EPID image by comparing it to pre-generated-EPID-quality DRRs of every breathing phases. A lung phantom with a tumor object (3cm diameter cylinder) on a moving platform was used for this test under conditions of amplitude reduction by 1/2 during (1) the entire delivery and (2) the duration when MLC blocked the tumor (hindering the phase determination). The dose delivered to the phantom was inversely reconstructed in the associated phase image of the phantom from each EPID image based on our documented inverse method of the 4D-DR; the phase-specific dose was integrated, generating the 4D reconstructed dose. Forward 4D Monte Carlo calculations were used as the 4D dose of a ground truth.

RESULTS: The reconstructed dose showed 98.1% gamma pass rate for 3%/3mm under regular breathing (i.e. no amplitude reduction) when compared with the forward 4D dose. When the irregular condition (#1) was adopted for treatment, the pass rate became 94.2%. The irregularity under the MLC block (#2) showed the pass rate of 91.3%. The two conditions did not affect the reconstruction noticeably in DVH plots of the tumor and lung-tumor.

CONCLUSION: As long as the extent of on-treatment motion is smaller than that of the planning CT, the 4D-DR can be done accurately. Otherwise, ontreatment 4D CT images are necessary.