Evaluation of Treatment Interruptions and Recovery during Biology-Guided Radiotherapy Delivery.

PURPOSE/OBJECTIVE(S): A Biology-guided Radiotherapy (BgRT) based device is designed to use Positron Emission Tomography (PET) signals to achieve tracked dose delivery. The goal of this study is to investigate the dose delivery accuracy in case of interruption during BgRT treatment, and resumption in a separate treatment session for a multi-target delivery, as the PET activity continues to decay.

MATERIALS/METHODS: A custom-built large anthropomorphic phantom (LAP) including a 26 mm spherical target with 3D independent motion and two 22 mm spherical targets with 1D sinusoidal motion embedded in water was used. All three targets were filled with FGD in an 8:1 target to background uptake ratio (41.52 kBq/ml in target and 5.19 kBq/ml in background). During BgRT delivery, the treatment was intentionally paused during delivery to the second target and the current treatment session was ended to generate a partial fraction. Then the partial fraction was continued in a new session, where the CT scan localization and PET pre-scan were repeated using the existing PET activity present in the phantom. The newly acquired PET pre-scan, was then used to determine if sufficient PET counts were present to resume treatment delivery. The interruption and recovery algorithm is designed to calculate the fluence that needs to be delivered to the remaining targets as well as the residual fluence to be given to the targets that have already received partial dose prior to the interruption. Once the new fluence is recomputed, the treatment is resumed. The delivered doses were captured using radiochromic film (EBT-XD) inserted in the target as well as post-treatment dose calculations based on the delivered beamlet sequence to evaluate the results in terms of dosimetric coverage and margin loss. The margin loss is calculated as the maximum difference between the distance from the Clinical Target Volume (CTV) contour to the 97% isodose contour in the treatment plan and the on the film. The dosimetric coverage is defined as the percentage of voxels within the CTV that lies within 97% and 130% of the prescribed dose.

RESULTS: As shown in the table below, a margin loss of less than 3 mm for all targets and 100% CTV coverage was achieved. After treatment interruptions, the PET safety evaluation based on the PET pre-scan helped to determine whether the treatment could be continued on the same day using the same injected PET activity (an NTS value ≧ 2 and AC value ≧ 5 kBq/ml).

CONCLUSION: This study demonstrated that the BgRT system is able to deliver the prescribed dose to all targets with independent motion, even when an interruption and resumption occurs during treatment. In case such an interruption if the remaining PET activity satisfies the BgRT safety evaluation, the treatment can continue to deliver the remainder of the BgRT doses.