Room: AAPM ePoster Library
Purpose: The Vero can perform real-time dynamic tumour tracking (DTT) with a gimbal-mounted linac head that pans and tilts to follow the tumour. No treatment planning systems model the beam’s panning/tilting, so DTT plans are optimized, calculated, and evaluated on one respiratory phase. Here we discuss a novel method for re-calculating the dose distribution on a different breathing phase by properly modelling the beam’s panning/tilting during DTT, and how this compares to another simpler method.
Methods: A script was developed in-house to transfer step-and-shoot intensity modulated radiotherapy (sIMRT) plans made on one breathing phase (eg. exhale) to another (eg. inhale). The script calculates a new gantry, ring and collimator angle, and an isocenter shift, for each beam to mimic its path when panning/tilting. sIMRT plans were created for 11 patients and re-calculated on the inhale phase CT using two different
methods: 1) simply translating the beam to simulate tracking, and 2) using our script. The inhale dose was deformed to the exhale CT and accumulated with the original dose, and the maximum dose for certain organs at risk (OARs) was compared.
Results: In total, 37 OARs were examined. Both methods found 3 OARS exceeded their dose limits when re-calculated on the inhale phase that met their limits on the exhale phase. Deforming the inhale dose to the exhale CT and accumulating it with the original dose shows 1 OAR exceeds its limit via the first method and 2 via the second method.
Conclusion: DTT plans optimized for one breathing phase leave an OAR vulnerable to exceeding its dose constraint during other breathing phases. Modelling DTT beam motion as a translation is simpler to implement but might produce different results than when the beam pans/tilts. Dose accumulation to a reference phase should be used to most accurately assess the OAR doses.