Room: AAPM ePoster Library
Purpose: To investigate the feasibility of newly designed overlapping half beam based multi-isocentric volumetric arc modulated therapy (OHMI-VMAT) method for pediatric total body irradiation (TBI)
Methods: The TBI techniques generally require a long-distance geometry to cover the whole body even if a patient would be a pediatric size around 120-?. We designed the OHMI-VMAT delivery technique to optimize TBI dose distribution to overcome geometric limitations. The anthropomorphic phantom was used for evaluation of the method as compared to conventional long-distance intensity-modulated TBI (LDIM-TBI). The OHMI-VMAT plan is composed of two 5-? overlapping between the different isocentric VMAT fields. And each isocentric VMAT beams were set to reverse half beam to avoid multi-leaf collimator (MLC) length limitation, which is 14.5 ? in travel direction. The half beam arcs in the VMAT planning were applied to effectively control dose constraints (90% of the prescribed dose of 12 Gy), especially in normal lung tissue. The dose distributions from the RTP system were analyzed to find the feasibility of OHMI-VMAT for pediatric TBI. To verify overlapping and half beams modulations, we performed radiochromic film dosimetry with solid phantoms (SP34, IBA Dosimetry, Germany).
Results: The OHMI-VMAT showed a competitive dose distribution as compared to the LDIM-TBI. Mean doses of the lung were 997.9 cGy and 1015.6 cGy for OHMI-VMAT and LDMI-TBI, respectively. There was shown severe under dose distribution in the segmented chest planning target volume (PTV) surrounding both lungs in the LDIM-TBI (>10 Gy, 78.3%) compared to in the OHMI-TBI (>10 Gy, 95.4%). Analysis of 5-? overlapping between the different isocentric VMAT fields with solid-slab phantoms al showed relatively good agreements in film dosimetry verifications.
Conclusion: The newly designed OHMI-VMAT method enables effectively to optimize dose distribution without any geometrical limitations in aspects of dose planning and delivery using the VMAT technique.