(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
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.
Keywords
TBI, Intensity Modulation, Radiochromic Film
Taxonomy
TH- External Beam- Photons: treatment planning/virtual clinical studies
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