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SU‐E‐J‐72: Dosimetric Study of Cone‐Beam CT‐Based Radiation Treatment Planning Using a Patient‐Specific Stepwise CT‐Density Table
Author(s) -
Chen S,
Le Q,
Mutaf Y,
Yi B,
D'souza W
Publication year - 2015
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4924159
Subject(s) - hounsfield scale , nuclear medicine , cone beam ct , dosimetry , cone beam computed tomography , medicine , pelvis , radiation treatment planning , head and neck , radiation therapy , computed tomography , radiology , surgery
Purpose: To assess dose calculation accuracy of cone‐beam CT (CBCT) based treatment plans using a patient‐specific stepwise CT‐density conversion table in comparison to conventional CT‐based treatment plans. Methods: Unlike CT‐based treatment planning which use fixed CT‐density table, this study used patient‐specific CT‐density table to minimize the errors in reconstructed mass densities due to the effects of CBCT Hounsfield unit (HU) uncertainties. The patient‐specific CT‐density table was a stepwise function which maps HUs to only 6 classes of materials with different mass densities: air (0.00121g/cm3), lung (0.26g/cm3), adipose (0.95g/cm3), tissue (1.05 g/cm3), cartilage/bone (1.6g/cm3), and other (3g/cm3). HU thresholds to define different materials were adjusted for each CBCT via best match with the known tissue types in these images. Dose distributions were compared between CT‐based plans and CBCT‐based plans (IMRT/VMAT) for four types of treatment sites: head and neck (HN), lung, pancreas, and pelvis. For dosimetric comparison, PTV mean dose in both plans were compared. A gamma analysis was also performed to directly compare dosimetry in the two plans. Results: Compared to CT‐based plans, the differences for PTV mean dose were 0.1% for pelvis, 1.1% for pancreas, 1.8% for lung, and −2.5% for HN in CBCT‐based plans. The gamma passing rate was 99.8% for pelvis, 99.6% for pancreas, and 99.3% for lung with 3%/3mm criteria, and 80.5% for head and neck with 5%/3mm criteria. Different dosimetry accuracy level was observed: 1% for pelvis, 3% for lung and pancreas, and 5% for head and neck. Conclusion: By converting CBCT data to 6 classes of materials for dose calculation, 3% of dose calculation accuracy can be achieved for anatomical sites studied here, except HN which had a 5% accuracy. CBCT‐based treatment planning using a patient‐specific stepwise CT‐density table can facilitate the evaluation of dosimetry changes resulting from variation in patient anatomy.