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SU‐F‐T‐258: Efficacy of Exit Fluence‐Based Dose Calculation for Prostate Radiation Therapy
Author(s) -
Siebers J,
Gardner J,
Neal B
Publication year - 2016
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 - 10.1118/1.4956398
Subject(s) - fluence , centroid , monte carlo method , computation , nuclear medicine , radiation treatment planning , image guided radiation therapy , dosimetry , mathematics , physics , medical imaging , radiation therapy , algorithm , computer science , medicine , optics , statistics , artificial intelligence , radiology , geometry , laser
Purpose: To investigate the efficacy of exit‐fluence‐based dose computation for prostate radiotherapy by determining if it estimates true dose more accurately than the original planning dose. Methods: Virtual exit‐fluencebased dose computation was performed for 19 patients, each with 9–12 repeat CT images. For each patient, a 78 Gy treatment plan was created utilizing 5 mm CTV‐to‐PTV and OAR‐to‐PRV margins. A Monte Carlo framework was used to compute dose and exit‐fluence images for the planning image and for each repeat CT image based on boney‐anatomyaligned and prostate‐centroid‐aligned CTs. Identical source particles were used for the MC dose‐computations on the planning and repeat CTs to maximize correlation. The exit‐fluence‐based dose and image were computed by multiplying source particle weights by FC(x,y)=FP(x,y)/FT(x,y), where (x,y) are the source particle coordinates projected to the exit‐fluence plane and we denote the dose/fluence from the plan by (DP,FP), from the repeat‐CT as (DT,FT), and the exit‐fluence computation by (DFC,FFC). DFC mimics exit‐fluence backprojection through the planning image as FT=FFC. Dose estimates were intercompared to judge the efficacy of exit‐fluence‐based dose computation. Results: Boney‐ and prostate‐centroid aligned results are combined as there is no statistical difference between them, yielding 420 dose comparisons per dose‐volume metric. DFC is more accurate than DP for 46%, 33%, and 44% of cases in estimating CTV D98, D50, and D2 respectively. DFC improved rectum D50 and D2 estimates 54% and 49% respectively and bladder D50 and D2 47 and 49% respectively. While averaged over all patients and images DFC and DP were within 3.1% of DT, they differed from DT by as much as 22% for GTV D98, 71% for the Bladder D50, 17% for Bladder D2, 19% for Rectum D2. Conclusion: Exit‐fluence based dose computations infrequently improve CTV or OAR dose estimates and should be used with caution. Research supported in part by Varian Medical Systems.