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Detectability of Dose Delivery Changes in Cancer Patients Based On Portal Dosimetry

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L Persoon

L.C.G.G. Persoon1*, E.G.C Troost1, M. Podesta1, S.M.J.J.G. Nijsten1, F. Verhaegen1, (1) Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology, Maastricht University Medical Centre, Maastricht, the Netherlands

SU-E-J-191 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: During the course of radiotherapy the dose delivery can be influenced by a number of factors, e.g., anatomical changes over time. This can result in discrepancies between planned and delivered dose. The electronic portal imaging device has been demonstrated to be valuable for 2D transit and 3D in-vivo dose verification. The aim of this study was to investigate what type of patient-related errors can be detected by quantifying the discrepancies based on 2D transit portal dosimetry.

Methods: For 25 cancer patients treated with volumetric modulated arc therapy, changes were artificially introduced in the planning CT. The artificially introduced changes were: 1) Development and reduction of atelectasis in the lung; 2) weight loss and gain; 3) shift of the patient, and 4) base-line shifts in anatomical structures. After introduction of the changes, 3D dose distributions were recalculated with the Eclipse treatment planning system and compared to the planned dose distribution using a 3D γ evaluation. The 2D portal dose images were predicted with in-house developed algorithms and compared with 2D γ evaluation and dose difference maps.

Results: Several of the introduced changes were found to lead to significant discrepancies in dose metrics derived from dose-volume histograms or 3D γ distributions, which could also be detected by 2D transit portal dosimetry. Clear dose discrepancy trends over time were observed with increasing geometrical aberration.

Conclusion: We have shown the relationship between 3D dose discrepancies and 2D portal dosimetry. Lower detection limits will be determined for the different anatomical changes introduced. This analysis may form the basis for decision making in adaptive radiotherapy.

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