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  4. Dosimetrically Triggered Treatment Adaptation: Replanning Rate as a Function of Flagging Criteria

Dosimetrically Triggered Treatment Adaptation: Replanning Rate as a Function of Flagging Criteria

A Barragan Montero1,2*, D Dumont3 , E Sterpin1,4 , X Geets1,5 , (1) Universite catholique de Louvain, Brussels, Belgium (2) University of Southwestern Texas, MAIA Lab, Dallas-TX, USA (3) Centre Hospitalier Universitaire Tivoli, La Louviere, Belgium, (4) KU Leuven, Department of Oncology, Laboratory of Experimental Radiotherapy, Leuven, Belgium (5) Cliniques Universitaires Saint-Luc, Department of Radiation Oncology, Brussels, Belgium

Presentations

(Sunday, 7/29/2018) 4:30 PM - 5:00 PM

Room: Exhibit Hall | Forum 6

Purpose: Despite many efforts, there is little consensus on how to best select patients requiring adaptive radiotherapy. This is due to the variety of criteria used to trigger adaptation, which are specific to each institution and eventually to the physician. This study analyses the influence of different dosimetric-based selection criteria on the replanning rate.

Methods: Daily MVCT images from 38 head-and-neck (H&N) and 37 lung patients treated with TomoTherapy were used to recompute the dose on every fraction and assess the dosimetric differences with respect to the planning dose. This was done automatically using the adaptive platform developed by 21ˢᵗ Century Oncology (Madison-WI), which registers the planning CT to the daily MVCT and uses the deformation fields for contour propagation and dose accumulation. In-house software was then used to flag dosimetric differences above (below) user-defined thresholds (t) for each organ at risk (target), e.g., t = 30 Gy for parotids. Two different approaches were used: a) soft constraint (orange flag), when the difference between the accumulated and planning dose was above 10% (Level-1) of t, and b) hard constraint (red flag), when the accumulated dose was above t AND the difference between accumulated and planned was above 10% (Level-1) of the planned value. Two more conservative tolerance levels were analysed: 5% (Level-2) and 2.5% (Level-3).

Results: If the presence of at least one red flag was considered sufficient to trigger adaptation, the number of H&N patients requiring replanning ranged from 1 (Level-1) to 4 (Level-3), which is less than 11%. For lung, the replanning rate remained low (2 patients – 5%) for the less conservative case (Level-1), but increased to 10 patients (27%) for tighter tolerance (Level-3).

Conclusion: This study confirms that even with very conservative criteria, adaptive replanning was only triggered in a small portion of the patient population.

Funding Support, Disclosures, and Conflict of Interest: A. Barragan Montero is supported by the Baillet Latour funds.

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