ESTRO 36 Abstract Book

S903 ESTRO 36 _______________________________________________________________________________________________

Results Thirteen patients had their RT plan adapted at least once during treatment. In the first adaptation (13 pts), the median decrease in the CTV receiving 95% of prescribed dose (V95%) and the planning target volume (PTV) was 0.2% [0-6.4%] and 5.2% [0-11.7%], respectively, see Fig2. The largest underdosage was related to interfractional baseline shifts in the diaphragm position (6 pts), with a median decrease in CTV V95% of 0.6% [0-6.4%] and PTV V95% of 8.6% [1.7-11.7%]. Target deviations registered as changes in SS (8 pts), were typically caused by swelling of the target area (7 pts), shrinkage or swelling of the mediastinum (4 pts) and/or compression or stretching of the target due to changed diaphragm position (1 pt). Changes in SS were pooled and showed a median decrease in CTV V95% of 0.2% [0-0.6%] and PTV V95% of 5.0% [0.5- 6.9%].

Four pts had a second adaptation during RT. For these pts, changes above tolerance were solely observed for SS. The median decrease in CTV V95% was 0.2% [0-2.1%] and PTV V95% was 4.5% [0.6-7.2%]. None of the twice adapted patients showed changes in anatomy which justified reverting to the original treatment plan – either the changes were further in the same direction or in a different region. Conclusion Target coverage during the chemoRT in EC patients was compromised in some cases due to interfractional anatomical changes. Changes observed during RT persisted and in some cases they increased, making adaptation of the RT plan necessary. EP-1662 Interfractional trend analysis of sinograms: a decision-making for adaptive radiotherapy S. Bresciani 1 , A. Garello 1 , A. Miranti 1 , A. Maggio 1 , A. Di Dia 1 , P. Gabriele 2 , M. Stasi 1 1 Candiolo Cancer Institute - FPO- IRCCS, Medical Physics, Candiolo TO, Italy 2 Candiolo Cancer Institute - FPO- IRCCS, Radiotherapy, Candiolo TO, Italy Purpose or Objective The aim of this study is to investigate how geometric and anatomical changes can be detected in daily sinogram informations and how this information can be used to examine interfractional trends, building toward a methodology to optimize treatment and support adaptive replanning. Material and Methods Sensitivity of detectors and sinograms complex to detect shift errors and anatomical variations was previously tested on thoracic phantom. In particular systematic variations in shifts (1-5 mm in lateral direction), anatomical variations (adding 1.25-2.5 cm bolus over phantom) were applied. Subsequently, a total amount of 106 patients treated with Tomotherapy and their related 1573 sinograms were analyzed. The sinograms, measured using Xenon detectors integrated in Tomotherapy unit, were compared with a reference one (usually the first fraction) using both