(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To evaluate direct proton therapy (PT) dose calculation on daily cone beam CT (CBCT) scans in esophageal cancer by comparing with gold standard dose calculations on planning and repeat CT scans.
Methods: The daily iteratively reconstructed 3DCBCTs and the mid-treatment repeat 4DCT of 7 patients treated with photons to 45 Gy in 25 fractions were retrieved. Multi-field optimized PT plans (2 posterior beams) were generated in RayStation on the average image of the planning 4DCT (robustness towards 7 mm setup and 3% range uncertainty). Deformable registration of the planning CT image to the daily CBCTs and to the repeat CT average image was performed for clinical target volume (CTV) and organ at risk (OAR) mapping. CBCT calibration was based on available Catphan phantom scans containing 6 tissue equivalent inserts. Plans were recalculated on all daily CBCTs (n=162) after correcting for the clinically applied couch shifts, and on the rigidly registered repeat CTs. Nominal doses were compared between CT and CBCT and the evolution during the treatment course was evaluated.
Results: Acceptable agreement of dose-volume parameters was obtained when comparing recalculations on repeat CT and on CBCT of that day, for CTV (average V95 difference of 3.8% (range: 0.0%-5.4%)) and OAR: mean heart dose (average 1.6 Gy (range: 0.1 Gy-3.8 Gy)), mean lung dose (average 0.2 Gy (range: 0.1 Gy-0.4 Gy)). CBCT doses indicated the need for adaptation (CTV V95<97% cut-off) in 4 patients, at time points varying between fraction 1 and 8. This was confirmed on the repeat CT in 3 of the 4 patients.
Conclusion: This work shows the potential of direct proton dose calculation on CBCT for an early detection of significant anatomical changes in esophageal cancer. High-quality CBCT could be as important in PT as in photon radiotherapy for objectively assessing the need for plan adaptation.
Funding Support, Disclosures, and Conflict of Interest: This project was supported by Kom op tegen Kanker (Stand up to Cancer), the Flemish cancer society.
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