(Monday, 7/30/2018) 3:45 PM - 4:15 PM
Room: Exhibit Hall | Forum 5
Purpose: To demonstrate the dosimetric impact of adaptive replanning for H&N cancer. To correlate anatomical variations of primary and nodal diseases to the dosimetric change on target coverage and organ sparing. To establish volume variation threshold to trigger adaptive replanning.
Methods: Eleven patients with primary tumor in the mid neck region (oropharynx, tongue, base of tongue, tonsil and larynx) were found to have at least one adaptive replan treated on our Elekta accelerator with Agility MLC, three with IMRT and eight with VMAT. The prescription ranged from 66 to 72 Gy, with seven delivered in simultaneous integrated boost and four with sequential boost after 60 Gy. The initial plan was recomputed on the rescan CT, with isocenter loaded to the same anatomical location. Target coverage was evaluated using D95 and V98, cord with maximum dose, and parotid and constrictor with mean dose.
Results: Five patients had nodal GTV enlargement >10% (up to 1092%), with three also having primary CTV increase >10% (up to 68%). Five patients presented primary CTV and/or nodal GTV shrinkage >10% (up to 28% for primary and 40% for nodes). On patient had complex change, with primary CTV shrinking 13%, left nodes growing 31% and right nodes recessing 32%. When target grew <20%, D95 and V98 degraded by <3% and 5%, respectively. Larger variation resulted in unacceptable loss of target coverage, but lack clear linear relationship. When the primary target reduced by >20% (in three patients), dose to key organs such as cord and parotid got out of tolerance. There was no clear relationship between nodal shrinkage and organ dose increase.
Conclusion: To maintain sufficient target coverage and organ sparing, we recommend a 20% threshold on volume variation for primary CTV or nodal GTV to trigger adaptive replan for patients with primary disease in mid neck.
Keywords
Anatomical Models, Simulation, Thresholding
Taxonomy
TH- External beam- photons: adaptive therapy
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