A margin-based analysis of the dosimetric impact of motion on step-and-shoot IMRT lung plans.
OBJECTIVE
Intrafraction motion during step-and-shoot (SNS) IMRT is known to affect the target dosimetry by a combination of dose blurring and interplay effects. These effects are typically managed by adding a margin around the target. A quantitative analysis was performed, assessing the relationship between target motion, margin size, and target dosimetry with the goal of introducing new margin recipes.


METHODS
A computational algorithm was used to calculate 1,174 motion-encoded dose distributions and DVHs within the patient's CT dataset. Sinusoidal motion tracks were used simulating intrafraction motion for nine lung tumor patients, each with multiple margin sizes.


RESULTS
D95% decreased by less than 3% when the maximum target displacement beyond the margin experienced motion less than 5 mm in the superior-inferior direction and 15 mm in the anterior-posterior direction. For target displacements greater than this, D95% decreased rapidly.


CONCLUSIONS
Targets moving in excess of 5 mm outside the margin can cause significant changes to the target. D95% decreased by up to 20% with target motion 10 mm outside the margin, with underdosing primarily limited to the target periphery. Multi-fractionated treatments were found to exacerbate target under-coverage. Margins several millimeters smaller than the maximum target displacement provided acceptable motion protection, while also allowing for reduced normal tissue morbidity.
Benjamin J
BJ
Waghorn
Department of Radiation Oncology, UF Health Cancer Center at Orlando Health, 1400 South Orange Avenue MP 730, Orlando, Florida 32806, USA. amish.shah@orlandohealth.com.
Amish P
AP
Shah
Justin M
JM
Rineer
Katja M
KM
Langen
Sanford L
SL
Meeks
D000465:Algorithms; D006801:Humans; D008175:Lung Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D009038:Motion; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided; D050397:Radiotherapy, Intensity-Modulated; D047368:Tumor Burden