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On the Range Dependence of the Dose Blurring Effect

D Mohatt1*, D Wiant2 , T Ma3 , N Islam4 , H Malhotra5 , (1) Roswell Park Cancer Institute, Buffalo, NY, (2) Cone Health Cancer Center, Greensboro, NC, (3) Roswell Park Cancer Institute, Buffalo, NY, (4) Roswell Park Cancer Institute, Buffalo, NY, (5) Roswell Park Cancer Institute, East Amherst, NY


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Stereotactic body radiotherapy (SBRT) requires highest levels of precision and accuracy. However, the success of treatment can be compromised by respiratory motion giving rise to the dose blurring effect. The objective of this study is to quantify the change in the dose distribution as a function of range of target displacement.

Methods: A Quasar phantom was used to simulate respiratory induced target motion up to 3 cm [± 15 mm]. The effects of target motion were compensated for via dose convolution using a multiple instance geometry (MIG) technique. Separate fields were created for each 10 phase GTV structure for which the total dose was weighted with respected to each field at 10 distinct isocenters. We compared DVH curves and planner dose distributions using gamma criteria for both conventional and SBRT tolerances. The blurring effect was verified using radiochromic film analysis.

Results: For target ranges of less than 1 cm [± 5 mm], the dose blurring effect is essentially unnoticeable when assuming conventional gamma tolerances (3%,3mm), yet degraded by -16%/cm for larger ranges of motion. However, when assuming SBRT criteria (3%,1mm) we observed a consistent degradation of -21%/cm for all ranges of motion. With the MIG correction, gamma pass rates for film were >99% (3%, 3mm), and >85% (3%,1mm).

Conclusion: In the treatment of a moving target, the mismatch between the predicted and actual dose distribution is range dependent. We have shown a nontrivial reduction of the SBRT gamma pass rate for even small ranges of target motion, with greater degradation as the range of motion increases. The dose blurring effect is independent of treatment modality, margin or computational algorithm and can be corrected for using the MIG technique.


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