TITLE

Is it important to use monte carlo calculations for radiosurgery of the clivus?

AUTHOR(S)
Ho, A. K.; Soltys, S. G.; Chang, S. D.
PUB. DATE
March 2013
SOURCE
Journal of Radiosurgery & SBRT;2013 Supplement 2.1, Vol. 2, p71
SOURCE TYPE
Academic Journal
DOC. TYPE
Article
ABSTRACT
Objectives: We sought to determine if dosimetric differences exist between Ray-Trace (RT) and Monte- Carlo (MC) calculation algorithms for radiosurgery for tumors of the clivus, given their proximity to air-tissue inhomogeneity. Methods: We retrospectively identified 6 patients with a tumor of the clivus where the target was located near an air-tissue interface. CyberKnife treatment was delivered in 1 to 5 fractions. Plans originally calculated with the RT algorithm were re-calculated with the same monitor units (MU) with the MC algorithm. Similarly, plans originally calculated with MC were recalculated with RT. MC calculations were performed using High Resolution with 2% uncertainty on the MultiPlan 4.6 planning system. If the RT plans were originally calculated using water- air density model, they were re-calculated with Body standard density model, and then compared with MC plans computed with Body standard density model. The maximum dose (Dmax) to the planning target volumes (PTV) and critical structures, minimum dose (Dmin) and coverage percentage for the PTV were calculated. Results: For the PTV, MC target coverage was a median of 4% (range, 0-13%) less than for RT. MC PTV Dmin was a median 6% (range, 1-10%) and Dmax a median 6% (range, 4-9%) higher than for RT. Similarly, the dose to critical structures (optic pathways and brainstem) was a median 9% (range, 3-27%) higher with MC as compared to RT. The degree of differences depends on the location of the target relative to the air-tissue inhomogeneity. Conclusion: Monte Carlo dose calculations are recommended for targets near tissue homogeneity such as the clivus. Assuming that Monte Carlo more closely approximates the true dosimetry, the use of Ray Trace could both overestimate target coverage and underestimate dose to critical structures. Analysis of the clinical correlates of worse tumor control and higher toxicity is ongoing. Disclosure: No significant relationships.
ACCESSION #
92611188

 

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