Proceedings of the 51st Annual ASTRO Meeting S699

comparing both the measured dose profile and the isodose lines with those calculated by the Eclipse TPS. It is possible to test theconsistency of the delivered dose from spots with 58 different energies in one exposure by using the step phantom with MatriXX.

Conclusions: The 2-D ion chamber array was found to be useful for quick real time QA checks of both the spot positioning ac-curacy and the consistency of dose delivery by multiple spots with different energies from a spot scanning proton therapy machine.All or some of the procedures explored in this study can become part of a periodic QA program for these machines.

Author Disclosure: N. Sahoo, None; X.R. Zhu, None; B. Arjomandy, None; G. Ciangaru, None; X. Ding, None; M. Gillin, None.

3151 Risk Estimate of Second Malignant Neoplasm Incidence and Mortality from Secondary Neutrons for Two

Children Who Received Proton Craniospinal Irradiation

P. J. Taddei, D. Mirkovic, A. Mahajan, D. Kornguth, A. Giebeler, R. Zhang, M. C. Harvey, S. Woo, W. D. Newhauser

The University of Texas M. D. Anderson Cancer Center, Houston, TX

Purpose/Objective(s): The aim of this study was to quantify secondary neutron dose and estimate second malignant neoplasm(SMN) risk for a 9-year-old girl and a 10-year-old boy of similar stature who received proton craniospinal irradiation (CSI) formedulloblastoma at our institution.

Materials/Methods: Proton radiotherapy plans were created using a commercial treatment planning system for the two patients.Simulations of 30.6-Gy CSI with a 23.4-Gy surgical bed boost treatment were performed using the Monte Carlo code MCNPX. Thegeometric models included a field-specific passive-scattering proton treatment unit and human phantoms based on computed to-mography (CT) images of each patient. Therapeutic absorbed dose from protons and equivalent dose from secondary neutrons werecalculated throughout the patients’ bodies. Mean equivalent doses from secondary neutrons were determined in organs and tissues,and effective doses (whole-body weighted averages) from secondary neutrons were calculated. Risks of SMN incidence and mor-tality due to secondary neutrons were predicted based on the equivalent doses to organs and tissues and age-, sex-, and organ-spe-cific risk models from the BEIR VII Report of the National Research Council.

Results: Effective dose from secondary neutrons was 428 mSv for the girl and 418 mSv for the boy. Approximately 21% of thisdose was produced by neutrons originating within the patient. These effective dose values are equivalent to the total dose fromapproximately 20 whole-body CT scans. The excess lifetime risks of developing a fatal SMN due to secondary neutrons was5.3% for the girl and 3.4% for the boy. The risk of developing any SMN, fatal or non-fatal, was 12.2% for the girl and 6.2%for the boy. Breast, lung, and thyroid cancers were the most likely radiogenic cancers in the girl, while lung and colon cancerswere most likely in the boy.

Conclusions: While the exposures to secondary neutrons were similar for the two patients, the carcinogenic risk was higher for thegirl than for the boy, which highlights the importance of using sex-specific phantoms and risk models when making risk predic-tions. Although the dosimetric advantage of treating with protons outweighs these risks, they are not negligible. Modifications tothe treatment delivery and to the treatment unit that would minimize neutron exposures should be investigated.

Author Disclosure: P.J. Taddei, None; D. Mirkovic, None; A. Mahajan, None; D. Kornguth, None; A. Giebeler, None; R. Zhang,None; M.C. Harvey, None; S. Woo, None; W.D. Newhauser, None.

3152 Reference RBE Method for Correcting RBE for Dose Dependence in Particle Beam Therapy

O. N. Vassiliev

M.D. Anderson Cancer Center, Houston, TX

Purpose/Objective(s): With the improvements in dose measurement and delivery, the RBE is becoming an increasingly importantsource of uncertainties in particle beam therapy. The present study addresses the uncertainty associated with RBE dose dependence.In treatment dose distributions doses vary continuously from a prescription dose to nearly zero. Experimental RBE data, however,is typically reported only for a limited number of dose levels. In proton beam therapy, for example, for the lack of sufficient data onRBE variations with dose and endpoint, the concept of a generic RBE was introduced, neglecting the variations. The purpose of thisstudy was to develop a method for calculating dose dependent RBE that can be readily applied to a dose matrix generated by treat-ment planning system.

Materials/Methods: The method has two components. The first one is the concept of a reference RBE (denoted RBEr) represent-ing an RBE for a specific endpoint measured at a dose dr of a particle radiation. The second component is the linear quadratic modelused to extrapolate the RBE to doses other than dr. The uncertainty of the method decreases with increasing number of referenceRBEs covering a broad range of doses. The model additionally assumes that parameter b is the same for particle and for gammaradiations. The assumption dates back to the theory of dual radiation action. It has been used in several recent publications.

Results: A mathematical formalism has been developed for RBE calculation that takes into consideration dose fractionation. Forexample, the RBE for a particle beam treatment relative to a gamma radiation treatment with the same number of fractions is givenby the formula: RBE = {[1+4b�RBEr+4b2+4b2�(RBEr

2-1)�dr/d]1/2-1}/(2b), where d is a particle beam dose per fraction at a pointof interest, b = bd/a is a dimensionless parameter, a and b are tissue and endpoint specific parameters of the linear quadratic modelfor gamma radiation. The method was tested on previously published experimental data on in vitro survival of mammalian cellsafter proton irradiation. RBEs extrapolated from doses of 7-8 Gy to 1-2 Gy agreed with direct low dose measurements either withinexperimental uncertainties or within 10%. The model was then applied to in vivo proton RBE data. It was noted that most of thepublished data was reported at doses exceeding the typical dose per fraction. In fact, the median dose per fraction of a representativesubset of the data was 11 Gy and the RBE was 1.1. According to the model, this translates into RBEs of 1.2 to 1.4 for the a/b ratiosof 10 to 2 Gy, at the dose of 1.8 Gy per fraction. This result, however, is associated with a large uncertainty owing to the scarcity oflow dose data.

Conclusions: A method has been developed for correcting RBE for dose dependence. The method provides simple formulae forconverting a 3D dose matrix into an RBE matrix.

Author Disclosure: O.N. Vassiliev, None.