Program Information
Evaluation of Dosimetric Impact of Electron Return Effect in MR LINAC On Pediatric Patients
E Han1*, J Shin2 , Z Wen1 , C Lee3 , (1) UT MD Anderson Cancer Center, Houston, TX, (2) Massachusetts General Hospital, Brookline, MA, (3) Radiation Epidemiology Branch, DCEG, NCI, NIH, DHHS, Rockville, MD
Presentations
PO-BPC-Exhibit Hall-7 (Saturday, March 18, 2017) Room: Exhibit Hall
Purpose: Secondary electrons exiting skin under 1.5 Tesla magnetic field can curve back and re-enter the skin, resulting in a dose increase at the beam exiting skin, which is called Electron Return Effect (ERE). The ERE can also occur at the lung-tissue interface. Pediatric patients have internal organs compacted in their smaller bodies and are often treated using large fields such as whole lung or cranial spinal irradiation (CSI), which make the ERE more prominent. So far, no dosimetric impact of ERE on pediatric patients has been evaluated in Elekta MR-LINAC. We characterized dosimetric impact in MR-LINAC on pediatric patients using Monte Carlo radiation transport method coupled with computational human phantoms.
Methods: International Commission on Radiological Protection pediatric phantoms representing 1 and 15 years old individuals were incorporated into TOPAS Monte Carlo code and Monaco treatment planning system. Two Monaco plans - whole lung irradiation and CSI were generated for each phantom and the same beam geometries were simulated in TOPAS with phase space beam data of MR-LINAC provided by Elekta. Skin and lung dose and dose profiles were evaluated with and without magnetic field. Plan quality was compared between TOPAS and Monaco.
Results: In CSI, more than 30 % dose enhancement was found at the skin due to magnetic field. In whole lung irradiation, the opposing beams did not completely compensate for the ERE. There seemed to be an overall agreement between TOPAS and Monaco plans with presence of the magnetic field, despite of some regional dose discrepancy.
Conclusion: We have computationally quantified the skin dose enhancement due to ERE in the whole lung and CSI plans, and thus concluded that the efforts to eliminate hot spots on the skin are required for better TCP and NTCP, such as, adding a bolus as an absorber for those returning electrons.
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