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Verification of Electron Return Effect in MR LINAC On Pediatric Patients for Whole Lung Irradiation and Bolus Effect

E Han

E Han1*, J Shin2 , Z Wen1 , C Lee3 , (1) UT MD Anderson Cancer Center, Houston, TX, (2) Massachusetts General Hospital, Brookline, MA, (3) National Cancer Institute, Rockville, MD


SU-F-205-8 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 205

Purpose: Electron Return Effect (ERE) can occur at exiting skin under 1.5 Tesla (T) magnetic field. Pediatric patients are often treated with large fields such as whole lung irradiation (WLI) or mantle field. The ERE may become more prominent in the large field. To reduce the ERE, it is proposed to add a layer of bolus as an absorber of the returning electrons. We quantitatively assessed the dosimetric impact of the ERE and a bolus effect by means of the phantom measurements and independent dose calculations from TOPAS Monte Carlo (MC) calculation.

Methods: ICRP reference pediatric phantoms representing 1-, 5-, 10-, and 15-year-old females were incorporated into Monaco treatment planning system. The WLI plan using antero-posterior and postero-anterior fields was generated for each phantom with and without 1.5T. Plans using bolus on the skin of posterior beam were generated and plan quality was compared with the plan of 0T. Female Rando phantom was used for measuring doses to lungs, skin and breast tissue. The WLI plan was generated and delivered with Elekta MR-LINAC. Dose was measured with Gafchromic film and thermoluminescent dosimeter and compared with two MC plans.

Results: Lung heterogeneity index increased up to 5%. Mean skin dose enhancement by 1.5T was 44%(1-year-old), 33%(5-year-old), 37%(10-year-old) and 30%(15-year-old female) and adding 1-cm bolus reduces the dose enhancement by 10-15%. Mean breast tissue dose enhancement was 11% for 10-year-old and 6% for 15-year-old. Using 1-cm bolus is better than 0.5-cm bolus for comparable plan quality and more efficient skin dose reduction.

Conclusion: Dosimetric impact of ERE on pediatric patients in WLI was assessed with two MC calculations which was validated by measurements. Since younger patients receive more skin dose by 1.5T, skin and breast tissue dose should be carefully reviewed. We confirmed that adding 1-cm bolus can significantly reduce skin dose.

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