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Out-Of-Field Radiation Dose Considerations in Partial Breast Irradiation with Elekta Unity MR-Linac - a Feasibility Study

M Muruganandham1*, B Lee2, S Vedam3, J Wang4, J Yang5, M Salehpour6, S Shaitelman7, (1) MD Anderson Cancer Center, Sugar Land, TX, (2) UT MD Anderson, Houston, TX, (3) UT MD Anderson Cancer Center, Houston, TX, (4) MD Anderson Cancer Center, Houston, TX, (5) MD Anderson Cancer Center, Houston, TX, (6) M.D. Anderson Cancer Center, Houston, TX, (7) MD Anderson Cancer Center, Houston, TX

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Partial breast irradiation (PBI) using MR-linac is of great interest due to superior soft tissue contrast for target localization. However, electron streaming effect- tendency of back scattered, secondary electrons to focus in an orthogonal magnetic field can deposit dose to skin surfaces beyond irradiated region. This study aimed to investigate the effects of magnetic field on dosimetry using planning CT of PBI patients previously treated with conventional linac and measure the out-of-field doses during MR-linac PBI treatments with an anthropomorphic female phantom and TLD.

Methods: Current study includes 7 patients. Dose of 7.5 Gy x 1 was prescribed to 95% of mean PTV_eval. 7 field IMRT plans were created, including the presence of magnetic field, in Monaco TPS5.4 for the Elekta Unity MR-linac. Target volume coverage, hot spots, and DVH were reviewed to evaluate plan quality for clinical acceptability. Similar plan was created on phantom and delivered using Unity MR-linac system. Doses outside the fields were measured without and with 1cm bolus by placing TLDs on phantom at chin, lips, and nose.

Results: Most plans exhibited 5-20% doses away from irradiated region. Location and extent of these doses depends on target location, depth and contours of irradiated surface. Upper central lesion plans displayed doses at shoulder, upper neck, chin, lips and nose. Upper outer lesion plans showed doses mainly at ipsilateral arm. TLD doses measured without bolus: chin 6 -13%, lips 5-12%, nose 5%, and were in good agreement with TPS calculations. These doses were reduced to less than 0.5% with 1 cm bolus.

Conclusion: PBI with Unity MR-linac results in significant doses to skin surfaces away from the irradiated region. TPS estimated out-of-field doses agree with TLD measured values. PBI plans should be reviewed carefully and efforts to mitigate out-of-field doses be addressed prior to patient treatments.

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Keywords

Not Applicable / None Entered.

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined (general)

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