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A Comprehensive Investigation of Dosimetric Uncertainties in Junction Region of Breast Superclavicular and Tangential Fields


T Han

T Han1*, R Tailor2, A Melancon3, R Howell4, F Mourtada5, M Salehpour6, (1) UT MD Anderson Cancer Center, Houston, TX, (2) UT MD Anderson Cancer Center, Houston, Texas, (3) MD Anderson Cancer Center, Houston, Tx, (4) UT MD Anderson Cancer Center, Houston, TX, (5) Christiana Care Hospital, Newark, DE, (6) M.D. Anderson Cancer Center, Houston, TX

SU-C-137-7 Sunday 1:00PM - 1:55PM Room: 137

Purpose:
The dosimetric uncertainties in junction regions are one of the major concerns in treatments involving field-junctions, especially tangential breast and supraclavicular fossa radiotherapy fields. The uncertainties are linked to dose calculation algorithms, set-up, jaw/couch position uncertainties, and choice of mono- versus multiple- isocentric techniques. The purpose of this study was to comprehensively investigate the effects of these common variants to understand dose non-uniformity in junction regions.

Methods:
To understand the effects from dose algorithms and set-up uncertainties, a typical tangential/superclav field combination was planned to an anthropomorphic Rando phantom using the Pinnacle-9.0 treatment-planning system and delivered 6 times on different days. 3 films (EBT2) were placed in coronal plane across the field-junction. Pinnacle CCC-9.0, Eclipse AAA-10.0 and AcurosXB-11.0 were used for calculation. To understand the effects from the linac's mechanical uncertainties, junctions were generated with two open fields using both mono-isocentric and two-isocentric techniques. Junction-fields were delivered on 6 different linacs to EBT2 in a solid water phantom. Impacts of couch position accuracy/reproducibility, and jaw-position accuracy/reproducibility were analyzed.

Results:
In general, the major contributor to the dose uncertainties in junction region is the jaw position accuracy. Among the 6 linacs investigated in this study, the uncertainty of jaw position can be up to ±3mm, subsequently leading to about 25% hot/cold of dose distributions in junction region. The impact of mono versus two-isocentric technique is relatively small (within 5%), because the patient-couch positioning was found to have good accuracy and reproducibility. All dose algorithms are found to be in close mutual agreement (within 5%) for most regions of the junction. Jaw positioning has good short-term reproducibility, but might change over months.

Conclusion:
Dose inhomogeneities at field-junctions are more strongly dependent on jaw-positioning accuracy compared to set-up, dose-calculation, and other source of errors.

Impact:
Guidance to improve dose uniformity in field-junction.


Funding Support, Disclosures, and Conflict of Interest: This work was funded by National Institutes of Health grants 2R44CA105806-02, CA010953

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