Room: AAPM ePoster Library
Purpose: Several trials are currently investigating the potential of external beam accelerated partial breast irradiation (APBI) to reduce the burden of radiation therapy after breast-conserving surgery in early stage breast cancer. Respiratory motion can degrade both coverage and dose homogeneity of treatment plan delivery. A few studies report the interplay effects of respiratory motion and multileaf collimator motion during breast intensity-modulated radiation therapy delivery, however, the effect on volumetric modulated arc therapy APBI (VMAT-APBI) has not been examined. This study investigated the amplitude of respiratory motion during VMAT-APBI delivery that significantly impacts dose distribution.
Methods: A total of 10 patients with planned VMAT-APBI were included. The prescribed dose was 28.5 Gy in 5 fractions. Results were measured by Delta4 diode array phantom, ionization chamber, and radiochromic film. A new breast phantom that models the average body shape of Asian women was created and it measures point dose and dose distribution. Point dose and dose distribution were measured under static and respiratory motion (2, 3, 5 mm in anterior-posterior and superior-inferior direction) conditions with ionization chamber and radiochromic film. Comparisons of measured and calculated point doses or dose distributions were evaluated by dose difference or gamma passing rates.
Results: For all measurements under static conditions, point doses, and dose distributions showed agreement. The average gamma passing rates with 3% and 2 mm criteria (average ± standard deviation) in the film measurement under the conditions of static, 2 mm, 3 mm, and 5 mm amplitude were 95.7 ± 1.1%, 93.9 ± 3.3%, 93.8 ± 2.9%, and 83.7 ± 5.8%, respectively.
Conclusion: We recommend that if the amplitude of respiration is greater than 5 mm, managements, such as breath-hold or shallow breathing, should be employed to avoid unintended dose distribution.
Funding Support, Disclosures, and Conflict of Interest: This work was supported in part by St. Luke's Breast Charity Fund.