Author: Jamie s Baker, Sam Beddar, Mahsa Dehghanpour, Ariel MT Hoang, Emma Holliday, Rachael M. Martin Paulpeter, Joshua S. Niedzielski, Luis Augusto Perles, Josie S Ruzek, Gabriel O. Sawakuchi 👨🔬
Affiliation: MD Anderson Cancer Center, The University of Texas MD Anderson Cancer Center, UT MD Anderson Cancer Center, niversity of Texas MD Anderson Cancer Center School of Health Professions, Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, University of Texas MD Anderson Cancer Center School of Health Professions 🌍
Purpose: This study aims to evaluate the dosimetric impact on organs at risk (OAR) of small versus large spot sizes when treating pelvic cancer patients with pencil beam scanning (PBS) proton therapy. The two different spot sizes are associated with the old and new Hitachi Proton machines available at UT MD Anderson CC.
Methods: This is a retrospective study involving patients enrolled in a clinical trial being treated for anal cancer with lymph node involvement. For each of the 8 patients, two plans were created in RayStation 12A, one utilizing large spots (FWHM: 3-4cm) and another with small spots (FWHM: 1.1-2.1cm). Both plans had identical beam configuration and were optimized to provide clinically acceptable target coverage while sparing OARs similarly. A paired t-test was used to compare mean doses to bowel bag, bladder, bone marrow, and genitalia between the two sets spot sizes.
Results: The plan with the small spot sizes significantly (p<0.001) reduced the OAR mean doses. The median reduction of the mean dose for bowel bag, bladder, bone marrow, and genitalia were respectively 21%, 27.7%, 15.8%, 39.9%.
Conclusion: There is a significant dose reduction in pelvic OARs when using the smaller pencil beam spot sizes.