Pencil Beam Scanning (PBS) Proton Therapy for Children Requiring Radiotherapy with Anesthesia (GA) for Pelvic Rhabdomyosarcoma (RMS)

Abstract

<h3>Purpose/Objective(s)</h3> Radiotherapy (RT) for pediatric pelvic RMS presents unique challenges, particularly for young children who require GA during RT, for whom bladder/rectal filling can be particularly variable. PBS may reduce dose to OARs and risk of second malignancy vs. double scatter (DS) protons; however, the precision of PBS requires highly reproducible daily setup. This study was undertaken to describe use of PBS in patients requiring RT for pelvic RMS with daily GA. <h3>Materials/Methods</h3> Pts with RMS of pelvic soft tissues treated with PBS from 1/2013-6/2020 with GA were included in this IRB-approved retrospective analysis. PBS proton plans were developed for all. DS plans were generated for all, and photon (VMAT) backup plans for 9/11 pts. Statistical analyses used 2-sided Student's t-tests. <h3>Results</h3> 11 total pts, median age 2.4 y (R 9 m – 5.9 y), 6 male patients, all had fusion-negative RMS (10 group 3 disease; 1 recurrent bladder/vaginal wall). Primary site included bladder (1), prostate (6), uterus (2), and vagina (1). During simulation/daily treatments under GA, Foley catheter with bladder filling to 75% expected capacity was used for 10. Rectal balloon was used for 2; aggressive management of constipation to promote empty rectum was used for all. All pts had IGRT (daily kV +/- CT [6]). RT dose was 41.4 Gy after delayed primary excision (DPE) (3) and 50.4 Gy without (8). Of 10 pts with FU data, all were without evidence of disease at median FU 1.3 y (range 0.3 – 5.6 y). Acute G3 toxicities included anorexia (3), diarrhea (1), and dermatitis (1), with no G4+ toxicities. One pt developed late RT cystitis. Among pts treated to 50.4 Gy, mean bladder dose (p<0.05) and body V5 (p<0.001) were significantly lower for PBS vs. photons. Max bladder, rectum, and body doses were significantly lower for PBS vs. DS (p<0.05 for each). <h3>Conclusion</h3> PBS proton therapy for very young children with pelvic RMS demonstrates acceptable rates of local control and toxicity when delivered under GA with deliberate management of bladder/rectal filling. Integral dose is reduced with PBS vs. VMAT, and OAR doses are reduced with PBS vs. DS. PBS should be considered in future trials as a standard RT approach for pediatric pelvic RMS.