PQA 08 - PQA 08 Genitourinary Cancer, Patient Safety, and Nursing/Supportive Care Poster Q&A
3328 - Optimizing Rectal Dose and Minimizing Toxicity in Prostate Radiotherapy in a Randomized Pivotal Trial: Assessment of Rectal Spacer Placement Effect on Rectal Dosimetry and Toxicity
Advanced Radiation Centers of New York New York, NY
S. H. Zimberg1, J. Shah2,3, V. Agrawal4, and D. Song5; 1Advanced Radiation Centers of New York, Lake Success, NY, 2Advanced Radiation Centers Of New York, New York, NY, 3Advanced Radiation Centers of New York, New York, NY, 4Harvard Medical, Boston, MA, 5Johns Hopkins University School of Medicine, Baltimore, MD
Purpose/Objective(s): The insertion of rectal spacers between the prostate and the rectum has been shown to reduce dose to the anterior rectal wall and late rectal injury. We hypothesize that optimizing the symmetry and spatial distribution of a rectal spacer can improve rectal dosimetry and decrease rectal toxicity. We report an evaluation of the association of symmetric distribution of a novel saline-filled pre-formed degradable balloon spacer to rectal dose distribution and rectal toxicity. Materials/
Methods: CT simulation images were analyzed from 222 patients enrolled in a multi-center, randomized controlled pivotal study from 2018 to 2022. Assessment of balloon symmetry relative to prostate midline was scored as: S (symmetrical), 1L/1R (1 cm left or right lateral to midline), 2L/2R (2 cm left or right to midline), or 0 (no balloon). Scoring was assigned at each of 3 axial slices (midgland, 1cm superior and 1cm inferior to midgland). A total symmetry score (SYM) was then calculated per patient (SYM-S for symmetric and SYM-1 to SYM-4 for variant degrees of asymmetry). Acute rectal toxicity was assessed using CTCAE v4.0. The effects of symmetry score on rectal dosimetry and toxicity were evaluated using t-test and chi-squared test. Results: A total of 141 patients had balloon spacer placement with CT images available for analysis. Overall symmetrical balloon distribution, defined as SYM-S, SYM-1, or SYM-2, was seen in 120 patients (85.1%). Mean balloon height at CTV center was 15.06 (±6.92) mm. Marginally symmetrical distribution (1L/R) was seen in 18 patients (12.8%) with a mean balloon height of 11.38 (±9.23) mm. Asymmetrical distribution (2L/R) was seen in only 3 patients (2.1%) with no observed balloon height. Compared to perfect symmetrical placement (SYM-S), variations of asymmetrical balloon distribution (SYM-1 to SYM-4) were significantly associated with less rectal dose reduction at all dosimetric thresholds, namely rV40 (61.8% vs. 46.7%, p=0.006), rV50 (76.1% vs. 58.2%, p<0.001), rV60 (86.1% vs. 67.7%, p<0.001), rV70 (92.3% vs. 75.6%, p<0.001 ) and rV80 (96.8% vs. 85.4%, p<0.001). Additionally, asymmetrical balloon distribution was associated with a trend for greater rectal toxicity at 6 months following spacer placement. Patients with SYM-S score reported lower rates of grades 1 and 2 rectal toxicities (10.4% and 2.6%, respectively), compared to patients with SYM-1 to SYM-4 score (15.6% grade 1 and 4.7% grade 2), p= 0.223. Conclusion: Rectal spacer spatial distribution significantly impacts rectal dosimetry and degree of rectal sparing. Symmetrical rectal spacing using a pre-formed saline-filled degradable balloon was associated with a trend to lower rates of acute rectal toxicity. Future clinical studies are needed to confirm these findings in larger cohorts, as well as evaluate long-term effects on rectal toxicity.
