3295 - Implications of Rectal Wall Gel Placement in Treatment Delay

G. Hixson¹, G. Y. Sun², V. Narayana³, C. Evans¹, E. P. Short¹, P. Heckman¹, J. Takayesu⁴, M. M. Cousins^4,5, and W. McLaughlin¹; ¹Department of Radiation Oncology, Assarian Cancer Center, Ascension Providence Hospital, Novi, MI, ²University of Michigan, Ann Arbor, MI, ³Ascension Providence Hospital, Southfield, MI, ⁴Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, ⁵Department of Radiation Oncology, Self Regional Healthcare, Greenwood, SC

Purpose/Objective(s): Knowledge of rectal anatomy in the prostate region is critical for proper hydrogel placement prior to definitive prostate radiation treatment. The rectourerthralis (RU) is a muscle independent of the rectal wall, running superior to the rectum and inferior to the prostate. It appears merged to the rectum on MRI and CT but is readily displaced with ultrasound probe and water dissection. After water dissection, gel can be placed within or inferior to the RU and anterior to the rectum. This location can give the appearance of gel placement within the rectal wall on post-placement cross sectional imaging. We explore the implications of this finding in a large series. Materials/

Methods: The study was an IRB approved retrospective review of technical outcomes in prostate radiotherapy through assessment of 300 patients undergoing definitive radiation who received hydrogel spacer placement under general anesthesia. Video tracking was implemented for the last 150 cases. Principles of placement included 1) optimization of probe position to increase the distance between prostate and rectum and decrease the risk of penetration, 2) avoid injection of water or gel against resistance, 3) goal separation of 1cm at base, mid, and apex. Gel position was classified into the following groups: 1) Superior to the RU with rectal exclusion (SRU RE), 2) superior to the RU with incomplete exclusion and lateral rectal extension around gel (SRU IE), 3) Inferior to the RU without rectal wall penetration (IRU), 4) Rectal wall penetration. Separation of the rectum and prostate was measured on axial imaging as minimal separation at base, mid, and apex Clinical consequences included severe pain and delay in therapy due to non-ideal placement. Rectal toxicity was scored by patient reported outcome instrument comparable to standard rectal toxicity scales.
Results: Of patients included in this analysis (n =300), 120 (40%) were classified as SRU RE, 106 (35.33%) as SRU IE, 74 (24.67%) as IRU. No patients had rectal wall penetration. Patients in groups 1-3 did not experience severe pain or require delay in treatment and did not develop complications during therapy based on a patient reported outcome survey. There was no statistically significant difference is separation at base, mid and apex by gel position (0.39, 0.86, and 0.79 respectively) based on one-way ANOVA.
Conclusion: Due to the appearance of gel in the rectal wall, radiation may be delayed. We classified the variant gel positions in relation to the RU and prostate. There was no significant difference in rectal separation among the major variants, and no delay was necessary for any variant.

Abstract 3295 – Table 1

Minimum separation between rectum and prostate	SRU RE (n=120)	SRU IE (n=106)	IRU (n=74)	All (n=300)
At the base [Mean (SD)]	1.33 cm (0.65)	1.26 cm (0.42)	1.21 cm (0.39)	1.28 cm (0.52)
At the mid-gland [Mean (SD)]	0.92 cm (0.68)	0.86 cm (0.46)	0.91 cm (0.56)	0.90 cm (0.58)
At the apex [Mean (SD)]	0.87 cm (0.46)	0.90 cm (0.44)	0.92 cm (0.41)	0.89 cm (0.44)