The Use of Interstitial Spacer to Improve Therapeutic Ratio in Stereotactic Radiation Therapy for Retroperitoneal Oligometastasis

Abstract

Perirectal interstitial hydrogel spacer (HS) is frequently used as a safe and effective method of limiting rectal dose in prostate cancer radiation therapy (RT). There are limited reports in the literature of its use outside the pelvis and no reports of its use for Stereotactic Body Radiation Therapy (SBRT) for abdominal oligometastases. We present a case involving a novel indication for the use of HS: to separate bowel from the high dose region in a patient receiving SBRT for a retroperitoneal oligometastasis from metastatic gastric adenocarcinoma. Using an aseptic technique, 5ml of 1% lignocaine was infiltrated subcutaneously. Under light intravenous sedation and CT guidance, a Chiba needle was used to access the space between the left retroperitoneal mass and adjacent bowel loops. Subsequently 50ml of normal saline was injected to achieve hydro-dissection of the tissue plane to create an anatomical space that was filled by 22ml of HS. Radiation therapy (RT) plans were prepared for pre-spacer and post-spacer anatomy. The prescription was for 40Gy/5# to be delivered using a volumetric modulated-arc technique (VMAT). Treatment planning and dose calculation was performed in Monaco version 5.11. Pre-and post-spacer RT simulation computed tomographies (CTs) demonstrated visibly appreciable separation of bowel loops lateral to the tumor mass (Figure – not shown in abstract). Dose-volume histogram parameters demonstrated a reduction in large bowel maximum dose (Dmax 44.8 Gy vs 35.8 Gy) and volume receiving ≥25Gy (28.3cc vs 18.2cc), exceeding and then meeting departmental tolerances on the pre- and post-spacer plans respectively. There was tumor progression between planning CTs - the pre-spacer planning target volume (PTV) was 300cc, the post-spacer PTV was 333cc. Thus, PTV dosimetry did not improve due to contact with bowel on the medial aspect of the tumor.Tabled 1Abstract 2624; TableTargetOrgans at RiskPTVCTVLarge BowelKidneySpinal CordSmall BowelD98D50D2DmeanD98D50D2DmeanDmaxV25 (cc)DmaxV23 (%)DmaxV23 (cc)DmaxV19.5 (cc)Pre-Spacer35.347.651.245.943.749.251.448.844.828.344.926.513.9034.917.0Post Spacer31.847.450.745.637.649.250.848.235.818.244.825.816.2033.630.5 Open table in a new tab In this case, HS was successful in separating adjacent bowel from the planning target volume and improving large bowel RT dosimetric parameters. HS insertion in the retroperitoneum is feasible and has the potential to improve the therapeutic ratio in appropriately selected patients where PTV dose is limited by an adjacent organ.