(OA22) Inter-Fractional Motion in Patients With Intact Cervical Cancer Treated on a MRI Guided External Beam Radiation Therapy System

Abstract

Treatment verification has improved significantly over the past decades, now with the ability to have on-board MRI guidance. MRI provides superior soft tissue contrast compared to CT-based imaging modalities and provides better delineation of pelvic organs and gross tumor volumes (GTV) for those with cervical cancer. Consensus contouring guidelines for intensity modulated radiation therapy (IMRT) for cervical cancer advise including the whole uterus in the target volume and adding large planning target volume (PTV) margins to account for inter- and intra-fractional uncertainties. The primary objectives of our analysis was to assess the inter-fractional GTV motion for those with an intact cervix using on-board, daily MRI and to assess the relationship between bladder and rectal volumes and GTV motion. We analyzed 125 daily set-up MRI’s from five patients with intact cervical cancer who received MRI-guided pelvic IMRT at our institution between September 2016 and October 2017. The GTV, bladder, uterus, and rectum were contoured on all 125 MRI’s, which included 25 daily set-up MRI’s for each patient. The daily set-up MRI was registered with the planning MRI to assess organ motion. Tumor motion was assessed using GTV centroid displacements baselined to the planning MRI. Tumor volume and organ volumes were determined for each fraction. Then, a linear regression analysis was utilized to evaluate the relationship between rectal and bladder volumes and GTV motion. Patients were advised to have comfortable bladder filling for treatment. The initial GTV’s ranged in size from 31.9 cc to 119.5 cc. All GTV’s decreased in size during the course of external beam radiation, ranging from 34.0% to 85.2% of the initial tumor volumes on the final day of treatment. The ranges of daily centroid GTV displacement for all patients were from 0.08 cm to 2.23 cm, and the median GTV displacement for each patient was 0.69 cm to 1.04 cm. On linear regression analysis, 4 out of 5 patients demonstrated a strong relationship between rectal volume and GTV displacement with 26.4% of these displacements larger than 1 cm. Rectal volumes had the strongest impact on anterior-posterior GTV motion. Rectal volume appeared to better correlate with GTV displacement than bladder volume. However, for large tumors (>75 cc), tumor shrinkage during the course of radiation therapy seemed to have the most significant impact on GTV displacement. GTV shrinkage for large tumors strongly correlated with inferior GTV shifts, which were up to 1.4 cm in magnitude. Inter-fractional target motion is considerable and is affected by a variety of factors, including target volume changes, rectal volume, and, to a lesser degree, bladder volume. Our data suggests GTV shrinkage for large tumors has the greatest impact on GTV displacement. Inclusion of the whole uterus within the target volume and large CTV to PTV expansions remain standard practice. However, since the GTV is easily visible on daily MR imaging, one could think that eventually the target volume may exclude the uterine fundus and may also allow for decreased PTV margins with the ultimate goal of decreasing treatment toxicity.