Assessment Of Intra And Interfractional Organ Motion During Adjuvant Radiochemotherapy In Gastric Cancer

15132 Background: Adjuvant combined chemotherapy and radiotherapy (RT) in gastric cancer improves survival, however acute toxicity is substantial. Toxicity may be improved with three-dimensional (3D) RT, but organ motion must be considered in planning target volume (PTV) delineation. Methods: Participants (n=22) had baseline free breathing planning CT (CT0) with BodyFix immobilization. Abdominal CTs in free breathing (FB), inhale (I) and exhale (E) states were obtained in weeks 1, 3 and 5 of RT. Datasets were fused to CT0 in Pinnacle3 6.2 planning system using bone registration. Volumes of interest (VOIs) [right (R) and left (L) kidney, liver, stomach, pancreas, celiac axis and porta hepatis] were contoured and points of interest (POIs) were placed at each centre of mass. POIs were manually placed at the left dome of diaphragm and splenic hilum. Organ motion was determined by the difference between POI positions in cranial-caudal (CC), anterior-posterior (AP) and right-left (RL) directions. Maximal respiratory motion was determined from the difference between I and E positions. Interfractional displacement in organs relative to bones at weeks 1, 3 and 5 was determined on FB scans as compared to baseline. Results: Interfractional organ motion was maximal in CC direction with median absolute displacements (range) in mm of: splenic hilum 10 (0–52), stomach 8 (0.4–27.2), liver 7.4 (0.5–23.6), diaphragm 6 (0–28), L kidney 5.7 (0–37.3), R kidney 5.3 (0.2–35.3), pancreas 5.7 (0.3–29.1), porta hepatis 4 (0–14) and celiac axis 1.7 (0–9.1). Median interfraction displacement (range) in CC, AP and RL in mm for all organs was: 5.7 (0- 52), 2.1 (0–23.1), 2.3 (0–15.9). Positional difference between I and E state (median for all organs) was: 16 mm CC, 5.9 mm AP, and 1.7 mm RL with maximal individual breathing excursions of 59.9, 30.2 and 21.1 mm, respectively. Conclusions: Interfraction organ displacement relative to bones can be quantified and used in the safe design of 3D conformal radiotherapy. Respiratory motion can be substantial in some individuals. Accounting for organ motion in 3D RT planning is necessary and may reduce the toxicity of treatment. No significant financial relationships to disclose.