Purpose and Objectives: To investigate POSTER No: 1064 Title: A study to evaluate CTV to PTV margins for Pelvic nodal region and CTV to ITV margins for Utero-cervical complex during cervical cancer radiotherapy Umesh Mahantshetty, Ankita Nachankar, Yogesh Gadi, Suresh Chaudhary, SV Jamema, Reena Engineer, Supriya Chopra, Deepak Deshpande, Shyamkishore Shrivastava Department of Radiation Oncology and Medical Physics, Tata Memorial Centre, Mumbai, India. Purpose and Objectives: To investigate - CTV to PTV margins for pelvic nodal regions, - Primary tumor regression and - CTV to ITV margins for the utero-cervical complex Material and methods: Patients: Histologically proven cervical cancer underwent standard radical radio (chemo) therapy. RT planning: Strict bladder filling, planning CT with contrast, contouring (targets & OAR) and 3D-CRT / IMRT planning. CTV to PTV: Anisotropic with 10 mm in SUP-INF & 7 mm in AP and bilateral Tumor Regression: Pre-treatment and mid-treatment (24 – 30 Gy) MR pelvis IGRT Protocol: CBCT / MVCT of pelvic region were performed daily before treatment delivery. CTV to PTV margins for pelvic nodal CTV: Matching was done for pelvic vessels /nodal region (Shifts Set I) CTV to ITV margins for primary CTV: Matching for utero-cervical complex was done (Shifts Set II) Difference of Shift Sets I & II was compiled as surrogate for organ motion.

CTV- PTV for Pelvic Nodal CTV CTV – ITV for Primary Cervical Tumor Shifts Set I Difference between Shifts Set I & II TABLE: Showing shifts in various directions for CTV to PTV for nodal CTV TABLE: Showing difference in shifts Set I & II for CTV to ITV margins for Primary cervical tumor Ant. 10.7 mm (median 9.5, range 2-48) Post. 12.9 mm (median:10, range: 3–25) Sup. 10.4 mm (median 8, range 3-38) Inf. 12 mm (median:10.5, range:1–31) RT. lat 8.7 mm (median 7, range of 2–22) LT. lat 8.8 mm (median 8.5, range of 1-43) Ant. – Post. 7.9 + 2.1 mm (range:2 –14.4) Sup. – Inf. 7.8 + 2.8mm (range 3.5–18.2) Lateral 5.5 + 1.7 mm (2.5–12.9)

TUMOR REGRESSION Pre Rx MR showing tumor topography GTV: 67cc Pre RT Mid Rx MR showing tumor topography GTV: 27cc AFTER 26 Gy Mean cervical tumor volume reduced from 58.4 cc (median 45.4; range 10.9-289.8) at diagnosis to 28.3 cc (median 14.9; range 4.91–217.6) at mid treatment. There was no correlation between the initial volume, FIGO stage, regression pattern and displacement of utero-cervical complex.

CONCLUSIONS: Our study suggests - An anisotrpoic PTV margins of 10-15 mm to pelvic nodal CTV - Mean utero-cervical complex motion are in the range of 6-8 mm which may be sufficient for CTV to ITV margins - Mid treatment tumor regression occurs by almost 50% irrespective of initial tumor volume, stage or regression pattern