ESTRO 2021 Abstract Book

S1265

ESTRO 2021

Conclusion This improved device provides greater ease of use, positioning flexibility and receive sensitivity which could enable range verification during pulsed delivery of a 2 Gy fraction. For many tumor types, the new device can be positioned at preferred locations distal to the Bragg peak, whereas the old system had to be within 7 cm and was often laterally offset (Fig. e). Although validation on phantoms in proton therapy vaults is required, nonintervention patient studies could quickly follow because "listening" for thermoacoustic signals generated during the normal course of therapy is entirely passive. PO-1541 Dosimetric impact of tumour regression in carcinoma cervix treated with radical IMRT P. Ahlawat 1 1 Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, Radiation Oncology, New Delhi, India Purpose or Objective Tumour regression during intensity modulated radiotherapy (IMRT) in carcinoma cervix can be substantial, and thus can have major impact on doses to targets and organs at risk (OAR). To deal with this problem one may require re-contouring and re-planning. Thus, adaptive radiotherapy (ART) is a possible solution. ART is defined as changing the radiation treatment plan during the course of radiotherapy to account for various uncertainties; the most clinically applied amongst all is to correct the target size, shape and position. The present study was therefore aimed to quantify the tumour regression during the course of IMRT, and its volumetric and dosimetric impact on target(s) and OAR using repeat planning computed tomography scan (CT) and magnetic resonance imaging (MRI) pelvis with intravenous contrast. Materials and Methods This was an observational study in which 35 patients with carcinoma cervix treated with radical IMRT were selected. Baseline MRI was done for all patients. Initial plan was created for 50Gy in 25 fractions. At 36Gy a repeat MRI pelvis and planning CT (with same simulation steps as was done for initial planning CT) were done. Target and OAR were delineated again and replanning was done again for 50 Gy in 25 fractions. Tumour regression was calculated. Dosimetric comparison was done between the initial plan and replan. The complete treatment was done with the initial plan only. Results Thirty five patients were studied. There was mean GTV regression of 4.62 cc, although small but statistically significant regression (p = 0.002). Twelve patients had GTV regression (>15 cc). There was significant decrease in clinical target volume (CTV) between initial plan and replan. There was significant increase in CTV coverage. There was a non-significant increase in bladder D2 (dose received by 2 % volume) by 0.01 Gy, bladder mean by 0.03 Gy, and bladder V50 (volume in % receiving 50 Gy) by 0.6 %. There was non-significant increase in rectum D2 by 0.68 Gy, rectum mean by 0.4 Gy and rectum V50 by 2.18 %. Similarly, there was non- significant increase in bowel mean dose by 0.01 Gy, bowel V15 by 3.57 % and bowel V45 by 0.52 %.