ESTRO 2020 Abstract book

S1084 ESTRO 2020

Center for oncology Paul Papin Centre in Angers, Radiation Physics, Angers, France

Poster: RTT track: Treatment planning and dose calculation / QC and QA

Purpose or Objective 4-dimensional scan (4DCT) is the standard imaging modality to account for respiratory motion during treatment planning. Its use remains very heterogeneous specially the repetition of the 4DCT in which indications are poorly studied. We evaluated how the use of a single 4DCT impacts the dosimetry for lung cancer treated with stereotactic radiotherapy (SRT). Material and Methods A retrospective analysis was performed on two scans over two different days (4DCT1 and 4DCT2) for 30 patients with primary or secondary lung cancer treated by SRT from April 2018 to September 2019. 15 patients were scanned on a GE Lightfield scanner (slice thickness: 2.5 mm) and the other half on a Phillips Big Bore scanner (slice thickness: 1 mm). The Gross Tumor Volume (GTV) was defined on each phase (0 to 90) of each 4DCT and the internal target volume (ITV) was created. The displacement of center of mass and ITV’s volumes were studied in anterior-posterior (AP), superior-inferior (SI) and right-left (RL) directions. New dosimetry from 4DCT1 was performed and compared to initial dosimetry (from 4DCT1 and 4DCT2). Several metrics were used: maximum dose (D2%PTV), median dose (D50%PTV) and minimal dose (D98%PTV) to the PTV and median dose to the ITV (D50%ITV). Results The mean displacement of the center of mass was reproducible (less than 3 mm) between 4DCT1 and 4DCT2 for 77 % of the patients. Deviations superior to 3 mm or greater in SI direction were however observed for 13 % of upper lobes and 33 % of lower lobes. The volumetric ITV generated from 4DCT1 was 15 % smaller than the ITV resulting from the union of the ITVs of 4DCT1 and 4DCT2. D2%PTV, D50%PTV and D50%ITV were similar on dosimetric plans based on 4DCT1 and the union of 4DCT1 and 4DCT2. The D98%PTV ratio was 0.91 (± 0,1) for all lesions but was close to one (0.98) for upper lobes. For lower lobes characterized by larger tumor movement the D98%PTV ratio was 0.84 enlightening a possible underdosage (up to 70 %) when only one 4DCT is realized (see figure 1). No difference was observed between the 2 scanners although slice thickness differed.

PO‐1852 Changes in Dose Distribution in VMAT Plan for T3/4 NPC cases after teeth removal Y. Li 1 1 Tuen Mun Hospital, Department of Clinical Oncology, New Territories, Hong Kong SAR China Purpose or Objective Removal of loose teeth after the head and neck planning steps like computed tomography (CT) images acquisition leads to changes in dose distribution of treatment plans. T3/4 nasopharyngeal carcinoma (NPC) treatment plans are usually resulted with marginal acceptable dose to organs at risk (OARs) due to the large tumor size. This work aims to assess the applicability of original treatment plans for the patient who has removed loose teeth after CT image acquisition to avoid re-planning and delaying the treatment. Material and Methods Fifteen T3/4 NPC patients undergoing volumetric modulated arc therapy (VMAT) were included in this retrospective study. Their upper and lower teeth were contoured as upper/lower left, anterior and right portions. Those teeth were overridden the densities to 0g/cm 3 respectively to simulate removal of teeth and recalculation of plans were performed. Dosimetric comparisons of brainstem (BS), spinal cord (SC), chiasm, optic nerves, temporal lobes, temporomandibular (TM) joints and brachial plexus were carried out among original plan and the 6 recalculated plans. Furthermore, correlation tests were performed to find the relationship between the distance from (1) the primary tumor to 5mm planning organ at risk volume (PRV) of SC or (2) the planning target volume (PTV) receiving 54Gy (PTV54) to 5mm PRV of SC and maximum dose (Dmax) of 5mm PRV of SC. Also, the correlation tests between their volumes and Dmax of 5mm PRV of SC were investigated. Results All recalculated plans showed with significant differences when compared Dmax of bilateral optic nerves, temporal lobes and TM joints with that of original plan. For the Dmax of BS, chiasm and left brachial plexus, only the plans with removal of low anterior teeth showed no significant difference. For SC, only the plans with removal of upper teeth showed with significant differences. There was no correlation found between the distance from primary tumor/PTV54 to 5mm PRV of SC and Dmax of 5mm PRV of SC. Furthermore, their volumes showed no correlation with Dmax of 5mm PRV of SC. 2 out of 15 cases showed with exceeded doses in Dmax of 5mm PRV of SC and all were related to upper portions (5000.5-5010.5cGy). The upper teeth were further divided into 6 groups for investigation, 1 of the cases showed no exceeded dose while another case showed exceeded dose in 2 upper portions (5000.1-5002cGy).

. Conclusion

For upper lobes it seems there is a small dosimetric impact to have a single 4DCT: metrics are in agreement within 10 %. For lower lobes with a tumor motion larger than 10 mm there is a risk of potentially significant difference for the D98%PTV, hence the interest of a second 4DCT.