ESTRO 2021 Abstract Book

S1589

ESTRO 2021

Figure 1: Minimum relative absorbed dose (top) and relative D98% (down) of the PTV for automatic and modified contour planning. Absorbed doses received by the 98% of the volume for each PTV are also displayed. The average is 97.34 ± 2.08 % and 97.36 ± 1.86 % for the automatic and the modified structure respectively.

Figure 2: Absorbed dose profile at the same cross-section for automatic (top right) and modified (down right) contour planning. In the dosimetric calculation, the variation of the threshold in the external structure implies that the minimum absorbed dose received by the PTVs is higher, on average. This average shows an increase of 13% as compared to the automatically segmented. Furthermore, there exist four cases were the auto-segmentation stand out due to a marked difference between the minimum doses. The discrepancies in D98% are not high, with no clear trend. The dose profile in figure 2 shows how the modification of the inferior HU threshold for body segmentation influences the calculation, in particular, for a prescribed 60 Gy dose PTV, whose boundary begins at the zero position of the horizontal axis. Conclusion The modification introduced for the external contour not only has dosimetric impact but also could affect the coverage of the PTV coverage. PO-1865 Simultaneous integrated boost IMRT versus sequential IMRT in head and neck cancers M.I. Sharief 1 , P. Ahlawat 1 , M. Gairola 2 , T. Erum 3 , S. Tandon 1 , S. Purohit 2 , N. Sachdeva 1 , K.S. Chufal 2 , I. Ahmad 1 , R.L. Chowdhary 2 , V. Antony 4 1 Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, Radiation Oncology, Delhi, India; 2 Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, Radiation Oncology, New Delhi, India; 3 ESIC Medical College & Post Graduate Institute Of Medical Sciences And Research, Rajajinagar, Bangalore, Anesthesiology Pain And Crtical Care, Bangalore, India; 4 Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, Radiation Oncology, New Delhi, India Purpose or Objective Intensity Modulated Radiotherapy (IMRT) is currently considered the preferred modality of radiotherapy for head and neck cancers (HNC). The two common strategies to deliver IMRT are simultaneous integrated boost IMRT (SIB-IMRT) and sequential IMRT (Seq-IMRT). In Seq-IMRT same fraction size of 2 Gy as a single fraction per day is delivered to initially treat the elective volume and then to finally treat the boost volume in sequential manner. In SIB-IMRT different dose levels (2 or 3) are treated simultaneously with different doses per fraction. The superiority of one technique over the other has not been proved. The current study aimed to do a dosimetric comparison between these two techniques.