ESTRO 2023 - Abstract Book

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ESTRO 2023

The largest and lowest differences were observed in, respectively, H&N and abdomen regions. Mean V95% dose differences were 4.8±4.7%, 0.3±0.5%, and 0.1±0.1%, for the same couples.

Conclusion The efficacy of internal guidelines in reducing the CTV_LN inter- and intra-variability was demonstrated. The high target coverage agreement of CTV_LN_Old vs. CTV_LN_RO1 revealed the historical CTV-to-PTV margins to be safe even if relatively low DSC was observed. The introduction of GL increased the CTV dose coverage could allow margin reduction.

PO-2356 therapeutic radiographer contouring of target volumes for low and intermediate risk prostate cancer

S. Jansen van Rensburg 1 , M. Collins 2

1 GenesisCare, Maidstone, West Malling, UK, United Kingdom; 2 Sheffield Hallam University, College of Health, Wellbeing and Life Sciences, Sheffield, United Kingdom Purpose or Objective To demonstrate whether a therapeutic radiographer trained dosimetrist is competent to independently contour clinical target volumes (CTVs) for radiotherapy to the prostate +/- seminal vesicles on CT. Materials and Methods A retrospective contour comparison of therapeutic radiographer outlined prostate and seminal vesicle contours across 55 datasets using CHHiP guidelines for Group 1 and 2 risk cohorts. Contours were compared in terms of absolute volume, and similarity using DICE coefficient and mean distance to agreement. This was completed across a purposive sample of low and intermediate risk prostate cancer patients treated with radiotherapy by 26 different clinical oncologists across the group. Both prostate and seminal vesicle contours were considered independently. Results Prostate contours alone were comparable to clinical oncologist contours across the sample with a mean DICE coefficient of 0.84. Contouring for seminal vesicles was less accurate with a mean DICE coefficient of 0.63. Mean distance to agreement across prostate contours was 1.45mm. Mean distance to agreement across seminal vesicle contours was 1.39mm. Mean absolute volume difference for prostate was 3.45cc. Mean absolute volume difference for seminal vesicles was 1.75cc. Conclusion Prostate contours were generally comparable to the clinically-delivered clinical oncologist volumes, which is in keeping with this being a fairly stable anatomical structure, and that in radiotherapy planning the prostate itself is contoured in its entirety in all cases. Confounding factors for this included the potential inclusion of MR imaging by the consultant clinical oncologists, and variations in practice amongst the clinical oncologists. Seminal vesicle contours had less overlap. Seminal vesicle contouring varies according to clinical risk factors, and although the therapeutic radiographer contoured according to CHHiP criteria as a chosen baseline, the clinical oncologist contours were not necessarily based on the same set criteria but rather their clinical judgement at the time of outlining the patient informed by clinical practice recommendations. This is a limitation of this study, and further work should aim to be prospective to allow a true “like for like” comparison. This study potentially opens the door for such work. This study demonstrates the ability of the therapeutic radiographer to accurately delineate the prostate structure itself on CT anatomy, and therefore suggests a start point for further development work to allow autonomous target volume delineation across the cohort, to include accurate and appropriate seminal vesicle contouring. 1 Salah Azaiez, radiotherapy, Tunis, Tunisia; 2 Salah Azaiez, radiotherapy, tunis, Tunisia; 3 salah azaeiz, radiotherapy, tunis, Tunisia; 4 salah azaiez, radiotherapy, tunis, Tunisia; 5 Salah azaiez, radiotherapy, tunis, Tunisia Purpose or Objective Nasopharyngeal carcinoma (NPC) is the second most common neoplasm of head and neck in Tunisia. It affects young people. In fact, the distribution is bimodal between |15- 20 years old] and a 2nd peak at 50 years of age. Frequently, the diagnostic is made in advanced stage of the disease. Our objective is to evaluate the dose received by the hippocampi among patients treated for NPC Materials and Methods Our study included 30 patients treated for NPC, in the department of radiotherapy at Salah Azaeiz institute. The dose delivered was 70 GY for high risk volume and 59.4Gy for prophylactic volume using the simultaneous integrated boost (SIB)- VMAT technique (volumetric modulated arc therapy). We delineated the hippocampi retrospectively using axial brain MRI fused to the radiotherapy planning CT scan. We evaluated the maximal dose (Dmax) to the hippocampi, the mean dose (Dmean) and the dose delivered to 40% of the volume (V40 %). Treatment planning was performed on the TPS Eclipse (Algorithm AXB13.7 and PO13.7, Varian medical system, PA, CA). PO-2357 Should we evaluate the hippocampi dose in all patients treated for nasopharyngeal carcinoma? A. Mousli 1 , S. Massoudi 2 , K. Ben zid 2 , A. Yousfi 3 , R. Abidi 4 , C. Nasr 5

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