ESTRO 2025 - Abstract Book

S115

Invited Speaker

ESTRO 2025

Differences do not exist between treatment modalities only, but the best dose summation method can be different in different localisations, too. In the case of the combined EBRT and BT of cervical cancer, the uniform dose conception (UDC) overestimates the total (EQD2) dose to the bladder by 12%, to the rectum by 55%, and to the sigmoid by 17.2% in average. With the dose summation using DIR, these overestimations are 27.3%, 59%, and 6%, respectively. The problem for DIR is the foreign body – namely the applicator and the Foley-catheter – in the BT images, which causes ‘artefacts’ in the deformed images. The target volume overlaps with the organs at risk if the warps of the deformation are larger than a cm. In this case, the total dose of the deformed organ will be equal to the prescribed dose, which is not the truth. Significant development of the DIR algorithm is needed to use it for dose summation in the clinical routine. In the combined EBRT and BT of the breast tumours, the warp vectors are small because of the similar breast anatomy in EBRT and BT, and the volume of the plastic interstitial catheters are tiny, so it cannot disturb the deformation algorithm. Manual dose summation and DIR result in similar total doses to the target volume and the OARs. However, the conventional UDC method overestimates the total dose to the lung by 54% and ribs by 28% on average, the side effects are mainly in the skin, where this fast and automatic method provides similar results as the manual method. Is the difficult and time-consuming DIR method needed for dose summation of combined breast radiotherapy at all? The situation of dose summation in combined EBRT and BT of prostate cancer is similar to the case of the breast because there are (metal) interstitial needles in the BT images only. The UDC method overestimates the total dose to the rectum by 37% and to the bladder by 5% on average, but the side effects occur mainly in the urethra, where the total dose is underestimated by 1% using the conventional dose summation method. So, the fast and automatic conventional method seems to be appropriate for dose summation. Additionally, performing DIR is challenging because the gold standard image modality is transrectal ultrasound (US) in prostate BT, and US images do not contain information about the tissue densities. Importing US images into the DIR software is impossible, or if it is possible, the dose calculation is not performed. Using US, the presence of the US probe can also be a problem for the deformation. Further, dose distribution can be significantly different in the post-BT plan without the needles and the US probe than in the live plan, so DIR of the CT of the EBRT and the CT after BT does not use the dose values from the real BT plan (dose gradient is high in BT!). So if the dose summation with DIR is needed in combined prostate radiotherapy, significant development of the DIR algorithm is also needed before.

4838

Speaker Abstracts Adaptive treatment planning: Should we replan based on accumulated dose? Tomas Janssen Radiation Oncology, Netherlands Cancer Institute, Amsterdam, Netherlands

Abstract:

Online adaptive radiotherapy (OART) adjusts the treatment plan at each fraction to account for daily anatomical changes, typically guided by magnetic resonance imaging (MRI) or cone-beam computed tomography (CBCT). While this approach reduces the need for large planning target volume (PTV) margins, intrafraction motion remains a source of geometric uncertainty. Treatment margins, using conventional approaches, are determined based on population statistics to ensure proper target coverage for 90% of the patients. This however automatically implies that for 90% of the patients the PTV margins are too large.