ESTRO 2023 - Abstract Book

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ESTRO 2023

assessment time <9min 59sec and >10minutes. The overall uncorrected interfraction data shows that M is not small compared to ∑ (M= 0.9mm, ∑ 1.5mm) in the longitudinal direction; this will not have impacted the patient as this is corrected. The intrafraction data shows that M is small compared to ∑ and ∑ < 1mm in all directions. Because of this, and small sample size, ∑ is assumed to be zero for margin calculation. Assuming a delineation systematic error of 1mm, and random error as shown in Table 1, the calculated CTV-PTV margin is 2.6mm in all directions, with β = 0.84 for hypofractionation and penumbra 0.3. Due to the small sample in the stratified data, data should be reviewed with caution, however, the random error indicates it is appropriate to use the same margin within these sites. Data also supports current department practice to re-image if CBCTs take longer than 10 minutes to assess. Longer-term data will be analysed once sample sizes are bigger.

Conclusion A CTV-PTV margin of 3mm has been found to be appropriate for this treatment site, and the random error associated with intra-fraction motion is small. As a result of this study, a departmental change in practice has occurred to omit post treatment imaging for this patient cohort. The evaluation has indicated there may be a potential systematic error in the treatment pathway impacting longitudinal setup which requires investigation.

PO-1613 Is lumbosacral plexus delineation a must in modern pelvic radiotherapy ?

Z. Dahbi 1 , F. Kouhen 2 , M. Moukhlissi 3

1 1-Mohammed VI University of Health Sciences - Cheikh khalifa university hospital, radiotherapy , Casablanca, Morocco; 2 Mohammed VI University of Health Sciences , radiotherapy, Casablanca, Morocco; 3 Mohammed VI university hospital, radiotherapy, Casablanca, Morocco Purpose or Objective Radiation-induced lumbo-sacral radiculopathy is an under-reported long-term side effect of external pelvic radiation therapy. It consists of a variety of symptoms that can have a serious and significant impact on the quality of life of patients after cancer treatment. Our aim is to evaluate a correlation between this clinical syndrome and the doses received at the lumbo-sacral plexus. First, we used the Medical Outcome Study Short Form 36 (MOSSF36) to evaluate symptomatic lumbosacral radiculopathy in 150 patients, who have been currently cancer free and under follow-up after being treated with pelvic radiotherapy in our department. Target volume delineation : Delineation of the lumbosacral plexus was performed on the same dosimetric scan used to treat these symptomatic patients. For this study, the same radiation oncologist had delineated the lumbosacral plexus on each dosimetric scan from the L4 L5 interspace to the level of the sciatic nerve, with the assistance of a neuroradiologist using registration tools with pelvic MRI when available, and anatomy textbooks. Planning procedure and statistical analysis: All plans used the anisotropic analytical algorithm (AAA version 10.0.28) for dose calculation with a 2.5-mm calculation grid and heterogeneity correction.We collected dosimetric data from the treatment plans, to assess correlation with clinical analysis using spearman rank correlation All our results were compared and analyzed using SPSS software, version 10.0, and a statistical significance level of 0.05 was used (p < 0.05). Materials and Methods Patients selection :

Results