Session Item

The advent of stereotactic body radiotherapy (SBRT) has led to a high standard of precision and accuracy in dose delivery. However, modifications in the patient anatomy and organ-positioning affect the actual dose distribution to the lesion and to neighboring organs at risk (OARs), with possible impact on oncological and toxicity outcomes. The purpose of this study is to derive geometric and volumetric indexes representative of inter-fraction organ motion.

This study considers a subset of patients enrolled within the prospective phase II study sponsored by AIRC RADIOSA trial (NCT03940235) who treated pelvic lesions. For each patient and treatment fraction, OARs for each patient were contoured by clinicians on the simulation CT (sCT). Successively, the CBCTs were imported in RayStation (Raysearch®) and registered with the sCT and all the relevant OARs inside the field of view were contoured. Organ motion was then estimated in five significant OARs close to the target (right femoral head, left femoral head, bladder, rectum, iliac wing) by (i) means of geometrical evaluations, by computing the Dice similarity coefficient (DSC) and the maximum distance to agreement (MAX_DA), and (ii) volumetric considerations, by evaluating the percentage volume (V%) of an OAR in the sCT that remains outside the union volume of all the possible positions of the same OAR contoured on the CBCTs (Table 1, right). The same analysis was repeated for the gross tumour volume (GTV).

A total of 17 patients were analysed; geometric indexes, namely DSC and MAX_DA, did not show any definitive trend of motion of organs (Figure 1). On the other hand, median V% highlighted bladder and rectum as the OARs more prone to motion (Figure 2a). Regarding GTV, median DSC resulted in 0.6065 (IQR 0.48075 – 0.7585); median MAX_DA indices, 0.562 (IQR 0.371 – 0.778), suggested limited levels of displacement, however a median V% of 88.40% (IQR 22.07 – 233.33) was identified as the worst among all structures. Bony structures as iliac wing and femoral heads showed smallest V% values.

Figure 1. Dice similarity coefficient (DSC) (1a) and maximum distance to agreement (MAX_DA) (1b) regard five considered organs at risk (OARs).

Figure 2. Median and interquartile range (IQR) of V%, DSC, and MAX_DA for all considered OARs (2a). Graphical representation of V% (2b - top) and relative formula (2b- bottom).

In conclusion, as expected bony structures resulted less prone to motion than the other OARs; GTV is found to be the structure with the greatest V%, meaning a great displacement with respect to its location in the sCT; thus in order to achieve safety and accuracy during stereotactic treatments also GTV motion has to be considered, and further highlights the importance of CBCT imaging. In the era of personalized medicine and tailored treatment, dealing with inter-fraction errors due to motion is still an open question and would lead to safer and more effective treatment approaches.