ESTRO 2023 - Abstract Book

S1777

Digital Posters

ESTRO 2023

The collapsed cone (CC) (v5.2) algorithm implemented in the RayStation (v6) TPS was evaluated using a PTW microDiamond (mD) detector and an Elekta linear accelerator. Different scenarios were investigated, from simple static fields to intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) clinical treatment plans. Clinical treatment plans were recalculated on the CT-data set of a nearly water equivalent pelvic phantom using the same beam arrangements and control points. Discrepancies and their confidence limits (CLs) between results of calculations and measurements – applying both local ( δ ) and global ( Δ ) normalization – were determined for various points of interest (POIs). CLs were calculated as the sum of the absolute value of the mean discrepancy and the standard deviation of the discrepancies multiplied by a factor of 1.96. Results were based on a single-institution experience for one clinical test case (prostate) and evaluated against internationally accepted criteria. Classification of nontarget dose was based on the definitions proposed by AAPM TG 158. Results The mD was shown to be suitable for dosimetry in nontarget dose regions. The δ for simple static fields with collimator at 45° depended upon whether jaws were in place (-21.7% ± 29.4%) or retracted (15.9% ± 35.4%) (p-value < 0.01). POIs receiving < 5.0% of the prescribed dose failed the recommend tolerance level for CL δ (40%) as compared to POIs receiving ≥ 5% (88.6% versus 21.7% for IMRT and 62.6% versus 13.6% for VMAT). In addition, the CL δ for POIs that receive only indirect radiation – and can thus be categorized as truly “out-of-field” – as compared to “in-field” nontarget POIs was also above the tolerance limit (40%) for both IMRT (85.4% versus 13.1%) and VMAT (69.2% versus 23.0%). The CL Δ was below the tolerance limit (4%) for both techniques. Overall, no action levels (7%) were exceeded. Conclusion This study provides data about the level of dose calculation accuracy and reliability that can be expected for modern prostate radiotherapy in nontarget regions with the CC dose calculation algorithm implemented in the RayStation TPS. Since the measured and calculated doses differed no more than the action level value, our TPS can be considered acceptable for clinical use. However, POIs located in the < 5% dose region or receiving only indirect doses push the algorithm to its limits. A dose calculation algorithm that can accurately compute nontarget doses could be of great value to more comprehensively understand the clinical impact of these low doses. G.C. Mazzola 1,2 , G. Marvaso 1,2 , G. Corrao 1 , D. Zerini 1 , S. Durante 1 , A. Vavassori 1 , M.G. Vincini 1 , M. Zaffaroni 1 , E. Rondi 1 , S. Vigorito 1 , F. Cattani 1 , B.A. Jereczek-Fossa 1,2 1 European Institute of Oncology, Radiation Oncology, Milan, Italy; 2 University of Milan, Oncology and Emato-Oncology, Milan, Italy Purpose or Objective To compare dose distribution plans between ultra-hypofractionated (UH) IMRT with VERO system (BrainLab AG, Tokyo, JP) and UH-SBRT with CyberKnife (CK) radiosurgery system (Accuray Inc., Sunnyvale, CA) using image-guided virtual fiducial markers tracking, for localized prostate cancer (PCa) with concomitant focal boost to the dominant intraprostatic lesion (DIL). Materials and Methods Fifteen patients affected by intermediate or high-risk PCa, with at least one visible DIL on previously performed mpMRI, were included in this in-silico planning study. Dosimetric results were compared between fifteen delivered VERO IMRT and thirty respective in-silico CK SBRT plans (15 CK plans with prescription to 95% isodose line (IDL) plus other 15 CK plans with prescription to 85% IDL), aiming to perform UHRT of 40 Gy in 5 fractions (8 Gy/fr) to the DIL while treating the whole prostate to 36.25 Gy in 5 fractions (7.25 Gy/fr) every other day, using a 5mm isotropic expansion margin, except 3mm posteriorly, for prostate gland planning target volume (PTV-p) and 3 mm isotropic margin for PTV of the DIL (PTV-d). Results A total of 45 plans were compared for this study. Both in-silico SBRT and delivered IMRT plans were able to create the dose gradient (+10.34%) needed for the SIB prescription and have reached the primary planning goal of D95% > 95% for both PTVs in every plans. However, the comparison of dose coverage objectives established that statistically significant difference exists. Comparing delivered IMRT vs in-silico SBRT plans with prescription to 85% IDL, showed better coverage of PTV-p and, especially, of PTV-d, where a significant increase of median D95%, D2%, D0.03cc, D50% V100% and V110% was observed respect to delivered plans. The same difference for PTVs dose coverage, was maintained in a lower degree when comparing the median value of delivered IMRT plans versus in-silico SBRT plans with prescription to 95% IDL. Concerning the organs at risk (OARs), the three techniques maintained the dose distribution well below all the prescribed OARs dose constraints in every plan. Overall, the in-silico SBRT CY plans with prescription to 85% IDL resulted to be the best technique able to reach a maximum dose of 47 Gy to the DIL while respecting all the OARs constraints applied in IMRT- delivered VERO plans. An example of dose distribution of the three tecniques can be found in Figure 1. PO-2005 CyberKnife UH-SBRT for localized PCa with dose-escalation to the DIL: in-silico planning study