Reports of practical oncology and radiotherapy : journal of Greatpoland Cancer Center in Poznan and Polish Society of Radiation Oncology · 2025
Quality of density deformations in patient anatomy during synthetic CT guided online adaptive radiation therapy for pelvic cancer.
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Szegedi Domonkos, Stelczer Gábor, Major Tibor, Takacsi-Nagy Zoltán, Pesznyák Csilla
Аннотация
BACKGROUND: Online adaptive radiotherapy (oART) with cone-beam computed tomography (CBCT) enables real-time plan adaptation to daily anatomical changes in the patient. Synthetic CTs (sCTs), generated by deforming planning CTs (pCTs) to CBCTs, are crucial for precise dose calculations. However, variations such as gaseous volumes in the pelvis may impact alignment accuracy.
MATERIALS AND METHODS: This retrospective study analyzed 90 treatment fractions for nine patients with cancer in the pelvic region, treated with CBCT-guided oART on the Ethos system. SCTs were generated and compared to corresponding CBCTs using similarity metrics, focusing on bony structures, external body contours, and gaseous volumes. Symmetric differences and Dice Similarity Coefficients (DSC) were calculated to evaluate the registration accuracy.
RESULTS: Bony structure deformations were minor (average volume change 0.2%, DSC 0.89). External body contours on sCTs aligned closely with CBCT (mean deviation 1.0 ± 0.4%), while gaseous volumes exhibited lower alignment (DSC < 0.5 in many fractions). Gaseous volume discrepancies were independent of the pCT and showed time-dependent variability, emphasizing the challenges in adapting density maps. Only a small portion of the gaseous volume differences overlapped with the planning target volume (PTV), suggesting limited direct impact on dose calculation.
CONCLUSIONS: CBCT-based sCT adaptation effectively aligns external contours but does not accurately handle gaseous volumes, which may vary in great extent. Minimizing the time between CBCT acquisition and treatment is crucial to maintain alignment accuracy. Advanced imaging solutions enabling direct dose calculation without involving sCT generation could further enhance dose reconstruction but may not resolve the time-sensitive nature of gaseous volume changes.
MATERIALS AND METHODS: This retrospective study analyzed 90 treatment fractions for nine patients with cancer in the pelvic region, treated with CBCT-guided oART on the Ethos system. SCTs were generated and compared to corresponding CBCTs using similarity metrics, focusing on bony structures, external body contours, and gaseous volumes. Symmetric differences and Dice Similarity Coefficients (DSC) were calculated to evaluate the registration accuracy.
RESULTS: Bony structure deformations were minor (average volume change 0.2%, DSC 0.89). External body contours on sCTs aligned closely with CBCT (mean deviation 1.0 ± 0.4%), while gaseous volumes exhibited lower alignment (DSC < 0.5 in many fractions). Gaseous volume discrepancies were independent of the pCT and showed time-dependent variability, emphasizing the challenges in adapting density maps. Only a small portion of the gaseous volume differences overlapped with the planning target volume (PTV), suggesting limited direct impact on dose calculation.
CONCLUSIONS: CBCT-based sCT adaptation effectively aligns external contours but does not accurately handle gaseous volumes, which may vary in great extent. Minimizing the time between CBCT acquisition and treatment is crucial to maintain alignment accuracy. Advanced imaging solutions enabling direct dose calculation without involving sCT generation could further enhance dose reconstruction but may not resolve the time-sensitive nature of gaseous volume changes.