Abstract
Purpose:We report our clinical experience using a non‐invasive transperineal (TP) ultrasound (US) probe dedicated to pre‐positioning and monitoring of prostate cancer patients. The accuracy of pre‐treatment positioning was compared to CBCT for prostate and post‐prostatectomy patients. Intrafraction motions were recorded for both localizations. The dosimetric impact of these displacements was finally investigated on prostate patients.Methods:Differences between CBCT/CT and TP‐US/TP‐US registrations were analyzed on 427 and 453 sessions for 13 prostate and 14 post‐prostatectomy patients, respectively. Ten prostate patients’ dosimetries were retrospectively planned using 2 different protocols: 80Gy in 40 fractions and 36.25Gy in 5 fractions with a 5mm CTV‐ to‐ PTV margin. The delivery time was measured in order to analyze ranges of intrafraction motions related to each protocol. Mean prostate displacements were calculated for each patient and applied to the treatment isocenter coordinates to evaluate the dosimetric impact of these motions.Results:CBCT and TP‐US shifts agreements at ±5mm were 76.6%, 95.1%, 96.3% and 90.3%, 85.0%, 97.6% in anterior‐ posterior, superior‐ inferior and left‐right directions, for prostate and post‐prostatectomy patients, respectively. Intrafraction motions were analyzed considering delivery times of 140 and 290s with an additional time of 120s for patient installation for doses of 2 and 7.25Gy, respectively. Intrafraction motions were patient‐dependent and were larger as the irradiation time increased. Larger displacements were observed for prostate compared to post‐prostatectomy localizations. Shifts above 3mm were observed on 17.6% and 4.5% of the 2Gy sessions against 30.6% and 7.3% of the 7.25Gy sessions in the anterior‐posterior direction for prostate and post‐prostatectomy localizations, respectively. Preliminary dosimetric results showed that intrafraction motions mainly impact the PTV coverage.Conclusion:4D TP‐US modality is a promising alternative to irradiating and/or invasive IGRT modalities for both inter and intrafraction motions management. Preliminary dosimetric results show that intrafraction monitoring is mandatory especially for hypofractionated treatments.M Fargier‐Voiron was supported by a PhD grant from Elekta
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