CTV to PTV margins based on CBCT method for prostate cancer of Patients treated with VMAT technique

Document Type : Original Paper


1 Laboratory of high energy physics Modelisation Simulation, Faculty of Sciences, University Mohammed V, Rabat, Morocco

2 Hassan First University of Settat, High Institute of Health Sciences, Laboratory of Sciences and Health Technologies, Settat, Morocco

3 Departement of Physics, Laboratory of High Energy Physics, Modelling and Simulation, Faculty of Science, Mohammed V Agdal University, Rabat, Kingdom of Morocco


Background: patient repositioning in treatment radiotherapy is the main factor of target messing to irradiation. However additional margin is necessary to consider the uncertainties created along and around X, Y and Z axis.

Methods: set-up and random errors were calculated in translational and rotational axis for a sample of 20 prostatic patients; using daily IGRT-CBCT method. The aim of this study was to determine the additional margin that should be added from clinical target volume (CTV) to prevent toxicity and increase the irradiation precision in radiotherapy. The van Hark formula (PTV margin =2.5Σ +0.7σ) was used for all patients to perform PTV margin for prostatic localization.

Results: the research performed for a sample of 20 consecutive patients. With respect to systematic error along the lateral axis, longitudinal and anterior-posterior was 2.32, 2.42 and 3.54 respectively. The Random error was 1.82, 2.19 and 1.76° along lateral axis, longitudinal and anterior-posterior respectively. The rotational systematic error was 1.49, 2.04 and 2.14° around lateral, longitudinal and anterior-posterior axis respectively. And the Random error was 1.78, 1.75 and 1.63° around lateral, longitudinal and anterior-posterior axis respectively. The calculated safety margin to caver clinical target (CTV) volume taking the prostate variability into account measured 7.55, 8.08 and 10.79 mm for lateral, longitudinal and anterior posterior respectively and 7mm would be enough in the posterior side. Rotational set-up errors for almost 95% of patients were between -2° and 2°.

Conclusion: The calculated safety margin in all direction was smaller than 1 cm except in anterior side that would be 1 cm or more.


Main Subjects

Articles in Press, Accepted Manuscript
Available Online from 17 July 2022
  • Receive Date: 21 November 2021
  • Revise Date: 11 July 2022
  • Accept Date: 17 July 2022
  • First Publish Date: 17 July 2022