Document Type : Original Paper
Laboratory of Physics of Matter and Radiation, Faculty of Sciences, Mohammed first University, Oujda, Morocco.
Physics departement, faculty of sciences, University Mohamed First, Oujda 60000, Morocco
LPMR, Faculty of sciences, University Mohamed 1st, Oujda, Morocco
National School of Applied Sciences, University Mohamed 1st, Oujda, Morocco.
Objective: To quantify the dosimetric effect of the lateral and longitudinal positioning errors on left sided breast cancer, for 3D conformal radiation therapy 3DCRT mono isocenter (MIT) and double isocenter technique (DIT) irradiations, and its frequency dependence.
Patients and methods: Treatment plans of 10 patients with left breast cancer were included in this study. For each patient two reference treatment plans were planned by both techniques. Then positioning errors of 2mm and 4mm in both directions (right and inferior) were applied to each reference plan, in different error repetition scenarios (error occurring 1time, 5times, 10times, and 25times) over the 25-fraction treatment planned. Student t test for paired samples was used for statistical analysis, with an error risk alpha of<0.05 accessing the statistical significance of the results.
Results: Dosimetric impact of positioning errors affects the MIT, and the DIT-TG (breast isocenter) plans for the heart, and the MIT and DIT-SC (supraclavicular isocenter) plans for the spinal cord. For the DIT-SC, the isocenter is near the spinal cord, so every small lateral movement of the isocenter would dosimetrically affect the spinal cord. The same goes for the heart and the isocenter position in the DIT-TG plans. Thus, the right direction affected dosimetric parameters of these organs-at-risk.
Conclusion: Even a millimeteric error could have consequences on the heart and spinal cord, likely leading to increased post-treatment toxicities, especially if the reference plan doses of DVH were close to the limit recommended. If DIT plan is used, it is necessary to control the repositioning accuracy of the patient at each treatment session. The MIT should be encouraged whenever possible.