Dosimetric Effect of Setup Errors on Left Sided Breast Irradiation, For the Mono and Dual Isocenter Techniques

This study quantifies the dosimetric impact of lateral and longitudinal positioning errors on left-sided breast cancer during 3D conformal radiation therapy, employing both mono isocenter (MIT) and double isocenter technique (DIT) irradiations, and explores the frequency dependence of these errors.
The study includes 10 left breast cancer patients, with two reference treatment plans created for each using both MIT and DIT techniques. Positioning errors of 2mm and 4mm in the right and inferior directions were simulated across varying error repetition scenarios (1 time, 5 times, 10 times, and 25 times) throughout the 25-fraction treatment period. Statistical analysis employed paired samples Student t-tests with a significance level of α<0.05.
Dosimetric impact was observed in MIT and DIT-TG (breast isocenter) plans for the heart, and in MIT and DIT-SC (supraclavicular isocenter) plans for the spinal cord. DIT-SC, being close to the spinal cord, demonstrated sensitivity to small lateral isocenter movements, impacting spinal cord dosimetry. Similarly, the heart and isocenter position in DIT-TG plans were susceptible to right-directional errors, affecting dosimetric parameters of these organs-at-risk.
Even minimal errors, measured in millimeters, can significantly influence heart and spinal cord dosimetry, potentially leading to heightened post-treatment toxicities, particularly when reference plan doses are close to recommended limits. The study advocates for vigilant repositioning accuracy control in DIT plans during each treatment session. Encouraging the use of MIT, when feasible, emerges as a crucial consideration to mitigate dosimetric variations and enhance treatment precision.