Surface guided radiotherapy for breast in deep inspiration breath hold: Sentinel® validation and dosimetric impact

Surface guided radiotherapy, a tracking technique that reconstructs the three-dimensional surface of the patient in real time using optical imaging, is nowadays used to improve patient positioning accuracy, speed up workflows, and potentially lower imaging dose by reducing cone beam imaging checks before treatment delivery. This thesis analyzes its use for left-sided breast treatment in deep inspiration breath hold in a retrospective cohort of 35 patients, asking if the system is reliable and if residual positioning errors have a dosimetric impact on treatment quality. The vertical elevation of the primary gating point recorded by the Sentinel system at simulation, defined as the difference in chest-wall elevation between free breathing and deep inspiration breath hold respiratory conditions, was validated against the elevation measured between the CT acquired in free breathing and deep inspiration breath hold. The two measures showed a high level of agreement, confirming the reliability of the Sentinel system for motion tracking. Residual positioning errors, verified with CBCT before treatment, were generally small but variable, with some shifts and rotations large enough to affect target coverage and organ at risk doses. The recalculation of the treatment plans using the average shifts found during the treatment course with the CBCT, confirmed that most cases maintained adequate dose coverage, while some patients showed underdosage of the breast and an increase in cardiac and contralateral breast doses. A 3D gamma analysis, a standard tool to compare planned and recalculated dose distributions, highlighted that while some patients retained near perfect agreement between the original and the recalculated plan, others experienced relevant degradation. Overall, SGRT provides accurate, non ionizing positioning, but inter-patient variability persists, suggesting a strategy of early imaging verification, robustness checks, and individualized imaging frequency.