Room: Exhibit Hall | Forum 5
Purpose: To investigate the dosimetric impact of the interplay effect for free-breathing intensity-modulated radiation therapy (FB-IMRT) of abdominal tumors.
Methods: Treatment deliveries of FB-IMRT for abdominal cancer patients were monitored by the CatalystTM system, which uses a novel nonrigid algorithm to allow simultaneous real-time tracking of the isocenter position. A hundred fractions of treatment deliveries were analyzed retrospectively. Interplay effect would underdose/overdose the planning target volume (PTV) or organs at risk (OARs). For each beam, the shift of dose-volume histogram (DVH) was evaluated by recalculation of original IMRT plan with the isocenter variation acquired by the CatalystTM system during beam delivery time. Accumulation of shifted DVHs from all treatment beams resulted in the actual fractional DVHs of PTV and OARs for each treatment delivery. Reproducibility of PTV dose delivery was evaluated by comparison of factional DVHs through treatment. Tolerance levels on intrafractional motion of isocenter were evaluated to achieve 95% coverage of PTV with prescription dose.
Results: The isocenter shifts during treatment were -0.1Â±0.3 mm, -0.2Â±0.7 mm and -1.2Â±1.1 mm in longitudinal, lateral and vertical directions respectively. Maximum real-time isocenter shifts resulted in -2.25Â±0.24% shift of PTV dose coverage in corresponding fraction. Accumulated variations of fractional DVHs showed that D95 of PTV was changed by 3.6Â±5.6% compared to planned D95. Tolerance level would be set as 3mm to achieve high quality treatment delivery.
Conclusion: Respiration introduced isocenter variation for abdominal IMRT could be monitored by the CatalystTM system. With this real-time surface imaging system, the dose delivery accuracy were estimated through all fractions. Reasonable tolerance levels on intrafractional motion would benefit dose coverage of PTV and treatment outcome.