SU-E-T-797: Variations of Cardiac Dose at Different Respiratory Status in CyberKnife M6â"¢ Treatment Plans for Accelerated Partial Breast Irradiation (APBI).

PURPOSE: Cyberknife robotic assisted radiation delivery has become a choice for accelerated breast RT, while a slightly increased cardiac dose has been reported. The dose dynamics throughout the respiration cycle has scarcely been explored. This study was designed to investigate the dose changes at each respiratory phase or status during respiration cycle.

METHODS: Six patients with 4DCT studies and six patients with a pair of free-breathing and deep breath-hold CT sets were used for dosimetry comparisons. 4DCT sets were obtained by Siemens™ CT and its respiratory gating system, comprising of 8 phases. Standard APBI plan at 340 cGy was done per fraction per NSABP B-39/RTOG 0413 and modulated with Cyberknife M6™ on MultiPlan™5.1.2. For the purpose of this study, the tumor volume was outlined in the media-lower quadrant of the left breast.

RESULTS: Except for D5cc in plans with 4DCT, cardiac doses are significantly different between respiratory phases in well inhaled breathing phases, and more significantly in plans with BH CT. Mean cardiac doses in 100% inhalation phase were often found to be 5-15% (p< 0.02) less than those in other phases.

CONCLUSION: Although ineligible cardiac doses are noted in APBI plans using 4D free-breathing CT and instantaneous free breathing CT series, a reduction in cardiac dose was seen for the well-inhaled phases. This provides practical guidance for cardiac dose reduction applicable with CK M6 APBR.