Author: Min Su 👨🔬
Affiliation: Mount Sinai School of Medicine 🌍
Purpose: To evaluate dosimetric impact of target coverage and organ at risk (OARs) sparing for whole breast patient setup roll/yaw change in 3D-conformal radiotherapy.
Methods: 8 whole breast cases, 4 from left breast and 4 from right breast, were included in this study. All breast treatment plans were generated with tangential field-in-field technique in Varian Eclipse V16.1 on a CT image set. Patient roll or yaw changes ±2° were simulated by spinning the planned field gantry or couch in planning system. Target breast coverage was evaluated by V95% in percentage. OARs were evaluated by heart Dmean in cGy; ipsilateral lung V2000cGy, V1000cGy, V500cGy, in percentage; contralateral lung Dmax and contralateral breast D10% in cGy. The patient treatment plan was used for the comparison. Varian Aria was the record/verify system. 211 breast/chest wall fraction initial setup roll/yaw data were collected to observe a patient initial daily setup rotation error.
Results: The results showed that maximum 2.0°/3.1° of initial setup roll/yaw error were seen. The comparison results showed that the maximum V95% changes were: 1.48%, 1.93% with average (0.41+0.39) %, (0.24+0.46) %; The maximum change of heart Dmean was 18.3 cGy, 5.4 cGy; of Ipsilateral lung V2000cGy was 0.52%, 0.84%; V1000cGy was 0.56%, 0.95%; V500cGy was 0.63%, 1.18%; of contralateral lung Dmax was 64.09 cGy, 59.09 cGy; of contralateral breast D10% was 9.00 cGy, 8.2 cGy. The ipsilateral lung dose change showed more sensitive to yaw changed and the heart Dmean from left breast plan showed more sensitive to the roll change.
Conclusion: To accurately deliver planned dose, an on-board image verification system and 6-DoF couch are needed. The peculiarities of whole breast tangential field treatment, which all the OARs are in one side of target, makes roll/yaw change maybe favorable to the planning aim.