Room: AAPM ePoster Library
Purpose: To evaluate intra-fraction motion and resulting dosimetric effects during stereotactic body radiation therapy (SBRT) of liver tumors under breath-hold (exhale and inhale) and free-breathing.
Methods: Pre and post CBCT images were acquired for patients receiving free-breathing (N=8), exhale (N=11) and inhale breath-hold (N=5) SBRT treatments. Dose prescribed was 33–50Gy in 5 fractions using SDX for active breath-control. Margins used were ITV or CTV plus 5mm axially and 8 or 10 mm superior-inferior (SI). CBCTs were co-registered by physicians with standard clinical alignment bias to the PTV using rigid registration in MIM 6.8.3 (MIM Software Inc.) and iso-center point shifts were computed in anterior-posterior (AP), left-right (LR) and SI directions. Dosimetry effects of intra-fraction shifts were estimated by rigid-transform of planned dose-distributions to evaluate target coverage and OAR doses. Margin appropriateness was assessed using Van Herk’s method.
Results: Mean absolute-value intra-fractional positional-variability were: 13.0±9.3mm SI, 6.5±5.0mm AP and 3.8±2.2mm LR for free-breathing; 3.4±2.0mm SI, 5.9±3.7mm AP and 3.0±2.4mm LR for exhale; 1.7±2.2mm SI, 4.5±1.6mm AP and 1.8±0.9mm LR for inhale. Margins computed with Van Herk’s method for exhale and inhale treatments are 3.0/9.8/3.8mm and 5.7/12.3/4.2mm in SI/AP/LR respectively. For free-breathing, GTV coverage was maintained during all fractions and mean liver dose threshold was exceeded in 4/8 fractions. For exhale breath-hold, GTV was under-covered (by more than 10%) in 4/11 fractions and small bowel dose to 0.5cc would be exceed in 2/11 fractions. For inhale breath-hold, GTV was under-covered in 1/5 fractions and large bowel dose threshold would be exceeded in 1/5 fractions.
Conclusion: Intra-fraction tumor position can be quite variable, beyond current PTV expansions anteriorly. On the other hand, SBRT liver treatments using breath-hold were less than 5.9mm. Larger margins could be needed in anterior-posterior direction to more accurately maintain GTV coverage and OAR thresholds.