Room: Exhibit Hall | Forum 9
Purpose: Convolution/superposition algorithms such as AAA ignore atomic-composition of matter, only differ in their electron-density scaling for dose calculation. Acuros-XB (AXB) algorithm maps material density and calculates dose in a voxel-medium (Dm). It was observed that for higher-density materials, such as cancellous/cortical bones, differences were up to 15%; due to higher energy-deposition ratio of bone. Stereotactic, single-dose of 18 Gy targeting spinal metastases is gaining popularity in our clinic. With materials composition mapping, major treatment planning systems provided both Dw and Dm reporting modes; often confusing which medium to use for a clinical plan, particularly, for single-dose of spinal-metastases.
Methods: Five spine stereotactic patients (3 T-spine/2 L-spine) who underwent single-dose of 18 Gy to PTVD95=100% with AXB-Dm in Eclipse were investigated. Average PTV was 35.8Â±24.3cc(5.7-59.7cc) derived from GTV on 2.5mm CT-cuts co-registered with MRI-images. Highly conformal coplanar-VMAT stereotactic plans were delivered with Truebeam with 6MV-FFF(1400 MU/min) beam using 1Ã—1Ã—1mmÂ³ grid-size. Plans were re-computed using AXB-Dw and AAA utilizing identical plan parameters. Original and re-computed plans were compared via RTOG-0631-criteria, target conformity/gradient index, including target volume encompassed by the prescribed-percent dose (Vp) and dose to organs-at-risk.
Results: For PTV coverage: Conformity-index, R50%, Vp and Dmax were higher by 10%/7%, 2.0%/5%, 5%/3% and 4%/1% with AXB-Dw/AAA, on average, (p<0.001) compared to AXB-Dm. Maximum dose and dose to 0.35cc of spinal cord were higher by 2.0%/3% with AXB-Dw/AAA, on average, (p<0.002); maximum dose to partial cord was higher by 2.5%/4.0% with AXB-Dw/AAA, on average, (p<0.002) compared to AXB-Dm.
Conclusion: AXB-Dw and AAA both significantly overestimated dose to target-coverage and dose to spinal/partial cord by up to 6%. While delivering single-dose of very high dose-gradient; differences between AXB-Dm, AXB-Dw and AAA may be of clinical interest, particularly, for a protocol or re-treatment patients. Stereotactic spine treatment with implanted vertebral hardware will be further investigated.