Dosimetric comparison of liver tumour radiotherapy in all respiratory phases and in one phase using 4DCT

Abstract

Purpose: Delineation of target volume could be performed on all respiratory phases and radiation is delivered during free breathing cycle, or on the basis of one respiratory phase and deliver gated treatment choosing a specific phase for irradiation. We performed the comparison of both techniques in terms of target and normal tissue dose distributions.

Materials and methods: We analysed 26 metastatic liver tumours using 4DCT to characterize tumour motion and to create treatment volumes: GITV4D was based on all 8 respiratory phases and GTVGAT was based on one specific phase. A 5mm automatic expansion was added to the GTVs to create PTVs. Two treatment plans were prepared to the total dose of 36 Gy in 3 fractions and dose-volume distributions were analysed for the target and organs at risk. Target motion along the superior-inferior direction was greatest with the mean of 1.1 cm ± 0.3, and in the lateral the mean was 0.7 cm ± 0.3.

Results: GTV and PTV volumes were larger in the 4D than in the GAT, mean 30.7 vs. 19.3 cm(3), and 66.7 vs. 45.1cm(3). We achieved similar dose coverage in PTV4D, for the 4D plan, and PTVGAT for the GAT plan, but a decrease in the average minimum dose to 17.8 Gy and the average mean dose to 35.3 Gy was found in PTV4D in the GAT plan. Radiotherapy delivered using GAT resulted in lower liver doses than using 4D with reduction of mean volume receiving 5 Gy by 6.5%±5, V(15Gy) by 4.5%±3.4, V(21Gy) by 3.4%±2.8 and reduction of mean kidney volume receiving 5 Gy by 9.1%±7.9, V(15Gy) by 4.1%±6.4, V(21Gy) by 3.2%±5.6. We also found correlations between PTV volume reduction with GAT, GTV motion and doses to normal tissues.

Conclusions: Gated radiotherapy could potentially allow a reduction in PTV volumes from those delineated on all respiratory phases, maintaining acceptable target coverage. Smaller target volumes improve doses distribution in normal tissue especially in the liver and kidney, but also spinal cord and intestine. A significant correlation has been found between dose and volume reduction in the OARs and both GTV motion and PTV volume reduction.