Dosimetric comparison of target volume coverage and sparing of organs at risk (OARs) between helical intensity modulated radiotherapy (H-IMRT) versus volumetric arc therapy (VMAT) for mid and lower thoracic oesophageal carcinoma.

Abstract

469 Background: To evaluate the effectiveness of H-IMRT and VMAT in covering target volume while adequately sparing the OARs for patients with mid and distal oesophageal carcinoma, on the basis of dosimetric analysis. Methods: The target areas and organs at risk in 30 patients with locally advanced carcinoma oesophagus undergoing neo-adjuvant chemo-radiotherapy were specified and transmitted to Eclipse, Version 13.6, Varian Medical Systems) & Accuray Treatment Planning System (Accuray Precision, Version 2.1.4, Accuray Medical System). Two plans (H-IMRT and VMAT) were generated for each patient for a total dose of 41.4 Gy delivered to the PTV in 23 fractions, and the Homogeneity Index (HI), Conformity Index (CI) and the dose distribution to the OARs (spinal cord, heart, lungs, kidneys and liver) were compared using dose volume histograms. Results: H-IMRT resulted in a more homogeneous dose distribution to the target (HI- 0.059) as compared to VMAT (HI- 0.07) [p-0.004]. The Conformity Indices showed no significant difference between the two techniques (H-IMRT – 0.989, VMAT – 0.987) [p-0.66]. VMAT resulted in a significantly less dose to the spinal cord (23.598 Gy vs .25.657 Gy) [p-0.021]. With VMAT plans, the heart mean dose (18.101 Gy vs. 20.031) [p- 0.00007], heart V40 (4.902% vs. 6.143) [p-0.002,] the averaged lung V20 [p-0.00005] and the mean dose to the left kidney (3.84 Gy vs. 4.721 Gy) [p- 0.010] were significantly less as compared to H-IMRT No statistically significant difference in both techniques with respect to heart V30, Mean Lung Dose (MLD), and mean dose to the righ and left kidney was observed Conclusions: VMAT proved to be better at sparing of OARs whilst providing almost the same Conformity as compared to a H-IMRT. H-IMRT has statistically better dose homogeneity but, it tends to deposit a slightly higher dose to the OARs. Whether the aforementioned differences in the dosimetric parameters translate into clinical benefits has to be evaluated by clinical outcome studies.