Dosimetric effects of repeat computed tomography scan during radiotherapy planning in esophagus carcinoma

The aim was to assess the potential reduction in the doses to organs at risk (OARs) and target organ volume by doing replanning on repeat computed tomography (CT) scan during the 4^th week of radiation therapy (RT).

Twenty-four histologically proven patients of inoperable esophagus carcinoma were studied. All patients received induction chemotherapy followed by concurrent chemotherapy and radiotherapy. CT simulation with proper immobilization was done, and images were transferred to the treatment planning system. Delineation of target volumes and OARs was done, and two plans were generated for 60 Gy in 30 fractions and 40 Gy in 20 fractions with intensity-modulated RT keeping the doses to OARs within the tolerance limits. Replanning for 20 Gy in 10 fractions was done on repeat CT scan during the 4^th week of radiotherapy treatment, and potential reduction in doses to OARs and target organ volume was assessed.

A total of 24 cases were analyzed for the adaptive plan with the coverage of the 95% prescription isodose for planning target volume. Statistical analysis was done by t-test. The difference in the doses received by the OARs was analyzed and was seen that due to re CT scan, the doses were reduced to the left lung V₂₀ (mean 19.23 Gy vs. 17.35 Gy) and D_mean (mean 16.03 Gy vs. 14.25 Gy), right lung V₂₀ (mean 18.38 Gy vs. 16.66 Gy) and D_mean (mean 15.70 Gy vs. 13.97 Gy), heart V₂₅ (mean 38.72 Gy vs. 35.32 Gy) and D_mean (mean 26.40 Gy vs. 22.74 Gy), and spine 1% volume (mean 36.54 Gy vs. 33.39 Gy) and D_max (mean 39.81 Gy vs. 34.34 Gy), gross tumor volume (GTV) (mean 67.37 cm ³ vs. 24.58 cm ³) and were all significantly smaller for the adaptive plan.

By doing adaptive radiotherapy in the 4^th week of treatment using repeat CT scan, along with the response evaluation, there is a significant reduction in the volume of GTV, and replanning of treatment on repeat CT scan also helps us in reducing doses to the OARs resulting in reduced toxicity.