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Evaluation of Variations in Gross Tumor Volume and Tumor Control Probability During Treatment Using KV Cone-Beam CT in Non-Small Cell Lung Carcinoma

I Ali*, O Algan, T Gunter, T Herman, S Ahmad, Oklahoma Univ. Health Science Ctr., Oklahoma City, OK

SU-E-J-193 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To investigate quantitatively the variations of gross tumor volume (GTV) as a function of time and cumulative dose during radiotherapy treatment and evaluate corresponding tumor control probability (TCP) in non-small cell lung carcinoma (NSCLC).
Methods and Materials: CT and cone-beam CT images for 20-patients NSCLC were reviewed, retrospectively, which included CBCT imaging that was used to setup seventeen patients treated with hypofractionated stereotactic body radiation therapy (SBRT) and three-patients treated with conventional fractions. The visible GTVs were outlined on CT from simulation and used in treatment planning to obtain optimized dose coverage. Furthermore, GTVs were outlined on CBCT images from daily setup with IGRT. The GTV variations were investigated as a function of time and cumulative dose delivered during the treatment course. TCP was calculated using GTV and mean dose from different CT imaging sets considering alpha-beta ratio of 10, survival fraction of 0.5 and colognes cancer density of seven millions for NSCLC.
Results: For the patients treated with conventional fractionation, the GTV decreased to 30% of initial volume as a function of cumulative delivered dose through the treatment course. In contrary, GTVs for SBRT patients increased up to 45% relative to the original volume as a function of cumulative dose delivered within few days. The volume shrinkage in conventional fractionation and enlargement in SBRT represented the response of tumor cells which was employed in determining TCP. Although, the variations in the mean dose were within 3%, the corresponding TCP varied by 10% during the course of treatment.
Conclusions: In NSCLC, GTV and corresponding TCP varied significantly during the treatment with SBRT or conventional radiation therapy. The GTV variations provided quantitative measures of tumor response to radiation therapy that can be extracted from volumetric on-board CBCT imaging which can be employed in treatment planning to perform adaptive radiation therapy.

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