Margin and PTV volume reduction using a population based library of plans strategy for rectal cancer radiotherapy

Day-to-day shape variation in the rectum CTV results in considerable geometric uncertainties during rectal cancer radiotherapy. To ensure coverage a large CTV-to-PTV margin is required. The purpose of this study was to increase the accuracy of treatment delivery by building a population based library of planning CTVs for rectal cancer patients and to evaluate its potential for rectum PTV margin and PTV volume reduction.

Analysis was done retrospectively on 33 early-stage rectal cancer patients with daily repeat CTs who received short-course pre-operative radiotherapy in 5 fractions of 5 Gy. We created signed distance maps from the planning rectum CTV to each of the repeat CTVs, from which we calculated the group mean, systematic and random error. The correlation between different regions of the rectum CTV was analyzed and used in combination with the distance maps to create the library of nine planning CTVs. For each of the repeat CTVs the best fitting CTV structure in the library was automatically selected defined by the plan that minimized the mean absolute distance between the repeat and library CTV. Residual distance maps were calculated from which a new PTV margin was constructed. Bootstrapping was performed on the margin difference to assess its significance.

Residual errors were found to decrease with the number of plans in the library, but adding more than five plans yields negligible further error reduction. Margin reduction of up to 50% was achieved at the upper-anterior site of the mesorectum. The average PTV volume decreased by 15.5% when a library is introduced.

A library of plans strategy for rectal cancer based on population statistics is feasible and results in a considerably reduced average rectum PTV volume compared to conventional radiotherapy.