Goal: register QA films to improve cumulative dose estimate

Jun Lian, Tim Cullip, Kathy Deschesne, S Harris, Mahesh Varia and Sha Chang

Purpose:Radical radiation therapy that combines external beam therapy (EBRT) and brachytherapy (BRT) is effective in managing local-regional confined cervical cancer. Depending on the cancer stage and disease volume, two therapies are used in a concurrent or sequential way. Although CT is widely used for EBRT treatment planning, traditional 2D film is still commonly used today in many institutions for BRT treatment planning. The incompatible image information between the BRT and EBRT leads to great difficulty in computation of the cumulative radiation dose from both treatments. To date, doses to target and critical structures are approximated by adding the point doses of BRT to the EBRT plan. This results in significant uncertainty of the dose evaluation of treatment plans. In this project, we propose to register the orthogonal films of BRT and CT of EBRT so as to accumulate the doses in a more accurate way.

Method and Materials:Five patients with cervical cancer were selected retrospectively for this study. Two in house made planning softwares were used in planning. BRT used two orthogonal films to reconstruct source positions. The doses on points “A” were prescribed at 4000 cGy. 3D conformal EBRT plan was generated based on CT to deliver 4500 cGy to pelvis. Two digitally-reconstructed radiographs (DRRs) were created with the same central ray and gantry angle as BRT films. A landmark-based image registration tool was developed to register films and DRRs of CT. The transformations, including translation, rotation and scaling, were calculated when registering two pairs of images (AP to AP, and LAT to LAT). The transformation matrix was applied on the dose grid of BRT and it was then merged with the dose grid of EBRT.

Results:A composite plan with accumulated doses of BRT and EBRT was created for each patient. The accuracy of the dose accumulation was tested in two means. First, the doses of relevant points, such as “A” points, were verified and they are correct. Second, the dose prescription of EBRT was intentionally set to zero before plan merging. The composite plan shows the isodose curves on CT are identical to those of original BRT plan. The curves are located in right anatomic regions of CT. The DVHs of target and critical structures were analyzed, and sampled points on DVHs agree with the estimation.

Conclusion:We have developed a method to accumulate the dose distributions from film-based brachytherapy and CT-based external beam radiotherapy. This yields a more realistic estimate of the cumulative dose received by the patient from both treatments. It also provides a more quantitative guidance for the design of subsequent radiotherapy plans.