Preliminary application of EPID three-dimensional dose reconstruction in in vivo dose verification of breast cancer intensity-modulated radiation therapy.

A preliminary study was conducted using electronic portal imaging device (EPID) based dose verification in pre-treatment and in vivo dose reconstruction modes for breast cancer intensity-modulated radiation therapy (IMRT) technique with known repositioning set-up errors. For 43 IMRT plans, the set-up errors were determined from 43 sets of EPID images and 258 sets of cone beam computed tomography images. In-house developed Edose software was used to reconstruct the dose distribution using the pre-treatment and on-treatment (in vivo) EPID acquired fluence maps. The maximum setup error was < 3.5 mm. For 43 pre-treatment cases, the γ pass rate (3 %/3 mm) is 98.49 % ± 1.15 %. The chest wall target ΔV_98%P, ΔV_95%P, andΔV_90%P are all < 5 %, while the majority of the ipsilateral lung ΔV_5Gy, ΔV_20Gy, and ΔV_30Gy are also < 5 %. For 258 in vivo cases, the γ pass rate is 90.98 % ± 6.53 %, with the chest wall target ΔV_90%P and ipsilateral lung ΔV_30Gy both < 5 %, while the other volume differences all exceed 5 %. The γ pass rate for in vivo verification is significantly lower than pre-treatment values. Although the in vivo γ verification satisfies the medical physics requirements, the reconstructed coverage of the chest wall target is far below the clinical dosimetry requirements. In vivo 3D dose reconstruction directly predicts changes in the planning target volume to aid clinicians better understand the actual dose received by patients with intra-fractional motion and anatomical changes.