Adaptive off-line protocol for prostate external radiotherapy with cone beam computer tomography

Purpose

The goal of this work was to prepare and to evaluate an off-line adaptive protocol for prostate teleradiotherapy with kilovoltage cone beam computer tomography (CBCT).

Patients and methods

Ten patients with localized prostate carcinoma treated with external beams underwent image-guided radiotherapy. In total, 162?CBCT images were collected. Position of prostate and pubis symphysis (PS) with respect to the isocenter were measured off-line. Using the CBCT scans obtained in the first three fractions the average position of prostate in relation (AvPosPr) to PB was calculated. On each CBCT scan, the position of prostate with respect to AvPosPr was calculated and cumulative histogram of prostate displacement with respect to AvPosPr was prepared. Using this data, the adaptive protocol was prepared in which (1) based on the CBCT made in the first three fractions the AvPosPr to PS is obtained, (2) in all other fractions two orthogonal images are acquired and if for any direction set-up error exceeds 0.2?cm the patient’s position is corrected, and (3) additionally, the patient’s position is corrected if the AvPosPr exceeds 0.2?cm in any direction. To evaluate the adaptive protocol for 30?consecutive patients, the CBCT was also made in 10th and 21st fraction.

Results

For the first 10?patients, the results revealed that the prostate was displaced in relation to AvPosPr >0.7?cm in the vertical and longitudinal directions only on 4 and 5?images of 162?CBCT images, respectively. For the lateral direction, this displacement was >0.3?cm in one case. For the group of 30?patients, displacement was never >0.7, and 0.3?cm for the vertical and lateral directions. In two cases, displacements were >0.7?cm for the longitudinal direction.

Conclusion

Implementation of the proposed adaptive procedure based on the on-line set-up error elimination followed by a reduction of systematic internal error enables reducing the CTV–PTV margin to 0.7, 0.7, and 0.4?cm for the vertical, longitudinal, and lateral directions, respectively.