A Precise Reirradiation Supporting Tool Initiative (PRISTIN) for Prescribing Absorbed Dose and Number of Fractions in...

This study aims to develop a supporting tool to calculate the most appropriate prescribing absorbed dose and number of fractions for precise reirradiation.

After deformable image registration of the initial computed tomography to the computed tomography at reirradiation, an initial biological effective dose (BED) taking into account the recovery from the initial irradiation is calculated voxel-by-voxel for each organ at risk (OAR). Using a commercial radiation therapy planning system, the clinical target volume for reirradiation (CTV2) is made. Keeping the BED_{tumor's α/β} to CTV2, cumulative BED_{OAR's α/β}(CBED_{OAR's α/β}) in each voxel of critical OARs is calculated by changing the number of fractions in a stepwise process. The most appropriate prescribing absorbed dose to the target and the number of fractions in reirradiation is determined by using CBED_{OAR's α/β}-volume histogram for critical OARs. The function of the tool was validated in silico using 3 scenarios in 2 patients: a patient with a lung cancer at the peripheral lung parenchyma and at the hilar lymphatic region at different times, and in a patient with a metastatic internal mammary lymph node relapsed after postoperative radiation therapy for breast cancer.

In scenario 1, giving 57 Gy in 22 fractions (57 Gy/22 Fr) to the CTV2 at the right hilum, the maximum CBED_α/β=2 was 124.078 Gy, and the mean CBED_α/β=2 of the whole lung parenchyma excluding gross tumor volume was 18.332 Gy. In scenario 2, 44.152 Gy/7 Fr to the target was suggested to be most appropriate. In scenario 3, 71.675 Gy/30 Fr proton therapy to the target was recommended in which the maximum CBED_α/β=2 in the aorta near the recurrence site was 145.796 Gy, and the volume of CBED_α/β=2 ≥ 100 Gy was 0.800 cm³, both are within the constraints.

The tool was suggested to be useful to find the most appropriate prescribing absorbed dose to the target as well as the number of fractions for precise reirradiation.