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

Abstract

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

Methods and materials: 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.

Results: 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.

Conclusions: 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.

Figure 1

Flowchart for prescribing absorbed dose and number of fractions in reirradiation. Abbreviations: 3DRTP = 3-dimensional radiation therapy planning system; CT = computed tomography; BED = biological effective dose; CBED = cumulative BED; DICOM = digital imaging and communications in medicine; DVC = dose-volume constraints; EBRT = external beam radiation therapy; RT = radiation therapy; TPS = treatment planning system; OAR = organ at risk. ^δThe normalized total absorbed dose, the ratio dividing the absorbed dose in each voxel by the prescribing total absorbed dose to the reference point of clinical target volume (CTV).

Figure 2

Positional relationship between each organ and target volumes used in scenario 1 and also in scenario 2. The gross tumor volume and clinical target volume at the peripheral right lung field and those at the right hilum are shown in red. Right main bronchus, left main bronchus, trachea, lung, and spinal cord are shown in blue, orange, magenta, green, and yellow. AP = antero-posterior; LR = left-right.

Figure 3

The absorbed dose, BED_α/β=2 map for the first plan (initial radiation therapy) and second plan (PRI), and CBED_α/β=2 map on transaxial images using PRISTIN in scenario 1. The color scales for absorbed dose maps are different from those for BED_α/β=2/CBED_α/β=2 maps. The upper 3 images are on the transaxial CT images at the level of the peripheral primary lung cancer and the lower 3 images are at the level of the hilar lymph nodes. Abbreviation: BED = biological effective dose; CBED = cumulative biological effective dose; CT = computed tomography.

Figure 4

The absorbed dose, BED_α/β=2 map for the first plan (initial electron therapy) and second plan (proton therapy), and CBED_α/β=2 maps on transaxial CT images using PRISTIN in scenario 3. The color scales for absorbed dose maps are different from those for BED_α/β=2/CBED_α/β=2 maps. The absorbed dose map of first plan overlapped on the CT scan before deformable image registration (DIR). Abbreviation: BED = biological effective dose; CBED = cumulative biological effective dose; CT = computed tomography.