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An accurate assessment of peripheral dose is necessary to estimate the risk of second cancer after radiotherapy. The calculation of dose to out-of-field organs, no matter how distant they are from the Irradiated Volume (IV), requires the knowledge of their shape and positions. Nevertheless, typical planning CTs (PCT) only consider a few cm superior and inferior to the IV. And yet, taking a whole-body CT of each patient is also not justifiable because of the extra whole-body exposure. This work aimed to use the already available PCT to generate a synthetic whole-body CT, which should approximately represent the unique geometry of each treated patient.

An interactive computer program, developed in MATLAB, takes the PCT as an input and transforms the ICRP110 adult reference computational phantom according to a rigid registration of both images. The user visually defines a subregion of the computational phantom that corresponds to the part of the patient included in the PCT. Several image pre-processing steps were tested to segment the bones on both images before the registration process. Finally, the best methods (the ones generating the highest Sørensen-Dice coefficients) segmentation/registration methodologies were selected and implemented in the code. The methodology was then validated using a published database (New Mexico Decedent Image Database) containing whole-body CT images.

A software termed IS²aR (Interactive Software for Image Segmentation and Registration) was created. It allows for registration of the patient’s PCT with the ICRP110 phantom. Figure 1 presents a screenshot of the program graphical interface. The results in a chest IV are shown in figure 2.

Figure 1 shows the interface for a pelvis case: patient's CT and ICRP110 (left column), their respective bone segmentations (center column), and the synthetic whole-body CT obtained by registering the subregions, together with their Sørensen-Dice index and transformation matrix (right column).

Figure 2 shows the validation in a coronal and sagittal view of the program, with the overlay of the whole-body CT (yellow) and the synthetic whole-body CT (red) generated by abdomen registration.

A user-friendly computational tool to generate synthetic patient-specific whole-body CTs was developed. It may be used, for example, for the accurate determination of peripheral dose during radiotherapy using our Periphocal 3D software (see abstract E22-0473). This is an important step towards personalized treatment planning that takes into consideration the probability of second cancer induction.

Acknowledgements:

Fondecyt N1181133

The Free Access Decedent Database funded by the National Institute of Justice grant number 2016-DN-BX-0144.