Session Item

For skin external radiotherapy, an adequate bolus conformation is necessary to achieve the desired skin dose escalation effect. However, it is sometimes difficult to achieve the skin conformation with classical or thermoformed boluses for particular skin applications (scalp, nose, extremities). 3D printing associated with patient CT images allows reaching this objective. As no marketed medical device fulfilled our needs, a specific process has been developed.

A patient CT scan is first performed and imported into the TPS (Varian Eclipse) to determine the bolus shape and location. A python graphical interface was developed to convert DICOM RT structures into .stl format used by the Raise 3D Pro2 printer. The printing material is polylactic acid (PLA), biocompatible and with a density close to water.  After printing, a new CT scan of the patient with the bolus in place is performed to verify the bolus conformity and used for treatment planning. The treatment process is then identical to that with classical or thermoformed boluses.
Before using the PLA bolus on patients, the dosimetric behavior was studied. A pluri-professionnal a priori risk analysis was also performed. Furthermore, 3D bolus patients have a specific follow-up to record any adverse clinical effects.
As the process uses a non-marketed equipment, a declaration to the French health product safety national agency (ANSM) is in progress.

The dosimetric behavior of the material is close to that of water (within 1%). No deformation was observed after an irradiation of 100 Gy in photons of 6 MV and electrons of 6 MeV. Since January 2021, 35 patients have benefited from the technique for ear, nose, scalp, foot, leg, finger and wrist locations. No complications have been observed to date. The printing time was variable, depending on the location and the thickness (from a few hours to several days). Depending on the shape, bolus cutting strategies have been developed for successful impressions. User feedback was excellent with better conformation of boluses to these complex surfaces compared to traditional boluses. Because of the need of two CT scans, generally spaced a week apart, this technique is not applicable to rapidly evolving lesions.

Despite a longer implementation time than with conventional or thermoformable boluses, 3D printing provides a technical solution to the bolus preparation for complex surfaces. The use of a non-marketed medical device required a special monitoring of patients and a declaration of the technique to the ANSM.