ESTRO 2024 - Abstract Book

S5547

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

Previous production of planned bolus consisted of MLC projection of the required bolus on the thermoplastic mask. The shape was cut to the required thickness and attached on the shell in mould room. This created irregularities in the position and shape of the bolus particularly in curvature areas on the patient. 3D printed bolus will enhance the accuracy as it will precisely match dosimetric requirements.

Evaluating 3D printing versus manually produced bolus will assess the comparability with the treatment plan and therefore will prove robustness on conformity and accuracy in bolus placement.

Material/Methods:

Initially, the Adaptiiv 3D software was commissioned. The density of the filament from 3D printer needed to be defined to ensure the correct dosimetry. The Hounsfield unit (HU) was defined by printing 5 solid blocks and CT scanning them. 1.0cm slabs were printed and dose measured on a linac. The implementation of the 3D printer was conducted in two phases.

In 10 retrospective manual bolus head and neck (HN) patients, 3D bolus was printed. Both types of bolus were CT scanned to evaluate its resemblance and the Hounsfield unit (HU) for density.

In the second phase, 10 HN patients were selected prospectively to have treatment using 3D printed bolus. 0.5cm bolus was delineated by the clinician and planned in the treatment planning system (TPS) by the dosimetrist. The bolus was converted into 3D printable format and exported to the 3D printer. The printed bolus was mounted onto the HN shell and a Cone Beam CT (CBCT) of the shell only was taken for size and positioning verification. Once the patient had finished their treatment, an additional CT scan of shell only was performed to confirm that the density of the printed bolus was consistent with defined HU. For each patient the mean HU was identified by outlining the 3D bolus from the post treatment scan and the HU was generated by the treatment planning system.

Results:

The HU for the 5 solid blocks were measured in TPS as 150HU. The dose verified for the 1cm slabs measured on the linacs agreed within 0.23% with predicted dose in TPS.

For the 10 retrospective patients, the results showed acceptable comparability between the two types of bolus.

For the 10 prospective patients, the bolus length and shape matched the bolus shape delineated by the clinician.

Bolus size and position marked onto the shell using an MLC light projection can be subject to light field ‘splash’ over curved surfaces and human interpretation in marking onto the shell. Manually created bolus will made to the marks on the shell however, 3D printed bolus is accurately created to the size defined by the clinician. The co-registration of the planned and printed bolus showed the overall difference in length averaged 0.15cm (Figure 1). Most of the 3D bolus resulted in longer length than the planned bolus. The results illustrated that there were discrepancies from the mean HU and standard deviation (SD) HU. For example, one patient had a mean 38.926HU with SD 191. Another patient had mean 103.3HU and SD 99.8. It is not yet known why a large standard deviation occurred, but it might be due to the location of the bolus on the patient. Since 150HU was defined from commissioning stage, it might be useful to collect data comparing 150HU to HU value close to bolus