Purpose: To investigate dosimetry of pre-plan vs. post-plan dosimetry for patients treated with 3D-printed brachytherapy surface applicators.
Methods: Patients treated in our clinic with surface brachytherapy undergo iterative pre-planning prior to the creation of a 3D-printed mold. The pre-plans are used to determine mold designs that result in the desired coverage and limited hot spots for each patient’s anatomy and treatment site. Pre-plans for treated patients were compared to post-plans obtained with the printed mold on the patient anatomy to determine the extent to which pre-plans predict the final plans as evaluated with the actual treatment CT-scan.
Results: To-date, five patients have been treated with 3D-printed molds for surface brachytherapy including three noses, one three-site forehead, and one cheek. For all patients, pre- vs. post-plan D90% and D95% were within 2.5% (average differences: 0.6% and 0.7% for D90% and D95%, respectively), where post-plans had higher coverage compared to pre-plans for all but one case. The post-plan V150% were all within 2% (average 0.6%) of the pre-plan, with most post-plans being slightly hotter than the pre-plan likely due to improved coverage in the post-plan. The average change in CTV minimum, mean, D90%, D95%, D100%, V100%, V110%, V120%, V130%, V140%, and V150% parameters were all less than ±1.5%, while average maximum point CTV difference was within 4% over all cases.
Conclusion: 3D-printed brachytherapy surface molds created for clinical use exhibit very good agreement between planned and final dosimetry on the patient. The pre-planning process for 3D-printed applicators allows for modifications to depth of catheters, intra-catheter spacing, and catheter orientation to achieve the desired dosimetry for surface brachytherapy. This process allows for dosimetric goals to drive the mold design, as opposed to mold design governing the achievable dosimetry which is often the case when using handmade or commercially available flap molds.