MPP05 Presentation Time: 4:45 PM

Purpose

Contact brachytherapy for non-melanoma skin cancers demonstrates very good treatment results. The main problem is to fit properly a standard applicator to a heavily pleated surface, such as the nasal or orbital region. In order to improve the reproducibility and quality of dose distributions, we have introduced individual custom-designed applicators manufactured on a 3D printer. The purpose of this study was to verify the effect of changing the contoured CTV volume between the pre-plan and the therapeutic plan.

Materials and Methods

For the study, 95 consecutive treatment plans were qualified for patients treated between 2021 and 2023 with individual contact applicators for skin brachytherapy. After the qualification, on the first visit in brachytherapy department, the patient had fiducial radiological markers surrounding the skin lesion placed and the CT scan performed. On the images, the physician contoured the volume of the CTV and critical organs. Based on the CT images and contours, medical physicist prepared the body of the applicator, the position of the catheters and the optimal source dwell positions in treatment planning system. The proposed dose distribution was consulted with the physician for verification. After approval, the DICOM files were exported to software converting DICOM files to printable stl files and the applicator was printed. At the next visit, the patient was CT scanned with the applicator in place and a treatment plan was prepared, based on the recontoured CTV and critical organs. We compared CTV volume values and dose distribution values in reconstructed critical organs and CTV for the pre-plan and approved treatment plan. We considered CTV volume, V100, V150 and D90. For OARs we compared doses in 0.1, 1 and 2 ccm.

Results

Since the parameters studied did not show conformity to the normal distribution (the Shapiro-Wilk test was used) we applied the Wilcoxon signed-rank test. The compared parameters for the evaluation of the treatment plan appeared to be consistent with each other within the limits assumed for the tests performed (α=0.05), except for the maximum doses in the lenses. The doses in 0.1 cc of lens in the realized plans were found to be statistically significantly lower than in the plans created at the time of applicator design. Left lens pre-plan D0.1 = 12.86% vs 11.48% (p=0,005441) in treatment plan, right lens pre-plan D.01 = 9.67% vs 8.02% (p=0,005694) in treatment plan. During the preparation of the final treatment plans, physicists suspected physicians to contour larger CTV volumes than during the pre-plan and the applicator design. A surprising result of this study was that although not statistically significant but the mean CTV volume in the pre-plan was higher than in the contour made for the final treatment plan (1.69 ccm vs. 1.61 ccm). The main investigator thought before performing the statistics that the relationship would be the opposite.

Conclusions

Treatment plans prepared for delivering skin brachytherapy using personalized applicators manufactured with 3D printing were not statistically different from those prepared during applicator design. Only the maximum doses deposited in the lenses were statistically lower in the treatment plans finally accepted for irradiation than in the pre-plans. Recontouring of the CTV and critical organs did not affect the dose distributions during the preparation of the final plan, which indicates that the individual applicators prepared on the 3D printer were properly fitted to the patient's body surface.