Radiation prescription for uveal melanoma: apical dose versus volume dose

Abstract

Purpose : Ruthenium-106 brachytherapy is commonly used to treat small to medium sized intraocular melanomas. The length of time the radioactive plaque remains sutured to the sclera (‘Treatment Time’) is usually determined by a proscribed dose to the tumour apex, and assumes that the centre of the plaque is aligned with the centre of the tumour. However, in certain clinical contexts plaques may be placed eccentrically. Furthermore, following trends in radiation oncology more broadly, D₉₈ – the total radiation dose delivered to 98% of tumour volume – has been recently shown in ruthenium brachytherapy to more closely correlate with tumour control than apex dose. However, how D₉₈ changes for these eccentric plaques is unknown. We hypothesised that as the distance between plaque and tumour edge decreased, the apex dose will decrease; and that differences in the apex and D₉₈ doses will increase.

Methods : A 7mm diameter dome-shaped tumour was treated with either a CCA or CCB plaque in Plaque Simulator (version 6.6.9, EyePhysics, LLC, Los Alamitos, CA, USA). Treatment time was determined by centrally placing each plaque and delivering a 100Gy apex dose for 2, 3, 4, 5 and 6mm thick tumours. Plaque edge and tumour edge were placed 0-4mm apart in 0.5mm increments to model multiple scenarios. For each plaque location and tumour thickness, we established the apex dose and D₉₈.

Results : For both CCA and CCB plaques, the apex dose and D₉₈ decreased as the distance from plaque edge to tumour edge decreased, with a greater rate seen in D₉₈. Apical dose and D₉₈ were broadly similar for thinner (2-4mm) tumours across all degrees of eccentricity, with D₉₈ greater than apex dose in most scenarios. D₉₈ was substantially greater than the apical dose for thicker (5-6mm) tumours at all eccentricities, especially when the plaque centre was more closely aligned with tumour centre (Figure 1, Figure 2).

Conclusions : Apex dose and D₉₈ decrease with increasingly eccentric ruthenium plaque placements. These changes depend on tumour thickness and plaque type, and may depend on tumour shape. Greater differences between apex dose and D₉₈ are seen in thicker tumours.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.

 

Figure 1: Apex dose and D₉₈ for CCA (A, C, E) and CCB (B, D, F) plaques: 2 mm thick tumour (A, B), 4 mm thick tumour (C, D) and 6 mm thick tumour (E, F).

 

Figure 2: Isodose lines for CCA plaques for 2mm thick (A) and 6mm thick (B) tumours, distance from plaque to tumour edge of 2mm