ESTRO 2023

Session Item

Brachytherapy: Physics

Session Code: 7016

Session Type: Poster (Digital)

Track: Brachytherapy

The effect of eccentric plaque placement on tumour dose in Ruthenium plaque brachytherapy

Claire Phillips, Australia

Presentation Number: PO-2163

Abstract

Abstract Title:

The effect of eccentric plaque placement on tumour dose in Ruthenium plaque brachytherapy

Authors:

Jeremy P M Flanagan^1,6, William H F Udovenya², Melvin A Astrahan³, Daniel McKay⁴, Claire Phillips⁵, John D McKenzie⁴, Roderick O'Day^6,4, Lotte Stubkjaer Fog^4,7

¹University of Melbourne, Ophthalmology, Department of Surgery, Melbourne, Victoria , Australia; ²University of Melbourne, Ophthalmology, Department of Surgery, Melbourne, Victoria, Australia; ³Keck School of Medicine, University of Southern California, Department of Radiation Oncology, Los Angeles, California, USA; ⁴Royal Victorian Eye and Ear Hospital, Department of Ocular Oncology, Melbourne, Victoria, Australia; ⁵Peter MacCallum Cancer Centre, Department of Radiation Oncology, Melbourne, Victoria, Australia; ⁶Centre for Eye Research Australia, Ocular Oncology Research Unit, Melbourne, Victoria, Australia; ⁷The Alfred Hospital, Alfred Health Radiation Oncology, Melbourne, Australia

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Purpose or Objective

Ruthenium plaques are used to treat small ocular melanomas. Treatment planning typically assumes that a plaque will be surgically centred under the tumour base. However, sometimes, surgical constraints require a plaque to be placed eccentrically. In this work, we investigate the dosimetric consequences of eccentric plaque placement by calculating the dose delivered to 98% of geometrically modelled tumour volumes (D98). D98 has been demonstrated to be related to tumour control probability [1].

Material and Methods

Dose distributions from two commonly-used plaque models (CCA and CCB) were calculated using Plaque Simulator (version 6.6.9 (current release is 6.8.9), EyePhysics, LLC, Los Alamitos, CA, USA; PS) [2] (fig 1). For the CCA plaque (diameter 15.3 mm), a 7 mm base diameter tumour was modelled; for the CCB plaque (diameter 20.3 mm), 7 and a 10mm base diameter tumours were modelled. The treatment time required to deliver a prescription dose of 100 Gy to the tumour apex for centred plaque placements was determined for each plaque and tumour. D98 was then calculated for plaque placements of increasing eccentricity, in 1 mm steps. Tumour heights of 2-6mm were modelled.

For each combination (tumour height, tumour base dimension and plaque), “safety margins” were calculated. These safety margins represent the eccentricities at which D98 drops below 95% and 90% of the prescription dose from plaque edge alignment with tumour edge.

Results

D98 decreases as plaque eccentricity increases for all scenarios.

The 95% safety margins range from 1.00 (CCB plaque, 4mm tall tumour with base diameter of 10 mm) to 4.00 mm (CCA plaque, 5 mm tall tumour with a base diameter of 7 mm). The 90% safety margins range from 0.33mm (CCB plaque, 4mm tall tumour with a base diameter of 10 mm) to 3.09mm (CCA plaque, 5 mm tall tumour with a base diameter of 7 mm) (table 1).

For the simple geometries studied, the safety margin is clearly related to plaque model, tumour base dimensions and tumour height. Safety margins are likely to depend on the geometric shape of the tumour base as well.

Conclusion

D98 in tumours treated with Ru-106 plaque brachytherapy decreases with increasing plaque eccentricity, and the severity of this decrease increases with tumor height. Treatment times that were calculated assuming central plaque placement should be modified to account for eccentric plaque placement in order to maintain the originally anticipated probability of tumour control.

References

[1] Acta Oncologica, 59:8, 918-925, DOI: 10.1080/0284186X.2020.1762925

[2] Int J Radiat Oncol Biol Phys 2005 , 61, 4, 1227–1242, 2005