Mitchell Lielkajis
University of Wollongong
PhD Student

Dean Cutajar
Lecturer
University of Wollongong

Joel Poder
Senior Medical Physicist
St George Cancer Care Centre

Ryan Brown
Medical Physics Registrar
St George Cancer Care Centre

Anna Ralston
Director of Medical Physics
St George Cancer Care Centre

Andrew Howie
Senior Medical Physicist
St George Cancer Care Centre

Introduction

Valencia applicators, incorporating an afterloaded Ir-192 source, are routinely used in the treatment of skin lesions, whereby a prescribed dose is delivered to a reference point 3mm underneath the skin surface. Dose calculations are performed through homogenous media algorithms only, resulting in no compensation for airgaps. This study was performed to analyse the effects of clinically relevant airgaps for Valencia applicator treatments.

Materials and Methods

EBT3 radiochromic film was used to analyse the dose distribution underneath a 3cm diameter Valencia FT applicator placed on a solid water phantom. To study the effects of airgaps underneath the applicator, linear airgaps were introduced of distance 0, 1 mm, 3 mm and 5 mm, with depth dose and horizontal profiles analysed. The effects of the applicator being placed at angles to the surface were investigated by placing the applicator at angles of 0, 2°, 3°, 4°, 5°, 6°, 10°, 15°, 20°, 25° and 30°, with depth dose and horizontal profiles analysed. Two clinical scenarios were also simulated, using the 3cm Valencia FT applicator placed on the nose of an anthropomorphic phantom.

Results

Linear translation of the Valencia FT applicator away from the surface of the phantom was shown to decrease the dose at the prescription point (3mm depth) by 9% for a 1 mm airgap, 27% for a 3 mm airgap and 35% for a 5 mm airgap. These dose decreases were calculated to mainly be due to the effect of increased distance from the source within the applicator. Placing the Valencia FT applicator at angles to the surface was found to decrease the dose at the prescription point by approximately 5% for shallow angles (less than 5°), 28% for 10° and 43% for 30°, however, only the shallow angles were clinically relevant. The anthropomorphic phantom measurements showed horizontal profiles at the prescription depth varying from 100% to 75%, indicative of approximately 3mm of induced airgap, with a majority of the treatment area underdosed.

Conclusions

Whilst it was desired that a model be produced to predict the effects of airgaps beneath the Valencia applicator during clinical application, which would allow for possible compensation and/or adjustment of the treatment, it was established that even the smallest airgap present would notably affect the treatment dose and possibly the treatment outcome. It is recommended that all efforts be made to eliminate airgaps during the use of the Valencia applicator for the treatment of skin lesions.


← Back to all abstracts