Dean Cutajar
Centre for Medical Radiation Physics, University of Wollongong
Research Fellow

Taghreed Al-sudani
PhD Candidate
Centre for Medical Radiation Physics, University of Wollongong

Andrew Howie
Senior Physicist
St George Cancer Care Centre, St George Hospital, Kogarah

Andrej Bece
Postgraduate Fellow
St George Cancer Care Centre, St George Hospital / St George and Sutherland Clinical School, University of New South Wales

Anatoly Rosenfeld
Director/Research Leader
Centre for Medical Radiation Physics, University of Wollongong

Background and Purpose

Accelerated partial breast irradiation (APBI) involves radiotherapy treatment of the lumpectomy bed after breast conserving surgery. High dose rate (HDR) interstitial brachytherapy is a modality used to provide the treatment. With the advent of the AAPM Task Group 186 (TG186) recommendations, complex dose calculations improving the accuracy near interfaces (skin, cavities, applicators) are being adopted in a paradigm shift for brachytherapy dose planning, away from the traditional Task Group 43 (TG43) protocol1. This study investigated the effect of the skin-air interface on the skin dose during interstitial brachytherapy APBI by performing phantom measurements in comparison with TG43 based dose planning.

Methods

A gelatine-based breast phantom was constructed. Nine interstitial catheters were implanted into the phantom through a template and six surface locations were localised with markers to indicate points of interest. The phantom was scanned using CT, and Oncentra Brachy was used to reconstruct catheter paths through the treatment volume and to optimise the source dwell points, as well as generate dose metrics for the treatment. A plan was generated with a prescribed dose of 3.4Gy to a theoretical volume. MOSkin dosimeters were placed on the surface of the phantom at the points of interest. The treatment was delivered to the phantom three times, with the MOSkin doses recorded after each full treatment delivery.

Results

The MOSkin detectors all recorded doses below treatment planning system determined doses at the points of interest. The relative discrepancies ranged from 2% to 29%, with an average discrepancy of 15%. The largest absolute dose discrepancy being 30cGy below the calculated dose of 193cGy (15%), from a point of interest directly above the centre of the target.

Conclusions

The Dose planning in interstitial brachytherapy APBI using would benefit from the adoption of TG186 based dose planning. A lower than predicted dose to the skin may be beneficial to the patient, however, indicates there may be coverage issues for treatment volumes close to the surface. The MOSkin, a dosimeter designed to measure the dose to a patient’s skin during radiation therapy, was used, providing a pilot study for the development of an in-vivo quality assurance program for breast radiation therapy, involving brachytherapy or external beam deliveries.

References

1. Beaulieu. L, et al, “Report of the Task Group 186 on model-based dose calculation methods in

brachytherapy beyond the TG-43 formalism: current status and recommendations for clinical

implementation”, 2012, Med. Phys. 39 6208–36


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