Session Item

To report results of in-vivo 2D dosimetry in HDR surface brachytherapy for patients treated with the Freiburg-flap (Elekta Brachytherapy) and custom-made surface applicators. To quantify the need for more accurate dose calculation engines in comparison to TG43-based planning. To highlight the advantage of radiochromic film 2D dosimetry in the assessment of delivered dose to the targets and organs-at-risk, and introduce adaptive planning based on the measurements.

In this study, data were analyzed from 87 film measurements taken during the delivery of 53 fractions of 21 HDR surface brachytherapy cases. Delivered doses were measured with pre-cut EBT3 film pieces (Figure 1(a)). Films were placed on patients’ skin (clinical target) or out-of-field and all cases were planned at the 3 mm depth from the skin surface. The prescription dose per fraction ranged from 250 cGy to 600 cGy depending on the diagnosis and treatment site. Treatment plans were calculated using TG43 formalism in Oncentra Brachy™ treatment planning system (Elekta Brachytherapy) and dose was also recalculated using advanced collapsed cone engine (ACE). For each fraction, measurements were compared to calculations at the skin surface to evaluate inter-fractional dosimetric variability and the need for planning adaptation. A histogram of all measurements normalized to the prescription dose was created to evaluate the percent increase in skin dose as a surrogate for dose delivered at the prescription depth.

A representative case (Figure 1(b)) shows histograms of measured doses recorded during five fractions of a partial scalp treatment and compared to expectations from TG43-based planning. Another representative two-fraction case is shown in Figure 1(c) to highlight differences between measurements and calculations. On average (for all cases) these differences were: -7.1% [range: -3% to -15%] w.r.t. TG43 and 2.4% [range: -2% to 6%] w.r.t. ACE. Dose adaptation was shown to be necessary in some complex cases and an example is shown in Figure 1(d) of a breast case (200 cGy × 23 fractions) with significant folds and air gaps that was adapted by increasing selected dwell times following the in-vivo dosimetry results of the first fraction. Figure 2 shows a histogram of all measured data in this study summarizing the dosimetric evaluation of skin dose in HDR surface brachytherapy (yellow highlighted area).

Obtained measurements reveal the need for dose adaptation and that model-based algorithms should be considered for planning complex HDR surface brachytherapy cases. Radiochromic film is an efficient in-vivo dosimeter that can be used in dosimetry of these cases and consequent adaptive re-planning.