Narrowing the difference in dose delivery for IOERT and IOBT for locally advanced and locally recurrent rectal cancer

Abstract

Purpose: Intra-operative radiotherapy (IORT) has been used as a tool to provide a high-dose radiation boost to a limited volume of patients with fixed tumors with a likelihood of microscopically involved resection margins, in order to improve local control. Two main techniques to deliver IORT include high-dose-rate (HDR) brachytherapy, termed 'intra-operative brachytherapy' (IOBT), and electrons, termed 'intra-operative electron radiotherapy' (IOERT), both having very different dose distributions. A recent paper described an improved local recurrence-free survival favoring IOBT over IOERT for patients with locally advanced or recurrent rectal cancer and microscopically irradical resections. Although several factors may have contributed to this result, an important difference between the two techniques was the higher surface dose delivered by IOBT. This article described an adaptation of IOERT technique to achieve a comparable surface dose as dose delivered by IOBT.

Material and methods: Two steps were taken to increase the surface dose for IOERT: 1. Introducing a bolus to achieve a maximum dose on the surface, and 2. Re-normalizing to allow for the same prescribed dose at reference depth.

Conclusions: We describe and propose an adaptation of IOERT technique to increase surface dose, decreasing the differences between these two techniques, with the aim of further improving local control. In addition, an alternative method of dose prescription is suggested, to consider improved comparison with other techniques in the future.

Keywords: dose distribution; dosimetry; intraoperative brachytherapy; intraoperative electron radiotherapy; intraoperative radiotherapy; locally advanced rectal cancer; recurrent rectal cancer; surface dose.

Fig. 1

Position of an IOERT applicator within the patient. IOERT applicator has been brought into the abdominal cavity and placed against the presacral region at the location of potential microscopic disease

Fig. 2

Image of flexible intra-operative template (FIT) showing 5 mm thickness and spacing of catheters at 1 cm within silicone flap. Additionally, an example of a cut corner is shown [18]

Fig. 3

Dose depths curves and profiles for IOERT and IOBT. A) Absolute depth dose curves for IOERT (dashed-line) and IOBT (solid line) at clinical axis, being the axis perpendicular to the tissue surface where the central axis of the beam intersects with the tissue surface. B, C) The relative dose profiles at a plane through the center of applicators for IOERT (B) and IOBT (C). For IOBT (C), the catheter points (red dots) and FIT (blue bar) are indicated

Fig. 4

Graph showing the current situations for IOERT and IOBT, in absolute dose depth for IOBT (solid line) and IOERT (long-dashed line). Short-dashed line illustrates the IOERT curve after applying a bolus, but before re-scaling. Dotted line shows the IOERT curve with bolus and re-scaling to deliver 10 Gy at 9 mm again