Total body irradiation: A transition from a Co-60 treatment unit to an IMRT lateral-field extended-SAD technique

Abstract

Purpose: The purpose of this work was to detail our center's experience in transitioning from a Co-60 treatment technique to an intensity modulated radiation therapy (IMRT) based lateral-field extended source-to-axis distance (e-SAD) technique for total body irradiation (TBI).

Materials and methods: An existing beam model in RayStation v.10A was validated for the use of e-SAD TBI treatments. Data were acquired with an Elekta Synergy linear accelerator (LINAC) at an extended source-to-surface distance of 365 cm with an 18 MV beam. Beam model validation measurements included percentage depth dose (PDD), profile data, surface dose, build-up region and transmission measurements. End-to-end testing was carried out using an anthropomorphic phantom. Treatments were performed in a supine position in a whole-body Vac-Lok at an e-SAD of 400 cm with a beam spoiler 10 cm from the couch. Planning was achieved using IMRT, where multi-leaf collimators were used to modulate the beam and shield the organs at risk. Beam's eye view projection images were used for in-room patient positioning and in-vivo dosimetry was performed for every treatment.

Results: The percent difference between the measured and calculated PDD and profiles was less than 2% at all locations. Surface dose was 83.8% of the maximum dose with the beam spoiler at a 10 cm distance from the phantom. The largest percent difference between the treatment planning system (TPS) and measured data within the anthropomorphic phantom was approximately 2%. In-vivo dosimetry measurements yielded results within the 5% institutional threshold.

Conclusion: In 2022, 17 patients were successfully treated using the new IMRT-based lateral-field e-SAD TBI technique. The resulting clinical plans respected the institutional standard. The commissioning process, as well as the treatment planning and delivery aspects were described in this work with the intention of supporting other clinics in implementing this treatment method.

Keywords: TBI commissioning; TBI treatment planning; extended‐SSD IMRT; total body irradiation (TBI).

FIGURE 1

Water tank set‐up for measurements at an e‐SSD of 365 cm and a gantry angle of 270° to prepare for PDD, profile, and transmission measurements. [LINAC depicted in the image is an Elekta Versa HD as the Elekta Synergy LINAC used to treat patients has been decommissioned].

FIGURE 2

(Left) Solid water blocks with a parallel‐plate chamber centered with respect to the intersection of the cross‐hairs and (Right) set‐up for surface dose and build‐up region measurements. [LINAC depicted in the image is an Elekta Synergy].

FIGURE 3

(Left) TPS BEV and (Right) light field projection of in‐room BEV for patient positioning before treatment delivery.

FIGURE 4

Anthropomorphic phantom made of solid water blocks, an acrylic cork block for the lungs, a CTDI phantom for the head and water saline bags to mimic the extremities of the human body. PTW Farmer chambers are identified 1 through 5 corresponding to the (1) right lung, (2), mediastinum, (3) left lung, (4) umbilical region, and (5) thigh. (6) represents the location of the diode.

FIGURE 5

TBI patient positioning set‐up.

FIGURE 6

PDD (top) and in‐plane (bottom) measurements between the TPS (orange dash) and measured data (blue solid) for a 10 × 10 cm² FS at isocenter at an e‐SSD of 365 cm at 3 cm depth. The PDD measurements are normalized at 10 cm depth and the in‐plane profile measurements are normalized on the central axis. PDD measurements did not include the build‐up region.

FIGURE 7

Surface‐dose and build‐up region to missing water tank PDD data, for differing beam spoiler distances from the solid water phantom—orange (10 cm), gray (20 cm), yellow (30 cm), and blue (no spoiler).

FIGURE 8

Diagonal profiles for a 40 × 40 cm² FS at isocenter at 10 and 20 cm depth for measured data (gray and yellow dash) and TPS computed after adjustments (blue and orange solid).