PO-0790: In-vivo dosimetry for kV radiotherapy: clinical use of micro-silica bead TLD & Gafchromic EBT3 film
2017
Purpose or Objective kV radiotherapy continues to be an important modality in
modern radiotherapy, but has received less research
attention in recent years. There remains a challenge to
accurately calculate and verify treatment dose
distributions for clinical sites with significant surface
irregularity or where the treated region contains
inhomogeneities, e.g. nose and ear. The accuracy of
current treatment calculations has a significant level of
uncertainty [1, 2]. The objective of this work was to
characterise two novel detectors, micro-silica bead TLDs
and Gafchromic EBT3 film, for in-vivo measurements for
kV treatments, and to compare measured doses with
conventional treatment calculations.
[1. Currie (2007) Australas Phys Eng Sci Med, 2. Chow
(2012) Rep Pract Oncol Radiother.] Material and Methods Micro-silica bead TLDs (1 mm diam.) and Gafchromic EBT3
film were calibrated against an NPL traceably calibrated
ionisation chamber using an Xstrahl D3300 kV radiotherapy
treatment unit. Energy response was evaluated over 70 to
250 kV and compared to 6 MV, useable dose range was
evaluated from 0 to 25 Gy, and uncertainty budgets
determined. Silica beads were cleaned, annealed, and TL
response individually calibrated. EBT3 film was used with
triple-channel dosimetry via FilmQAPro® with procedures
to reduce uncertainties. Commissioning tests were
undertaken in standard conditions using Solid Water blocks
and in simulated clinical treatment condition using a
custom made ‘wax face with nose’ phantom. Pilot in vivo
measurements were made for a consecutive series of eight
clinical patient treatments, including cheek, ear, nose and
rib sites, over 70 to 250 kV, and 4 to 18 Gy. Results for the
two dosimetry systems were compared to conventional
treatment planning calculations. Results Energy response varied by 460% for beads and 9% for film,
from 70 kV to 6 MV, necessitating energy-specific
calibration. Both dosimeters were useable up to 25 Gy.
Standard uncertainty was 3.1% for beads, 2.1% for film.
The figure shows typical film and bead positions within the
lead cut-out of a kV treatment to the cheek. The table
provides calculated and measured doses. Average
deviation over 6 patients was -1.3% for beads, -0.9% for
film. 3 patients had larger deviations; See table note 1:
tumour sitting over the maxillary sinus may reduce dose.
Note 2: beads placed along surface of tumour into ear,
most distal bead received dose -17.5% from prescription,
doctor made compensation. Note 3: Increased uncertainty
due to curved surface, film required offset to corner as
patient sensitive to contact. Note 4: Uncertainty
increased due to large respiratory motion at treatment
site. Conclusion Both micro-silica bead TLDs and EBT3 film were
characterised as suitable for in vivo dosimetry in kV
radiotherapy, providing assurance of delivered doses. Film
is simpler to prepare, use and read. A line of beads allows
conformation to irregular anatomy across the field. A
clinical service is now available to verify dose delivery in
complex clinical sites.
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