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S400
ESTRO 36
_______________________________________________________________________________________________
1
Samsung Medical Center- Samsung Biomedical Research
Institute- Sungkyunkwan University School of Medicine,
Radiation Oncology, Seoul, Korea Republic of
Purpose or Objective
A new technique for manufacturing a patient-specific
dosimetric phantom (PSDP) using three-dimensional
printing (PSDP_3DP) was developed, and its geometrical
and dosimetric accuracy was analyzed.
Material and Methods
External body contours and structures of the spine and
metallic fixation screws (MFS) were delineated from CT
images of a patient with MFS who underwent stereotactic
body radiation therapy for recurrent thoracic spine
metastasis. Contours were converted into a
STereoLithography file format using in-house program. A
hollow, four-section PSDP was designed and manufactured
using three types of 3DP to allow filling with a muscle-
equivalent liquid and insertion of dosimetric film and grass
dosimeters in the axial and coronal planes, respectively.
To evaluate the geometrical and dosimetric accuracy of
PSDP_3DP, CT images were obtained and compared with
patient CT data for volume, mean density, and Dice
similarity coefficient (DSC) for contours. The dose
distribution in the PSDP_3DP was calculated by applying
the same beam parameters as for the patient, and the
dosimetric characteristics of the PSDP_3DP were
compared
with
the
patient
plan.
Results
In comparing between the patient and PSDP_3DP, the
percent differences in volume for the external body,
spine, and MFS were -4.1%, 6.4%, and 10.0%, while the
DSCs were 0.98, 0.91, and 0.89, respectively. The
differences in density between the external body and
spine were 7.5% and 15.5%, respectively. In the axial plane
at the target center, Large dose differences were
observed at the border of the external body contour (low-
dose region), while most of the center region (high-dose
region) was in good agreement, with a dose difference
within 5%. The DHVs of both plans were well matched.
Specifically, the mean differences in dose for GTV, CTV,
spinal cord, and external body were -0.5%, -0.5%, 4.0%,
and -2.8%, respectively.
Conclusion
The physical accuracy and dosimetric characteristics of
the PSDP were comparable with patient data. The ability
to manufacture a PSDP representing an extreme patient
condition was demonstrated.
PO-0759 Validation of the influence of M512 substrate
resistivity on sensitivity degradation of radiation
N. Stansook
1
, M. Petasecca
1
, K. Utitsarn
1
, M. Carolan
2
, P.
Metcalfe
1
, M.L.F. Lerch
1
, A.B. Rosenfeld
1
1
Wollongong University, Center for Medical Radiation
Physics CMRP, wollongong, Australia
2
Wollongong hospital, Illawarra Cancer Care Centre,
Wollongong, Australia
Purpose or Objective
The diode detector has been wildly used as a quality
assurance (QA) tools in radiotherapy. However, the
detector is affected by accumulative radiation damage
leading to degradation of the sensitivity and dose per
pulse dependence. The objective of this study is to
investigate the influence of the substrate resistivity on
sensitivity degradation of radiation and dose per pulse
dependence of M512.
Material and Methods
The M512 is a monolithic 2D 512 diode array detector
fabricated on p-type Si substrate. The detector active
area is 52x52 cm
2
with 2 mm pixel pitch. M512 was
developed at the Center for Medical Radiation Physics
(CMRP) for quality assurance in SRS and SBRT. In this
study, two types of Si substrate including M512-Bulk and
M512-Epi were investigated. The M512-Bulk has been
fabricated on low resistivity bulk silicon with thickness 470
µm while the M512-Epi has been manufactured on an
epitaxial high resistivity p-silicon with 38 µm thick grown
on a low resistivity of 370 µm thick substrate. Both
detectors were irradiated on the
60
Co source in the total
dose ranging from 0 to 40 kGy for M512-Bulk and 0 to 60
kGy for M512-Epi detectors. The 6 MV photon beam was
used to investigated the sensitivity degradation and dose
per pulse dependence. To evaluate the sensitivity
degradation, the detector response was measured after
irradiation with dose increments of 10 kGy. The dose per
pulse dependence was determined by varying the SSD from
100 to 370 cm corresponding to the dose per pulse ranging
from 0.278 to 0.021 mGy/pulse. The PDD was measured
using a square field size 10x10 cm
2
by fixing the SSD at 100
cm and varying detectors depth in a phantom from 0.5 to
30 cm and comparing with the CC13 chamber.
Results
M512-Epi demonstrates excellent radiation stability with
the sensitivity degradation of 0.3 %/10 kGy while M512-
Bulk shows the degradation of 1%/10 kGy. The detector
response decreases with the dose per pulse decrease.
M512-Bulk shows less dose per pulse dependence compare
with the M512-Epi with the sensitivity response (pC/Gy)
decreasing about 2% while the sensitivity of M512-Epi
decreased by 8% at a dose per pulse change of 10 times.