ESTRO 2021 Abstract Book

S1514

ESTRO 2021

Table 1. Relative risks of SMN

OAR

Relative risk of SMN

SFUD/3D-CRT

SFUD/IMRT

3D-CRT/IMRT

LE 1

P 2

LE

P

LE

P

In-field total(- CTV)

0.26

0.32

0.21

0.28

0.82

0.87

Solid tumor

0.16 0.16 0.41 0.32 0.49 0.01 0.19

0.21 0.32 0.44 0.39 0.46 0.01 0.15

0.12 0.14 0.38 0.31 0.35 0.01 0.16

0.18 0.26 0.44 0.35 0.41 0.01 0.13

0.77 0.83 0.91* 0.95 0.70 0.85 0.84

0.85 0.82* 1.01* 0.88 0.90* 0.84 0.83*

Skin

Esophagus

Lung

Breast

Stomach Thyroid

1 Linear-Expontial model, 2 Plateau model

*P >0,05, otherwise P < 0,05

Conclusion The use of proton beam radiotherapy in the treatment of thymic tumors could reduce the frequency of radiation-induced secondary cancers.

PO-1789 Use of water-equivalent diameter to assess organ doses of adult patients from CT body scans A. Abuhaimed 1 , C. Martin 2 1 King Abdulaziz City for Science and Technology (KACST), The National Center for Applied Physics , Riyadh, Saudi Arabia; 2 University of Glasgow, Department of Clinical Physics and Bioengineering, Glasgow, United Kingdom Purpose or Objective There are several methods used to estimate organ doses of patients undergoing CT scans, some of which are based on physical measurements, using Monte Carlo simulations, or applying conversion factors. The aim of this work is to establish size-specific and scanner-independent dose conversion coefficients (DCCs) to estimate organ doses of adult patients resulting from body CT scans based on the patient size. Materials and Methods A total of 193 adult male and female phantoms of various sizes that represent a wide range of patient sizes were used. The phantom weights and lengths were in the range of 40 – 125 kg and 15 – 19 m with the body mass index being 16.5 – 49.6 kg/m 2 . Six different scan regions of the trunk were investigated: (1) chest, (2) abdomen, (3) pelvis, (4) chest-abdomen, (5) abdomen-pelvis, and (6) over all the regions of the trunk (CAP). Establishment of the DCCs requires two parameters to be identified, the patient size and organ doses. First: the patient sizes of each region were measured in terms of water equivalent diameter (D w ) that is recommended due to its accuracy as compared to other metrics that are based on patient dimensions. A MATLAB code was written to calculate D w for each phantom using a spectrum of 120 kV. Values of D w for each region were measured as an average of D w value for slices of a given region. Second: organ doses were assessed using the same phantom library with the NCICT software based on Monte Carlo simulations. A tube potential of 120 kV and a pitch of 1.0 were applied to assess organ doses for standard scan lengths. Subsequently, each organ dose was normalized with respect to the corresponding CTDI vol , from which scanner- independent DCCs were obtained. The DCCs were then linked to the patient size, D w , to obtain size-specific coefficients. Only organs that were fully or largely inside the scan length were considered. Results Generally, good correlations were found between the values, where R 2 of most organs were more than 0.9. Table 1 gives coefficients for the best-fit curves for the correlations, from which DCCs of specific sizes can be determined. This can be used for any scanner, and only depends on CTDI vol of the scan, where dose of an organ of interest can be estimated by multiplying the DCC by CTDI vol .