ESTRO 2020 Abstract book

S805 ESTRO 2020

materials. There is a strong need to continuously monitor annual occupational exposures to evaluate lifetime cancer risks. PO-1424 Fast model for evaluation of the thyroid dosimetry during chest tumor radiotherapy Y. Wang 1 , L. He 2 , M. Zheng 1 1 University of Electronic Science and Technology of China., School of Medicine-Sichuan Cancer Hospital & Institute, Chengdu, China ; 2 Sichuan Center for Disease Control and Prevention, Institute of Radiation Protection, Chengdu, China Purpose or Objective Due to the reported high incidence of thyroid cancer (7.5 / 1 million cases per 1 cGy absorbed dose to the thyroid) resulted from radiotherapy, dose assessment is significant to prevent thyroid late effects. To date, thyroid dosimetry was evaluated either by entrance skin dose (ESD) in terms of thermo-luminescent dosimeter (TLD) arrays or by absorbed dose (AD) computed with treatment planning system (TPS). However, the correlation between thyroid ESD and AD is hardly reported in any published works. Moreover, the reported measurement procedures for thyroid ESD are usually inefficient. The purpose of this study is to provide a fast model for efficient acquisition of thyroid ESD and analyzing the coherent relationship We have studied the thyroid dosimetry by the Chengdu dosimetric phantom, which was irradiated by a Varian 23EX linac with intentionally delineated esophagus or lung cancers (PTV) as shown in Fig. 1. Cases A-C corresponded to the esophagus cancers with lengths 9cm, 11cm and 14cm, while cases D-F corresponded to the lung cancers located at different positions of chest wall. The prescription was 60 Gy in 30 fractions delivered to the PTV with 100% of the volume covered by ≥95% of the prescribed dose. Four radiotherapy techniques, including 2D-CRT, 3D- CRT (5 fixed fields), IMRT (7 fixed fields), and VMAT, were delivered. We have measured the thyroid ESD with pre- calibrated TLD (chips: LiF (Mg. Cu. P), TLD reader analyzer system: RGD-3B) at five different points, including the right upper pole, the right lower pole, the left upper pole, the left lower pole, and the middle gorge of thyroid gland, while computed the thyroid AD by the Oncentra Masterplan TPS. between ESD and AD. Material and Methods

Oncology Lahore INMOL- Pakistan, radiation dosimetry, Lahore, Pakistan ; 3 Institute of Nuclear Medicine & Oncology Lahore INMOL- Pakistan, Dosimetry, Lahore, Pakistan ; 4 Institute of Nuclear Medicine & Oncology Lahore INMOL- Pakistan, Chairman of INMOL, Lahore, Pakistan ; 5 National University of Computer & Emerging Sciences NUCES- Lahore Pakistan, Civil Engineering Department, Lahore, Pakistan Purpose or Objective Nuclear medicine personnel are exposed to chronic low- dose ionizing radiations (IR) from radioactive sources, radiopharmaceuticals (Tc-99m and I-131) and from imaging modalities. This cohort evaluated the lifetime fatal/non-fatal (FCR/NCR) cancer risks and compared them with the risks of other carcinogens. Material and Methods A high-capacity TLD reader (Harshaw model 8800) was used to calculate whole-body AAED (annual average effective dose) (mSv) in nuclear medicine staff from 2015- 2019. The management of the dosimetry was done in software RaDLab. We used probability coefficients for stochastic effects by considering 35 years as working lifetime limit to assess lifetime fatal/non-fatal cancer risks as mentioned by ICRP (1990) and UNSCEAR (Lochard in IAEA 2002). The coefficient for FCR is 4.0×10 -2 (detriment/Sv) and for NCR is 0.8 × 10 -2 (detriment/Sv). The average annual risk (AAR) was also calculated by taking 60 years of average life expectancy. All risks were compared with lifetime occupational exposure (35 years) risks from other carcinogen substances (Nickel, Arsenic, Benzene & Asbestos) through Mann-Whitney U test. The lifetime risks for these substance were calculated from their limit of average exposure values (AEV) along with their respective risk coefficients as mentioned by Lochard in IAEA (2002). Results A declining trend was observed in AAED (from 0.561-0.274 mSv) and in lifetime cancer risks from IR (from 7.672×10 -5 to 7.854×10 -4 ) from 2015-2019. The range of lifetime risks was 4.33×10 -2 to 0.2×10 -2 for other carcinogens. The values of all risks from IR during 2015-2019 were found fairly lower than maximum allowable limit (20 mSv) IR exposure’s risk value 2.80×10 -2 and the risk values of other substances. A significant differences (p value 0.015) were found between mean IR risk values of following FCR, NCR, FCR-AAR & NCR-AAR, and between the mean risk values of other carcinogen substances.

No. of worke rs in respe

AAED (mSv) Min.

AAED (mSv) Max

AAED ±SD (mSv)( Mean) 0.561±0 .428 0.457±0 .472 0.383±0 .404 0.336±0 .381 0.274±0 .320

FCR AAR-

FCR NCR AAR- NCR

ctive year

26 (2015) 27 (2016) 32 (2017) 34 (2018) 34 (2019)

1.309 ×10 -5 1.066 ×10 -5 8.936 ×10 -6 7.840 ×10 -6 6.393 ×10 -6

1.570 ×10 -4 1.279 ×10 -4 1.072 ×10 -4 9.408 ×10 -5 7.672 ×10 -5

2.618 ×10 -6 2.132 ×10 -6 1.787 ×10 -6 1.568 ×10 -6 1.278 ×10 -6

0.08 1.40 7.854 ×10 -4 0.09 1.99 6.398 ×10 -4 0.01 1.83 5.362 ×10 -4 0.01 1.58 4.704 ×10 -4 0.01 1.36 3.836 ×10 -4

Results The ESD at the middle gorge of thyroid exhibited significant linear correlation with those measured at other points, as was displayed in Fig. 2(a)(b). A regressive model was proposed to predict thyroid AD from ESD at the middle gorge in Fig. 2(c). The 2-sided Mann–Whitney U-test was applied to verify the difference between the predicted and TPS computed ADs, where nonsignificant difference was observed with P =0.75 in Fig. 2(d).

Conclusion The nuclear medicine IR exposed risks are found lesser than the other carcinogens and they were declining each proceeding year due to the careful practice of occupational exposures in INMOL. The maximum medical IR exposure risk value 2.80×10 -2 (20 mSv) can be used as an indicator to compare other AAED risk exposures as well as all risks from all other cancer causing occupational