Dose reduction for normal tissue from daily CBCT doing radiation treatment for young patients
PO-2342
Abstract
Dose reduction for normal tissue from daily CBCT doing radiation treatment for young patients
Authors: Troels Lautrup1, Jolanta Hansen2, Rune Hansen2
1Aarhus universitetshospital, Stråleklinik, AUH, Aarhus, Denmark; 2Aarhus universitetshospital, Medicinsk fysik, AUH, Aarhus, Denmark
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Purpose or Objective
The aim of this study is to investigate a method to reduce radiation dose for normal tissue and calculate the LAR (Lifetime Attributable Risk) from CBCT (Cone beam computed tomography) for young women with a lymphoma/sarcoma in the thorax region.
Material and Methods
All CBCT scan was done a Varian Truebeam accelerator using the clinic’s standard thorax protocol (125kV, 270mAs), same ISO center with different scan lengths 17.5 cm, 15.5 cm, 13.5 cm, 11.5 cm, 9.5 cm, 7.5 cm and 5.5 cm.
Too measure the radiation dose a Capintec PC-4P ionsering chamber and a Perspex fantom with a diameter of 32 cm was used after which the CTDI and effective dose was calculated.
Too measure the organ dose in breast and thyroid tissue TL dosimetry (TLD) tablet was inserted in slice 9 and 12 in a CIRS Alderson anthropomorphic fantom
Estimating the risk of cancer was done using risk assessment models used in ICRP103 and BEIR VII rapport.
Results
As shown in table if the distance from the thyroid to the edge of the CBCT scan was increase from 1 to 5 cm the radiation dose was reduced by 7.9% and 91.4%. If the distance from breast tissue to the edge of the CBCT scan was increased by 0.61 and up to 4.61cm the radiation dose was reduced by 33.9% and 88.7%.
LAR of deadly cancer for the age group 10-19 years old calculated based on ICRP103 shows a reduction in radiation dose of 22.9 % and 57.1 % if the CBCT scan length is reduced by 4 and 10 cm. Calculation based on BEIR VII rapport, shows that the risk of LAR for the age group 15-20 years old was reduced if the scan length was reduced from 17.5cm down to 7.5 cm
| TLD measurement | TLD measurement
| TLD measurement
| Distance between CBCT edge and TLD tablet measured in cm.
| Distance between CBCT edge and TLD tablet measured in cm.
|
| CBCT length (CM) | Breast (mGy) | Thyroid (mGy) | Breast | Thyroid |
| 17,5 | 6,2 | 14,6 | 0 | 0 |
| 15,5 | 5,6 | 13,0 | 0 | 0 |
| 13,5 | 4,1 | 12,9 | 0,6 | 1,0 |
| 11,5 | 1,9 | 7,4 | 1,6 | 2,0 |
| 9,5 | 1,3 | 1,4 | 2,6 | 3,0 |
| 7,5 | 1,3 | 1,8 | 3,6 | 4,0 |
| 5,5 | 0,7 | 1,2 | 4,6 | 5,0 |
Conclusion
If the CBCT scan length is reduced it is possible to reduce the risk of LAR in OAR (Organ at risk) in the thorax region. If the distance from the OAR and to the edge of the CBCT is increased by 3 cm, the radiation dose can be reduced up to 85%. If it is not possible to spare the OAR based on the irradiated area, a daily ISO centre move before CBCT can be used to spare other OAR.