5 Radiobiology of LDR, HDR, PDR and VLDR Brachytherapy

Radiobiology of LDR, HDR, PDR and VLDR Brachytherapy

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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part I: The Basics of Brachytherapy Version 1 - 22/10/2015

Practical example 8: EQD2 calculation of a standard cervix cancer treatment with HDR boost A standard schedule for cervix cancer treatment is external beam RT of 45 Gy in 1.8 Gy fractions, with concomitant cispla- tin ,followed by an intracavitary (interstitial) boost of 4 times 7 Gy HDR. The total absorbed dose of 45 Gy delivered with EBRT corresponds to an EQD2 of 43.2 Gy assuming α/β = 3 Gy and 44.3 Gy assuming α/β = 10 Gy. The 4 fractions of 7 Gy corre- spond to an EQD2 of 56 Gy assuming α/β = 3 Gy and 39.7 Gy assuming α/β = 10 Gy. As the EQD2 delivered by EBRT and BT are additive, the total EQD2 is thus 99.2 Gy assuming α/β = 3 Gy and 84 Gy assuming α/β = 10 Gy. Practical example 9: EQDS calculation of a standard cervix cancer treatment with PDR boost A standard schedule for cervix cancer treatment is 45 Gy in 1.8Gy fractions with concomitant cisplatin, followed by a PDR BT boost delivering 40 Gy in 80 pulses of 0.5 Gy. The duration of the single PDR course is 3.5 days. As in the calculation of EQD2, only the dose per fraction and T 1/2 are taken into account. The EQD2 of the single PDR is 41 Gy assuming α/β = 3 Gy and 40.4 Gy assuming α/β = 10 Gy. A T 1/2 value of 1.5 h is used for the computation. The total EQD2 doses of combining EBRT and BT are 84.2 Gy assuming α/β = 3 Gy and 84.7 Gy assuming α/β = 10 Gy.

11. KEY MESSAGES

• Brachytherapy (BT) differs from external beam therapy (EBRT) in two main ways: the distribution of the absorbed dose and the time-dose patterns. • The dose is prescribed to an isodose surface encompassing a CTV volume of ~20 - 200 cm 3 . The GTV receives about >150% of the prescribed dose. • Brachytherapy uses different dose rates, divided, somewhat arbitrarily, into three ranges: low-dose rate (LDR) < 1 Gy h -1 , medium dose rate (MDR) between 1 and 12 Gy h -1 and high-dose rate (HDR) >12 Gy h -1 . • The LQ formalism and EQDX allow comparison of the predicted effects of a particular BT schedule with other BT and external beam schedules, with regard to both tumour control and normal tissue effects. This formalism can be safely applied within a range of doses per fraction from 0.5 Gy to 6-10 Gy; it might, however, potentially overestimate the effects at higher doses per fraction. • Because of historical precedents and clinical experience EQD2 referring to photon doses of 2Gy /fraction is commonly used. For protocols involving different tissues assumed values of α/β and half time of repair (T 1/2 ) are required for both radiation types and must be specified. • Changing dose rate in the MDR range causes the most pronounced changes in biological effect.