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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part I: The Basics of Brachytherapy
Version 1 - 22/10/2015
Radiobiology of LDR, HDR, PDR and VLDR Brachytherapy
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Fig 5.4: The Four R’s of Radiobiology : Repair, Redistribution, Reoxygenation , Repopulation in
function of Dose Rate or Treatment Time to deliver 2 Gy (after Steel 1986).According to ICRU
LDR ranges from 0.6 to 3.3 cGy / minute, HDR starts from dose rates over 20 cGy / min. (ICRU
1985)
Fig 5.5. Cell survival according to dose rate. Increasing the dose rate leads to more cell kill for
the same Total Dose. Line A corresponds to a dose rate level where no repair of sublethal dam-
age is done. Line B corresponds to a dose rate so low that not only repair but also repopulation
is possible. The clinical zones of LDR (40- 200 cGy/h) and HDR (> 12 Gy /h) are indicated
Fig 5.6 : Increasing biological effectiveness with increasing dose rate. Modified from Hall 1964
HeLa cell survival in function of dose rate (Hall). The arrows indicate the separation between
the LDR, MDR and HDR dose rates according to the ICRU 38 definitions (ICRU 1985)
Fig 5.7 : Variation of radiobiological effect in function of changes in dose rate. The highest dose
rate effect variation is noticed in the MDR range (Van Limbergen 1987)
Fig 5.8: The effects of sublethal damage, progression in cell cycle, and repopulation on survival
rate, according to dose rate.
the less time is left for recovery, leading to higher tissue effects
at higher dose rates (as demonstrated for an in-vitro situation)
up to a dose rate where no recovery is possible at all (line A in
Fig 5.5). Further increases in dose rates lead to overkill and no
further effects. Lowering the dose rate allows for a progressive
increase in recovery, down to a dose rate where maximum recov-
ery takes place (line B in Fig 5.5).
The differences in biological effectiveness according to dose rate
effects are also illustrated in an RBE/dose rate plot (Fig 5.6).
This dose rate effect leads to a substantial increase in cell kill by
a factor of 2- 4when comparing the effect of LDR to HDR. The
variation of biological effect with changing dose rate is smaller
(with a slower slope) in the LDR and HDR ranges, and signifi-
cantly larger (with a steeper slope) in the MDR range (Fig 5.6).
Altering dose rate will hence result in a larger dose rate effect in
this MDR region in and around a radioactive source (Van Lim-
bergen 1987) (Fig 5.7).
Repopulation
(see above) is the slowest process.
The relative importance of recovery, repopulation, and reoxy-
genation is dependent on the dose rate. Repopulation is possible
during low dose rate irradiation or in protracted schedules in
combination with external beam irradiation, while recovery has
a dominant effect in all the dose rate ranges used in BT. In case of
very low dose rate, repopulation of clonogenic tumour cells may
play an important role. (Fig 5.8)