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212 Microsphere deposition, dosimetry, radiobiology at the cell-scale
1
bed
NTCP
1
KF
d)
i
ii
()

toxicity observed in EBRT and that observed in liver radioembolization. e two modalities are characterized by a dierent dose rate: instanta­neous 1.5–2 Gy doses spaced in time (>8 hours) in EBRT and exponentially decreasing dose rate in liver radioembolization (half-life = 2.7 days for 90Y). e biological eective dose (BED) con­cept based on a linear-quadratic model (LQM) has been introduced to account for the dose rate (Fowler, 1989). is concept succeeded in unifying the renal toxicity observed in EBRT and in peptide receptor radionuclide therapy (PRRT) (Barone et al., 2005; Wessels et al., 2008) and is now also considered in liver radioembolization (Cremonesi et al., 2008; Strigari et al., 2010). Regarding the irradiation itself, both the photons used in EBRT and the β-particles used in liver radioemboliza­tion are low linear energy transfer (LET) particles and thus share the same radiobiology eectiveness (RBE) (ICRP, 2007).
Partial liver irradiation in EBRT (Dawson et al.
2001) showed that NTCP = 0.5 is obtained by kill­ing 40% of the liver volume or by irradiating 100%, 80%, or 66% of the liver volume with a BED of 77, 95, or 115 Gy, respectively. ese results can be described by the following the sigmoid curve for the lobule nonrecovery probability (Walrand et al. 2014b):
e liver NTCP as a function of the killed lobule
fraction (KF) is (Dawson et al. 2001)
NR bed
1
KF
93.8
0.4
1
(9.9)
2.12
8.29
(9.10)
Equation 9.2 with the glass microsphere distribu­tion obtained using the microsphere transport MC simulations corresponding to dierent BED deliv­ered to the liver parenchyma. e best agreement with studies in glass and resin microsphere radio­embolization (Chiesa et al., 2015; Strigari et al., 2010, respectively) was obtained using the asym­metric probability of 69%–31% for the microsphere bifurcation partition (Figure 9.11). is value is not far from the 64% to 36% value giving the best agree­ment with the cluster size distribution observed (Högberg, 2015). Note that a misunderstanding occurred in Walrand et al. (2014b) where for the MC simulations the median toxic dose (TD50) was given in the targeted liver region as done in EBRT, while clinical TD50 data observed in radio­embolization were reported averaged on the whole liver (Chiesa et al., 2015; Strigari et al., 2010). is explains why 60%–40% was the previous optimal microsphere partition probability.
An important benet of the MC simulation is the prediction of the hepatic toxicity as a func­tion of various parameters. Figure 9.12 shows the WLTD50, i.e., the dose averaged over the whole liver giving NTCP = 0.5, as a function of the targeted liver fraction and of the microsphere­specic activity (msA). For resin microspheres, the WLTD50 is almost constant for a targeted liver fraction larger than 65%, reducing the drawback of mixing whole and right liver radioembolizations in the same NTCP reporting (Strigari et al., 2010). Note that Equations 9.9 and 9.10 derived from EBRT involve that the WLTD50 become innite when the targeted liver fraction is lower than 40%.
where KF can be computed using
ν
where ν(BEDi) is the fraction of lobules receiv­ing a dose BEDi. Note that in EBRT, Equation 9.11 reduces to
where Vf is the irradiated liver fraction.
Walrand et al. (2014b) computed the ν(BEDi)
fractions by convolving the Russell dose kernel
=KF (bed)NR(be
(9.11)
(9.12)
9.13 APPLICATION OF THE HEPATIC TOXICITY MODEL
For convenient use, the WLTD(p,Vf,msA), i.e., the whole-liver dose providing a NTCP = p when tar­geting a liver volume fraction Vf using 90Y loaded microspheres of specic activity msA, is tted by
pF
0.869(msA)
=
(9.13)
Vf
F
WLTD ,,msA 47.1Gy
×
where the dimensionless scale factor F(msA) is
pVf
+
10.457  (msA)
()

Vf p
Kf()
()
1.2
(a)
SPECT–based VOI drawing method
NTCP
200
0
0.0 0.2 0.4 0.6 0.8 1.0
50 100
Liver BED (Gy)
(b)
150200
03555
Liver D (Gy)
Liver NTCP
72 87
MC simulation
Model 95% CI Exp
0.8
0.6
9.13 Application of the hepatic toxicity model 213
1
0.4
0.2
0
–0.2
20 40 60 80 100
0
120 140 160 180
D [Gy]
MC simulation
NTCP (D) FIT NTCP (D)
95% CI
Figure 9.11 NCTP comparison between clinical observations and prediction (squares) from Equations
9.9 and 9.10 using 69%–31% as the microsphere partition probability in the MC calculation of ν(BEDi). (a) Glass microspheres in right liver radioembolizations (Vf 0.66) (With kind permission from Springer Science + Business Media: Eur J Nucl Med Mol Imaging, Radioembolization of hepatocarci­noma with (90)Y glass microspheres: Development of an individualized treatment planning strategy based on dosimetry and radiobiology, 42, 2015, 1718–1738, Chiesa et al.). (b) Resin microspheres in mixed whole and right liver radioembolizations (Vf 0.80). (Reprinted from Strigari et al., J Nucl Med, 51, 1377–1385, 2010. With permission of the Nuclear Medicine Society). Squares were added by the author. Doses are averaged over the whole liver parenchyma.
214 Microsphere deposition, dosimetry, radiobiology at the cell-scale
msA/0.0471kBq
3
F
()
()
=−
1
p
p
200
(a)
WLTD50 [Gy]
q: Strigari et al. 2010
120
(b)
WLTD50 [Gy]
Vf = 0.66
2.50
180
160
140
120
100
80
ms A
2.5 kBq
0.94 kBq
0.45 kBq
0.15 kBq
0.94 kBq: Chiesa et al. 2015
60
40
110
100
90
80
70
60
50
40
0.5
0.00
0.05 kBq
0.6 0.7 Targeted liver fraction
0.50 1.00 Microsphere specific activity (kBq)
0.05 kB
0.8 0.9 1
Vf = 0.8
Vf = 1.0
1.50 2.00
Figure 9.12 Circles: WLTD50 (i.e., dose averaged on the whole liver parenchyma giving NTCP = 0.5) derived from Equations 9.9 and 9.10 using a 69%–31% microsphere partition probability in the MC simulation of ν(BEDi). Straight lines: t with Equations 9.13 and 9.14. (a) WLTD50 as a function of the targeted liver fraction for different microsphere-specic activities (msA). (b) WLTD50 as a function of the msA for different targeted liver volume fractions (Vf).
msA1e
(9.14)
And Kf(p) is the inverse of the relation
(Equation 9.10), i.e.,
Note that, for a xed total radioembolization
0.4
p
=
activity, the cube root in F(msA) is a quantity strongly proportional to the mean intermicro­sphere distance in the liver.
()
8.29
(9.15)
References 215
MC
MC
)
fit
MC
MC
)
fit
limWLTD, ,msA 0
pVf
()
=
Kf(0.5) = 0.4 and Kf(0.05) = 0.28, which means
that ablating by irradiation 40% and 28% of the liver induces a NTCP of 50% and 5%, respectively, as observed in EBRT (Dawson et al., 2001).
In the tted domain {msA  (0.05,0.15,
0.45,0.94,2.5) kBq, p  (0.15,0.30,0.50), Vf  (0.55,0.66,0.80,1.00)} the mean absolute relative deviation of the t versus MC simulations (i.e.,
was 2.9% with extreme relative devia-
tions (i.e.,
of –4.97% and 4.60%. It is
quite remarkable that such a simple expression of F accurately takes into account the impact of mSA.
As the microspheres are localized in the por-
tal triads, the WLTD(p,Vf,msA) value when msA vanishes does not approach that of EBRT. Indeed, TD50 = 43 Gy is the dose of 90Y corresponding to BED50 = 77 Gy observed in whole-liver EBRT. e simulation in Table 9.1 shows that, in the case of trans- lation invariant microsphere distribution (which is expected when the microsphere number increases), 43 Gy in the triad arteries corresponds to a liver paren­chyma dose of 40 × 43/118 = 15 Gy, i.e., BED = 19 Gy, fourfold smaller than the 77 Gy observed in EBRT.
As there is no clinical study available in order to validate the MC simulation for microsphere activ­ity below 0.05 kBq, a conservative expression of F(msA) was chosen such as
(9.16)
Equations 9.13 and 9.14 could be used to compute the needed absorbed dose and also the activity in order to achieve a desired NTCP = p as a function of the targeted liver fraction volume and mSA. Note that this set of equations was obtained by tuning one parameter of the model (the microsphere parti­tion probability at the arterial tree nodes) in order to t the clinical hepatic toxicity observed in glass (Chiesa et al., 2015) and in resin (Strigari et al., 2010) radioembolization. us, the prediction accuracy of Equations 9.13 and 9.14 does not only depend on the goodness of the model but also on the goodness of the observed clinical hepatic toxicity.
9.14 CONCLUSIONS
MC simulations, using a very simple model of the hepatic arterial tree and microsphere transport,
reconcile the liver toxicities observed in EBRT and in radioembolization using glass and resin microspheres. More interestingly, these MC sim­ulations can be tted with an analytical formula (Equations 9.13 and 9.14) that allows performing of individualized radioembolization planning as a function of the liver fraction targeted and of the mSA used.
is model contains an adjustable param­eter, i.e., the microsphere partition probability at the arterial nodes, which was tted in order to reproduce the liver toxicities observed in clinical radioembolization studies (Strigari et al., 2010; Chiesa et al., 2015). However, the obtained parti­tion probability 69%–31%: (1) is within the range of probabilities predicted by microsphere transport dynamic simulations (Kennedy et al., 2010), (2) is close to the value 64%–36% tting the observed microsphere cluster distribution (Högberg et al.,
2015), (3) ts both the hepatic toxicity reported in glass and resin microsphere radioembolization, and (4) unies hepatic toxicity observed in radio­embolization and in EBRT. ese four facts sup­port the coherence of the whole model and also of the reported clinical hepatic toxicity as well.
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PART 4
Following Patients Treated withRadioembolization
10 Postradioembolization imaging using bremsstrahlung 90Y SPECT/CT 221
S. Cheenu Kappadath
11 Quantitative postradioembolization imaging using PET/CT 229
Marco D’arienzo, Luca Filippi, and Oreste Bagni
12 Image-based three-dimensional dosimetry following radioembolization 251
Alexander S. Pasciak and S. Cheenu Kappadath
13 Diagnostic reporting using postradioembolization imaging 263
Yung Hsiang Kao
14 The use of postprocedural imaging in the medical management of patients 281
Austin C. Bourgeois, Marcelo S. Guimaraes, Yong C. Bradley, Christopher Hannegan, and Alexander S. Pasciak
10
Postradioembolization imaging using
90
bremsstrahlung
S. CHEENU KAPPADATH
Y SPECT/CT
10.1 Background and rationale 221
10.2 Challenges associated with
90
YSPECTimaging 222
10.3 Approaches to imaging 222
90
Y bremsstrahlung
10.1 BACKGROUND AND RATIONALE
90
Y radioembolization is used in the management of patients with unresectable primary and metastatic liver cancers. gated albumin ( used, at least in terms of patient safety, as a surrogate radiopharmaceutical for treatment planning since the inception of the therapy, as described in Chapter
4. Planar scintigraphy of
primarily to evaluate the lung shunt fraction and estimate the mean absorbed dose to lung aer the radioembolization treatment (Ho et al., 1997). MAA uptake in the lung consequently aects the prescrip­tion of administered vent radiation pneumonitis. MAA distribution in vivo is also used to assess extrahepatic distribution and judge the adequateness of tumor perfusion from the catheter placement. It has been demonstrated that the assessment of MAA distribution with single­photon emission computed tomography/CT (SPECT/ CT) is superior compared with SPECT, which in turn is superior to planar imaging (Ahmadzadehfar et al.,
99m
Technetium-labeled macroaggre-
99m
Tc MAA) has been successfully
99m
Tc MAA has been used
90
Y microsphere activity to pre-
90
Y
10.3.1 Planar imaging 222
10.3.2 SPECT and SPECT/CT imaging 223
10.4 Summary and conclusions 225 References 226
2010). Overall accuracies of 72%, 79%, and 96% for planar, SPECT, and SPECT/CT, respectively, have been reported (Hamami et al., 2009).
ere are a number of qualitative studies that have suggested that MAA distributions observed during planning oen match the distributions aer therapy. Concordance between MAA and that increases in condence when selective segmen­tal or lobar therapies are planned (Kao et al., 2013).
However, as reviewed in Chapter 4, several studies have also shown that the distribution of MAA dur­ing treatment planning may not be a consistent and reliable indicator of the distribution of the spheres aer the administration of treatment (Ilhan et al., 2015). Dierences of greater than 20% uptake between 43% (97/225) of cases (Wondergem et al., 2013).
Furthermore, especially in the past decade, in vivo used in conjunction with dosimetry models such as the medical internal radiation dose (MIRD) (Gulec et al., 2006) or partition (Ho et al., 1996) models to calculate the tumor and normal liver doses (see, e.g., Chiesa et al., 2015; Garin et al., 2013).
90
Y has been reported (Chiesa et al., 2015)
99m
Tc MAA and 90Y have been reported in
99m
Tc MAA distributions have begun to be
90
Y microsphere
90
Y micro-
221