Title: BIOLOGICAL EFFICIENCY OF A THERAPEUTIC PROTON BEAM: A STUDY OF HUMAN MELANOMA CELL LINE
1BIOLOGICAL EFFICIENCY OF A THERAPEUTIC PROTON
BEAM A STUDY OF HUMAN MELANOMA CELL LINE
- I. Petrovic1, A. Ristic-Fira1, D. Todorovic1, L.
Koricanac1, - L. Valastro2 and G. Cuttone2
- 1Vinca Institute of Nuclear Sciences, Belgrade,
Serbia and Montenegro - 2Istituto Nazionale di Fisica Nucleare, LNS,
Catania, Italy
2Advantages of protons compared to conventional
radiation - targeting radiation dose precisely
into the tumour, - sparing neighboring healthy
tissue.
3- Physical qualities of protons
- small lateral scattering,
- energy loss per unit length linear energy
transfer (LET) - increases while the proton slows down,
- range directly proportional to energy,
- depth-dose distribution
- - slow increase of dose plateau region,
- - rapid build-up to a sharp maximum almost at
the - end of range the Bragg peak,
- - distal swift fall-off.
4Practical approach to deliver uniform dose over
large volume at a given depth
- Spread-out Bragg peak (SOBP) modulation of
proton energy at the price of a slight increase
of the entrance dose. - Modulation of proton energy, i.e., range, is
achieved by degrading initial proton energy which
results in superimposition of a number of
monoenergetic proton beams of closely spaced
energies, thus the position of the Bragg peak is
pooled back towards the beam source as energy is
reduced. - The Bragg peak and SOBP have a higher LET than
the beam entering the tissue.
5National Association for Proton Therapy, USA
6Goal
- Evaluation of physical and radiobiological
parameters of the CATANA (Centro di Adro Terapia
e Applicazzioni Nucleari Avanzati) proton beam
facility, used for the treatment of eye melanoma. - Assessment of parameters describing the level of
cell radio-sensitivity and efficiency of
different radiation qualities (highly ionising
radiation - protons vs. conventional radiation -
?-rays) needed to analyse and predict success of
therapeutic irradiations.
7The CATANA (Centro di Adro Terapia e
Applicazzioni Nucleari Avanzati) treatment
facility, INFN - LNS, Catania, Italy
8Irradiation conditions
- Irradiations at 6.6, 16.3, 25.0 and 26.0 mm in
Perspex (Polymethyl methacrylate - PMMA) within
the SOBP of the 62 MeV proton beam (produced by
the superconducting cyclotron at the CATANA
treatment facility, INFN, LNS Catania). - Reference dosimetry plane - parallel PTW 34045
Markus ionization chamber calibrated according to
IAEA code of practice (IAEA-TRS-398 2000). - Single doses delivered to the cells 2, 4, 8, 12
and 16 Gy, at dose rate of 15 Gy/min. - Irradiations with ?-rays, at the same dose
levels, were performed using 60Co source at the
Vinca Institute of Nuclear Sciences in Belgrade,
at average dose rate of 1 Gy/min. - All cell irradiations were carried out in air at
room temperature.
9Bragg peak
- Figure 1. Depth dose distribution of the Bragg
peak in Perspex of the 62 MeV proton beam
produced at the CATANA treatment facility in the
INFN-LNS, Catania.
10SOBP
- Figure 2. Depth dose distribution of the
spread-out Bragg peak in Perspex of the 62 MeV
proton beam produced at the CATANA treatment
facility in the INFN-LNS, Catania. Arrows
correspond to irradiation positions at 6.6 mm
(A), 16.3 mm (B), 25 mm (C) and 26 mm (D).
11Table 1. Irradiation position parameters in SOBP
for HTB140 cells
Irradiation Depth in
Dose E position
Perspex (mm) ()
(MeV)
A 6.6
87.242.61 50.904.33
B 16.3
99.420.58
34.882.15 C
25.0 102.213.43
11.741.23 D
26.0
32.124.27 5.991.36
mean energy
12Cell culture conditions
- Irradiation of exponentially growing HTB140 human
melanoma cells. - Plating efficiency (PE) for HTB140 cells -
approximately 60 70 . - Doubling time (Td) for HTB140 cells - 242,7 h.
Biological assays
- Cell viability
- - clonogenic assay (CA),
- - microtetrasolium (MTT) assay,
- - sulforhodamine B (SRB) assay.
13Assessment of radiobiological effects of protons
- Surviving fraction as a function of dose and/or
depth. - Surviving fraction at 2 Gy (SF2) level of
radio-sensitivity. - Relative biological effectiveness (RBE)
inactivation capacity of irradiated cells - RBE(2Gy, ?) - ratio of 2 Gy ?ray dose and the
proton dose generating the same inactivation
level as that of ?-rays, - strongly depends on LET, reaching its maximum
value at 100 keV/µm, corresponding to proton
energy of 65 keV.
High-LET radiations, i.e., protons and heavy
ions, have more efficient biological
effectiveness than low-LET radiations, such as
X-rays or ?-rays.
14CA
Figure 3. Dose dependent surviving fractions,
estimated by clonogenic assay, of HTB140 melanoma
cells irradiated with ?-rays and protons.
Irradiation position within the proton spread-out
Bragg peak correspond to 6.6 mm (A), 16.3 mm (B),
25 mm (C) and 26 mm (D) depth in Perspex.
15CA
SRB
MTT
Figure 4.
16Table 2. SF2 values at different depths in SOBP
for HTB140 cells
Irradiation CA?
MTT?? SRB??? position
?-rays single 0.8740.074
0.8750.078 0.8480.054 protons A (6.6
mm) 0.8250.061 0.7740.033
0.8060.018 B (16.3 mm)
0.7480.103 0.6910.015
0.7220.022 C (25.0 mm)
0.5620.036 0.5200.015
0.5840.081 D (26.0 mm)
0.5780.064 0.5320.036
0.5960.011
? clonogenic assay, ?? microtetrasolium assay,
??? sulforhodamine B assay.
17CA
Figure 5. Surviving fractions of HTB140 melanoma
cells irradiated at 2, 4, 8, 12 and 16 Gy as a
function of depth, estimated by clonogenic assay.
Irradiation position within the proton spread-out
Bragg peak correspond to 6.6 mm (A), 16.3 mm (B),
25 mm (C) and 26 mm (D) depth in Perspex.
18CA
MTT
SRB
Figure 6.
19Table 3. RBE(2Gy, ?) values at different depths
in SOBP for HTB140 cells
Irradiation CA?
MTT?? SRB??? position
A (6.6 mm) 1.390.06 1.890.09
1.320.07 B (16.3 mm)
2.140.11 2.760.18
2.020.14 C (25.0 mm) 4.630.23
5.280.31 3.320.21
D (26.0 mm) 4.260.28
4.890.37 3.080.27
? clonogenic assay, ?? microtetrasolium assay,
??? sulforhodamine B assay.
20Conclusions
- Surviving fractions at 2 Gy (SF2), throughout the
whole spread out Bragg peak (SOBP), indicated
high level of radio-resistance of HTB140 cells. - For the dose range comprising small and
therapeutic doses, relatively high level of
survival was revealed, moderately decreasing when
changing irradiation position from the proximal
to the distal end of SOBP. - A rather important rate of the fall of survival
at small doses (2 and 4 Gy) was significantly
reduced for therapeutic doses (8 to 16 Gy),
implying even relatively higher level of
radio-resistance than pointed out by SF2.
21Conclusions
- RBE values indicated significant level of proton
induced cell inactivation, even though it was
shown that HTB140 cells are among the most
radio-resistant cells. - RBE values considerably increased when
approaching the distal end of SOBP. - At the distal declining edge of SOBP, where the
dose intensity was less than half of the full
dose intensity of SOBP, the killing ability of
protons was close to that observed at the distal
end of SOBP.