chapter 6 analysis of high dose rate brachytherapy...
TRANSCRIPT
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CHAPTER 6
ANALYSIS OF HIGH DOSE RATE BRACHYTHERAPY
DOSE DISTRIBUTION RESEMBLANCE IN CYBERKNIFE
HYPOFRACTIONATED TREATMENT PLANS OF
LOCALIZED PROSTATE CANCER
6.1 INTRODUCTION
6.1.1 Hypofractionated Radiotherapy for the Cancer of Prostate
Generally the tumour cells have higher �/� than the normal cells.
However cancerous prostate cells are exception for this. The �/� value of
prostate is between 1.5Gy and 3Gy (Fowler 2001, Fowler 2006, Brenner and
Hall 1999, Carlson et al 2004). This is lower than the �/� value of
surrounding normal cells of the organs at risk (OAR), the bladder and the
rectum. The �/� value of rectum for late rectal toxicity is between 3Gy and
5Gy (Marzi et al 2009, Van der Kogel et al 1988, Brenner et al 1998).
Similarly, the �/� of bladder is estimated as 7Gy (Marzi et al 2009). As this
low �/� value of the prostatic cancer cells encourages the therapeutic gain
factor in hypo fractionated radiotherapy; hypo fractionation has become a
vital fractionation scheme for the treatment of localized prostate cancer.
Hypofractionation in prostate has been implemented in several ways.
Hypofractionated High Dose Rate (HDR) brachytherapy, Hypofractionated
stereotactic body radiotherapy (SBRT), Hypofractionated intensity modulated
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radiotherapy (HIMRT) are the few forms of prostate hypofractionation
treatment.
6.1.2 Brachytherapy for Prostate Cancer
Prostate brachytherapy has been performed either as Low Dose
Rate (LDR) permanent implants with Iodine -125/ Palladium-103 seeds
(Cosset and Haie-Meder 2005) or as HDR temporary implants (Yoshioka et al
2006, Murali et al 2010, Martinez et al 2010, Grills et al 2004, Demanes et al
2011, Fröhlich et al 2007) with Ir-192 sources. The fractionation regimen
followed in HDR brachytherapy is an accelerated hypofractionation. Due to
the therapeutic gain advantage hypofractionation has been implemented in
IMRT as well (Vesprini et al 2011). The fractionation regimens followed in
HDR brachytherapy are also not unique. Murali et al (2010) follow the
protocol of 30Gy in 3 fractions (10Gy per fraction) for HDR brachytherapy of
localized prostate while Martinez et al (2010) and Grills et al (2004) followed
38Gy in 4 fractions (9.5Gy per fraction). Demanes et al (2011) followed two
biologically equivalent fractionation regimens 42Gy in 6 fractions and 38Gy
in 4 fractions.
6.1.3 Hypofractionated CyberKnife Radiosurgery for Prostate
Cancer
CyberKnife robotic radiosurgery treatments are performed in
hypofractionation alone. Prostate hypofractionation performed in CyberKnife
is a SBRT technique. Though the hypofractionation is adopted in several
modes the fractionation regimen followed are not unique. The
hypofractionation regimen for CyberKnife is still under research. CyberKnife
clinical trials of localized prostate cancer by King et al (2003) show a positive
outcome for a fractionation regimen of 36.25Gy in 5 fractions (7.25Gy per
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fraction). Clinical trials by Katz (2010) show that the 35Gy in 5 fractions
(7Gy per fraction) regimen is resulting better results than the 36.25Gy in 5
fractions regimen.
6.1.4 CyberKnife Radiosurgery Vs. HDR Brachytherapy in Prostate
Cancer
The HDR brachytherapy hypofractionation has been accepted as a
successful monotherapy mode for localized prostate cancer (Martinez et al
2010, Grills et al 2004, and Demanes et al 2011). It is imperative to assess
the degree of resemblance of a HDR brachytherapy dose distribution with a
CyberKnife hypofractionated dose distribution as the CyberKnife stereotactic
radiosurgery for prostate is in a forward progression in the modern day
radiotherapy. Fuller et al (2008) made a trial to create HDR like distribution
in CyberKnife treatment plan. However the HDR equivalent dose
distributions in CyberKnife treatment plans of prostate cancer are not yet
studied in depth. The present study intends to analyze the CyberKnife
hypofractionated dose distribution in terms of the accelerated dose regimens
of HDR brachytherapy to know the degree of resemblance of CyberKnife
dose distribution with the HDR brachytherapy of localized prostate cancer.
For this comparison purpose the HDR fractionation regimen followed by
Murali et al (2010) was taken as the reference.
6.2 MATERIALS AND METHODS
6.2.1 Treatment Planning and Plan analysis in CyberKnife
Radiosurgery
Thirty four localized prostate cancer patients who had undergone
CyberKnife radiosurgery were considered for this retrospective study. The
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contouring was done in the dedicated CyberKnife Multiplan treatment
planning system. Computed tomography (CT) images of 1mm slice thickness
of the pelvic region were acquired and fused with magnetic resonance (MR)
images to draw the Planning Target Volume (PTV) of the prostate target and
the OARs (rectum and bladder). The treatment plans were generated with
appropriate goals and evaluated. The prescription dose was 36.25Gy in 5
Fractions (7.25Gy per fraction).
The volume dose received by the entire target V100% was also taken
for the analysis. The target doses were evaluated for conformity and
homogeneity through conformity index and homogeneity index.
Conformity index was calculated using the formula,
Conformity index (CI) = (VRI/TVRI) x (TV/TVRI),
(Paddick 2000, Nakamura et al 2001)
Where VRI is the overall volume including the PTV, receiving the prescription
isodose,
TVRI is the volume of the target alone which receives the
prescription isodose,
Volume of the PTV is taken as TV.
Similarly the homogeneity index was calculated using the formula,
Homogeneity index (HI) = Dmax/DPI, (Shaw et al 1993)
Where Dmax is the dose maximum within the PTV and DPI is the prescription
or reference isodose value.
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For the OARs rectum and bladder the maximum level of doses
received by 2%, 5% and 10% volumes of the respective OAR were analyzed
in terms of D2%, D5% and D10%.
Similarly the dose volumes V100%, V90%, V50% and V30% of the
OARs were also analyzed.
6.2.2 HDR Fractionation Equivalent Dose of the CyberKnife
Hypofractionated Dose
The HDR fractionation equivalent doses were calculated from the
formula derived from the Linear Quadratic (LQ) model by equating the
biological effective doses (BED) of two fractionation regimen (Fowler 2005).
BEDCK = DCK [1 + dCK/ (�/�)]
The two fractionation regimens can be equated in the following
way.
DHDR [1 + dHDR/ (�/�)] =DCK [1 + dCK/ (�/�)]
Where DHDR is the HDR fractionation equivalent dose, DCK is the CyberKnife
hypofractionated dose, dCK is the dose per fraction in the CyberKnife dose
regimen, dHDR is the fractional dose in HDR equivalent dose regimen, and �/�
is the tissue/tumor specific ratio.
Now, DHDR = BEDCK / [1 + dHDR/ (�/�)]
But DHDR = ndHDR, Hence, ndHDR= BEDCK / [1 + dHDR/ (�/�)]
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Solution for this equation can be obtained as,
dHDR = [- �/� ± � (�/�) 2
-4*((-�/� x BEDCK)/n)]/2
The positive root of this solution is taken in this study.
In the present study �/� value of 1.5Gy was taken to estimate the
HDR equivalent doses of the prostate target. The HDR fractionation
equivalent doses were estimated for a fractionation regimen of 3 fractions. For
the OARs rectum and bladder �/� value of 4Gy was considered for estimating
the HDR equivalent dose in this study.
6.3 RESULTS
6.3.1 CyberKnife Hypofractionated Dose Distribution
The CyberKnife hypofractionated dose distributions within the
target and in the OARs were analysed and the dose values were estimated
using the dose volume histograms (DVH). The mean target volume doses
corresponding to different volumes are shown in Figure 6.1. D98% and D5% are
the indicators of the minimum and maximum dose within the target,
respectively. The mean value of D98% and D5% were 35.12± 0.85 Gy, 41.48 ±
1.75 Gy respectively.
The target conformity, homogeneity indices and V100% are shown in
Table 6.1.The conformity index was ranging between 1.15 and 1.73 with the
mean value of 1.29. Similarly the homogeneity index was ranging between
1.05 and 1.28 with the mean value of 1.17. The mean value of V100% was
95.29%. The V100%, V90%, V50% and V30% volumes of the rectum and the
bladder are also tabulated in Table 6.1.
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6.3.2 The Dose Conversion to HDR Equivalents
The percentage doses and the BED values corresponding to the
volume doses of the PTV are shown in Table 6.2 and Table 6.3 respectively.
The HDR fractionation (3fractions) equivalent doses corresponding to the
CyberKnife hypofractionation were estimated. The HDR fractionation
equivalent target doses D98%, D90%, D80%, D50%, D10% and D5% are shown in
Table 6.4.
The dose covering almost the entire target was estimated in this
study by the doses D98% and D95%. The mean HDR equivalent dose value
D98%was 27.80 ± 0.66 Gy.
Figure 6.1 The graph between the mean dose and the corresponding
percentage volume
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Table 6.1 The summary of dose distribution around the PTV, Rectum
and the Bladder
Structure Index Mean Min Max
PTV
Conformity Index 1.29 1.15 1.73
Homogeneity Index 1.17 1.05 1.28
V 100% in percentage 95.29 84.89 99.15
Rectum
Volume of the Rectum in cc 79.28 23.36 140.27
Dose volumes in cc
V100% 0.09 0 0.49
V90% 1.89 0.03 10.83
V50% 17.29 2.84 53.79
V30% 33.07 7.91 79.73
Bladder
Volume of the Bladder in cc 145.79 47.71 523.26
Dose volumes in cc
V100% 0.75 0.01 3.34
V90% 5.18 0.34 29.11
V50% 36.00 6.78 134.58
V30% 72.50 31.22 215.47
Figure 6.2 A sample dose distribution in Cyber Knife
Hypofractionated radiosurgery for prostate cancer.
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Table 6.2 The relative percentage doses of the PTV with respect to the
prescription dose of 36.25Gy
Study
Number
PTV hypofractionated dose in percentage
D98% D95% D90% D80% D50% D10% D5%
1 96.39 101.21 104.83 108.44 113.24 116.86 118.07
2 98.90 100.00 101.10 102.23 104.44 108.88 110.01
3 95.01 101.24 104.99 108.74 115.01 120.00 121.24
4 98.92 100.00 100.00 101.08 103.26 106.51 107.61
5 100.00 101.08 101.08 102.15 103.23 105.38 106.46
6 97.82 100.00 101.08 102.18 104.36 106.51 107.61
7 103.53 105.88 109.41 111.75 114.12 115.28 116.47
8 98.87 101.13 102.26 104.55 106.81 110.23 111.37
9 98.90 101.10 102.23 104.44 106.68 108.88 110.01
10 96.47 101.19 103.53 105.88 109.41 112.94 114.12
11 92.22 95.56 97.77 101.10 104.44 107.78 108.88
12 98.90 101.10 103.34 104.44 106.68 108.88 110.01
13 96.25 101.24 104.99 108.74 117.49 121.24 122.51
14 94.87 101.27 105.13 110.26 119.23 124.28 125.63
15 93.74 101.24 103.75 108.74 116.25 121.24 122.51
16 96.66 98.90 101.10 103.34 105.54 108.88 110.01
17 97.66 100.00 102.34 104.72 110.59 114.12 115.28
18 97.49 101.24 104.99 108.74 115.01 118.76 120.00
19 99.45 100.63 104.30 106.73 112.17 115.17 116.41
20 96.66 98.90 101.10 102.23 104.44 106.68 107.78
21 94.04 100.00 103.59 107.14 111.92 115.48 116.66
22 94.65 99.45 102.84 105.41 108.44 111.23 112.39
23 97.66 101.19 103.53 107.06 110.57 112.94 114.12
24 97.66 101.13 103.53 105.88 109.41 112.94 114.18
25 96.50 98.81 101.19 104.72 108.25 112.94 114.15
26 96.47 100.00 103.53 105.88 109.41 114.12 115.28
27 96.44 100.00 102.37 105.96 110.70 114.29 115.48
28 91.72 95.34 98.98 102.59 109.82 115.86 117.08
29 96.55 100.00 102.29 105.74 108.06 111.50 112.63
30 95.28 100.00 103.53 107.06 110.59 112.88 114.12
31 98.10 103.12 106.15 109.60 110.23 110.43 111.67
32 97.49 103.75 107.50 111.26 116.25 121.24 122.51
33 98.84 102.29 104.61 106.90 109.19 111.50 112.63
34 94.12 97.66 101.19 105.90 111.78 114.12 115.28
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Table 6.3 The biological effective doses corresponding to the
hypofractionated PTV doses
Study
Number
PTV Biological Effective Dose
D98% D95% D90% D80% D50% D10% D5%
1 197.71 216.18 230.53 245.35 265.73 281.61 287.05
2 207.21 211.46 215.75 220.19 228.98 247.19 251.94
3 192.59 216.29 231.20 246.61 273.43 295.80 301.50
4 207.32 211.46 211.46 215.64 224.23 237.37 241.91
5 211.46 215.64 215.64 219.86 224.12 232.77 237.15
6 203.11 211.46 215.64 219.97 228.64 237.37 241.91
7 225.33 234.78 249.38 259.32 269.57 274.64 279.89
8 207.11 215.85 220.29 229.42 238.62 252.87 257.67
9 207.21 215.75 220.19 228.98 238.05 247.19 251.94
10 198.02 216.07 225.33 234.78 249.38 264.42 269.57
11 182.44 194.63 202.91 215.75 228.98 242.60 247.19
12 207.21 215.75 224.56 228.98 238.05 247.19 251.94
13 197.20 216.29 231.20 246.61 284.44 301.50 307.38
14 192.08 216.39 231.76 252.98 292.28 315.65 322.06
15 187.93 216.29 226.21 246.61 278.91 301.50 307.38
16 198.75 207.21 215.75 224.56 233.44 247.19 251.94
17 202.49 211.46 220.62 230.09 254.38 269.57 274.64
18 201.86 216.29 231.20 246.61 273.43 290.16 295.80
19 209.33 213.92 228.42 238.28 261.09 274.16 279.65
20 198.75 207.21 215.75 220.19 228.98 238.05 242.60
21 189.04 211.46 225.55 239.98 260.03 275.49 280.75
22 191.27 209.33 222.59 232.88 245.35 257.08 262.04
23 202.49 216.07 225.33 239.64 254.27 264.42 269.57
24 202.49 215.85 225.33 234.78 249.38 264.42 269.81
25 198.13 206.90 216.07 230.09 244.54 264.42 269.69
26 198.02 211.46 225.33 234.78 249.38 269.57 274.64
27 197.92 211.46 220.73 235.12 254.85 270.29 275.49
28 180.66 193.81 207.53 221.60 251.12 277.20 282.59
29 198.33 211.46 220.40 234.22 243.74 258.26 263.11
30 193.61 211.46 225.33 239.64 254.38 264.18 269.57
31 204.16 223.68 235.91 250.19 252.87 253.68 258.96
32 201.86 226.21 241.46 257.20 278.91 301.50 307.38
33 207.00 220.40 229.64 238.96 248.46 258.26 263.11
34 189.34 202.49 216.07 234.90 259.44 269.57 274.64
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Table 6.4 The three fraction equivalent HDR doses of the PTV
corresponding to the hypofractionated doses of the
CyberKnife radiosurgery
Study
Number
DHDR (3Fractions)
D98% D95% D90% D80% D50% D10% D5%
1 27.66 29.02 30.04 31.05 32.40 33.42 33.76
2 28.37 28.68 28.99 29.31 29.93 31.18 31.50
3 27.27 29.03 30.08 31.14 32.90 34.30 34.65
4 28.38 28.68 28.68 28.98 29.59 30.51 30.82
5 28.68 28.98 28.98 29.28 29.59 30.19 30.49
6 28.07 28.68 28.98 29.29 29.91 30.51 30.82
7 29.67 30.33 31.33 31.98 32.65 32.98 33.31
8 28.36 29.00 29.32 29.96 30.60 31.56 31.88
9 28.37 28.99 29.31 29.93 30.56 31.18 31.50
10 27.69 29.01 29.67 30.33 31.33 32.32 32.65
11 26.49 27.43 28.05 28.99 29.93 30.87 31.18
12 28.37 28.99 29.62 29.93 30.56 31.18 31.50
13 27.62 29.03 30.08 31.14 33.60 34.65 35.01
14 27.24 29.04 30.12 31.57 34.09 35.51 35.89
15 26.92 29.03 29.73 31.14 33.25 34.65 35.01
16 27.74 28.37 28.99 29.62 30.24 31.18 31.50
17 28.02 28.68 29.34 30.01 31.66 32.65 32.98
18 27.97 29.03 30.08 31.14 32.90 33.95 34.30
19 28.52 28.86 29.89 30.57 32.10 32.95 33.30
20 27.74 28.37 28.99 29.31 29.93 30.56 30.87
21 27.00 28.68 29.69 30.69 32.03 33.03 33.37
22 27.17 28.52 29.48 30.20 31.05 31.84 32.16
23 28.02 29.01 29.67 30.67 31.65 32.32 32.65
24 28.02 29.00 29.67 30.33 31.33 32.32 32.67
25 27.69 28.35 29.01 30.01 31.00 32.32 32.66
26 27.69 28.68 29.67 30.33 31.33 32.65 32.98
27 27.68 28.68 29.35 30.36 31.69 32.70 33.03
28 26.35 27.37 28.39 29.41 31.44 33.14 33.48
29 27.71 28.68 29.32 30.29 30.94 31.91 32.23
30 27.35 28.68 29.67 30.67 31.66 32.30 32.65
31 28.14 29.56 30.41 31.38 31.56 31.61 31.96
32 27.97 29.73 30.79 31.84 33.25 34.65 35.01
33 28.35 29.32 29.98 30.62 31.26 31.91 32.23
34 27.03 28.02 29.01 30.34 31.99 32.65 32.98
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Table 6.5 The hypofractionated doses of the OARS in the CyberKnife
dose distribution
Study
Number
CyberKnife Hypofractionated dose in Gy
Rectum Bladder
D2% D5% D10% D2% D5% D10%
1 30.57 24.02 17.91 37.56 30.57 26.68
2 35.44 34.64 33.83 35.85 34.24 31.42
3 32.17 29.45 26.73 34.44 31.27 27.19
4 35.46 34.67 33.49 35.86 35.46 34.67
5 33.91 31.18 26.12 31.18 24.56 17.15
6 34.28 32.7 29.95 35.46 34.67 33.49
7 31.99 29 24.31 34.54 30.71 24.31
8 32.54 28.84 24.3 30.89 25.54 21.01
9 30.61 26.99 22.56 33.03 29.81 25.78
10 29.85 27.29 23.88 34.54 30.71 26.87
11 26.58 22.96 18.93 31.82 27.39 21.75
12 32.63 29.4 24.97 30.61 23.77 16.92
13 28.55 24.47 19.03 31.27 27.64 23.56
14 31.14 27.42 23.7 35.32 33 30.21
15 31.72 28.09 23.11 29.45 22.2 15.86
16 33.83 31.42 27.79 35.04 32.22 27.79
17 31.56 28.15 24.35 30.28 26.02 21.75
18 28.55 24.02 19.48 33.08 29.91 25.83
19 31.23 27.58 23.8 31.54 27.96 23.43
20 31.82 28.6 24.17 29.4 21.75 15.31
21 32.37 29.78 25.46 35.82 34.52 32.37
22 32.57 29.98 25.52 34.24 30.69 26.95
23 31.99 29.43 25.16 32.41 30.28 27.72
24 32.09 30.28 26.44 31.99 27.29 22.6
25 31.13 26.87 20.9 24.74 18.77 14.93
26 30.28 25.59 17.91 28.15 19.19 13.22
27 34.09 31.94 28.48 34.09 32.37 29.78
28 33.25 30.63 27.13 34.56 33.25 31.06
29 30.42 25 18.33 32.5 28.75 23.75
30 31.33 27.72 27.31 34.97 32.84 31.13
31 31.81 28.07 26.54 34.18 31.03 28.12
32 29.91 24.92 19.94 34.44 31.72 28.39
33 31.67 29.18 26.26 37.5 36.25 34.58
34 27.73 21.75 16.21 35.82 34.12 31.13
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Table 6.6 The three fraction equivalent HDR doses of the OARs
corresponding to the Hypofractionated doses of the
CyberKnife radiosurgery
Study
Number
DHDR in for �/�=4Gy
Rectum Bladder
D2% D5% D10% D2% D5% D10%
1 27.92 22.03 16.53 34.19 27.92 24.42
2 32.29 31.57 30.84 32.66 31.21 28.68
3 29.35 26.91 24.47 31.39 28.55 24.88
4 32.31 31.60 30.54 32.67 32.31 31.60
5 30.92 28.47 23.92 28.47 22.52 15.84
6 31.25 29.83 27.36 32.31 31.60 30.54
7 29.19 26.51 22.29 31.48 28.04 22.29
8 29.69 26.36 22.28 28.21 23.40 19.32
9 27.95 24.70 20.72 30.13 27.24 23.61
10 27.27 24.97 21.91 31.48 28.04 24.59
11 24.33 21.08 17.45 29.04 25.06 19.99
12 29.77 26.87 22.89 27.95 21.81 15.63
13 26.10 22.44 17.54 28.55 25.29 21.62
14 28.43 25.09 21.74 32.18 30.10 27.60
15 28.95 25.69 21.21 26.91 20.39 14.68
16 30.84 28.68 25.42 31.93 29.40 25.42
17 28.81 25.74 22.33 27.66 23.83 19.99
18 26.10 22.03 17.94 30.17 27.33 23.66
19 28.51 25.23 21.83 28.79 25.57 21.50
20 29.04 26.15 22.17 26.87 19.99 14.18
21 29.53 27.21 23.33 32.63 31.46 29.53
22 29.71 27.39 23.38 31.21 28.03 24.67
23 29.19 26.89 23.06 29.57 27.66 25.36
24 29.28 27.66 24.21 29.19 24.97 20.75
25 28.42 24.59 19.22 22.68 17.30 13.84
26 27.66 23.44 16.53 25.74 17.68 12.29
27 31.08 29.15 26.04 31.08 29.53 27.21
28 30.32 27.97 24.83 31.50 30.32 28.36
29 27.78 22.91 16.91 29.65 26.28 21.79
30 28.60 25.36 24.99 31.87 29.96 28.42
31 29.03 25.67 24.30 31.16 28.33 25.72
32 27.33 22.84 18.36 31.39 28.95 25.96
33 28.91 26.67 24.05 34.14 33.02 31.52
34 25.37 19.99 14.99 32.63 31.10 28.42
122�
The doses to 2%, 5% and 10% volumes of the rectum and the
bladder are given in Table 6.5. The HDR equivalent doses corresponding to
the Cyberknife hypofractionated doses of the rectum and the bladder are
shown in Table 6.6. The mean HDR equivalent dose of D95% was 28.81 ±
0.65Gy. The mean HDR equivalent of D5% was 32.73 ± 1.36 Gy.The
CyberKnife doses and the HDR fractionation equivalent CyberKnife doses for
the rectum and the bladder are given in Tables 6.5 and 6.6, respectively. The
mean HDR equivalent doses of D2% of the rectum was 28.86 ± 1.79Gy.
Similarly, the HDR equivalent doses for D2% of the bladder were having a
mean value of 30.22 ± 2.49Gy. CyberKnife Hypofractionated dose
distribution in an example of a prostate case is shown in Figure 6.2.
6.4 DISCUSSION
6.4.1 Target Dose Analysis
Murali et al (2010) have compared the dose distribution between
IMRT (conventional 2Gy fractionation) and HDR brachytherapy (30Gy in 3
fractions). According to that study the D98% of the PTV is 91.4%. The mean
D98% in the present study is 96.89%. It shows a better coverage in CyberKnife
plans at 98% coverage level. The HDR doses D90%, D80%, D50%, D10% and D5%of
the PTV reported by Murali et al (2010) are 102.4%, 110.7%, 130.6 % ,
225.5% and 306.7% respectively. These volume doses in our present study
are 103.1%, 105.9%, 109.9%, 113.2% and 114.4% respectively. These
comparisons show that the HDR dose distributions are comparable with the
HDR equivalent dose distributions of the CyberKnife hypofractionated doses
up to 80% of the target volumes. Studies by Fröhlich et al (2007) have
reported the HDR dose D90% as 107%. However the CyberKnife target dose at
90% coverage level is very much comparable with the HDR dose quoted by
Murali et al (2010). There is a substantial difference in the high dose regions
especially the dose received by 10%, and 5% volumes. This is mainly due to
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very high dose accumulation nearby the brachytherapy implants. Those very
high dose regions do not exist in CyberKnife dose distribution.
The dose received by the entire target volume V100% in the present
HDR equivalent dose distributions of CyberKnife is 95.29%. According to the
studies by Fröhlich et al (2007) it is 94%. Murali et al (2010) reports the
V100% as 92.2%.
These results show a similarity in the distribution of the
prescription dose between the HDR and HDR equivalent CyberKnife dose
distributions. The virtual HDR CyberKnife treatment analysis of Fuller et al
(2008) is reporting the V100% as 96.7% in the simulated HDR plan of
CyberKnife.
6.4.2 OAR Dose Analysis
As far as the OAR rectum is concerned the dose volumes V100%,
V90%, V50% reported by Murali et al (2010) are 0.1cc, 0.4cc, and 7.5cc
respectively. In the present study it is 0.09cc, 1.89cc, 17.29cc respectively.
This clearly shows the dose fall off within the rectum is better in the HDR
dose distribution when compared with the CyberKnife HDR equivalent dose
distribution. Similarly the bladder dose volumes V100%, V90%, and V50% of
Murali et al (2010) study are 1.5cc, 2.5 cc, and 12.3 cc respectively. In the
present study it is 0.75cc, 5.18cc and 36.12 cc respectively. This comparison
shows that the dose fall off within the bladder is much better in the HDR than
the HDR equivalent dose distributions of CyberKnife treatment plans.
Though this comparison shows a suggestion about the magnitude of
dose spread in a HDR dose distribution and HDR equivalent CyberKnife dose
distribution, the patient data sets are not the same. Furthermore, study must be
made on same set of patient data to ensure the results of the present study.
124�
6.5 CONCLUSION
The HDR equivalent doses of CyberKnife hypofractionation are
showing a resemblance with the HDR dose distribution up to 80% volume
doses which are encompassing the prostate target.
However, there is an appreciable difference observed in the high
dose regions. The rectum and the bladder dose fall off in CyberKnife are
slightly inferior to that of the HDR dose distributions reported in the
literature.
This study must be extended for further analysis on same set of
patient data to ensure the results of the present dosimetric study.