109�

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

110�

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

111�

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

112�

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.

113�

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/ (�/�)]

114�

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.

115�

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

116�

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.

117�

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

118�

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

119�

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

120�

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

121�

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

123�

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.