Analysis of high–dose rate brachytherapy dose distribution resemblance in CyberKnife hypofractionated treatment plans of localized prostate cancer
Introduction
Generally, the tumor cells have higher α/β than the normal cells. However, cancerous prostate cells are exceptions to this. The α/β value of prostate is between 1.5 Gy and 3 Gy.1, 2, 3, 4 This is lower than the α/β value of surrounding normal cells of the organs at risk (OAR), the bladder and rectum. The α/β value of the rectum for late rectal toxicity is between 3 Gy and 5 Gy.5, 6, 7 Similarly, the α/β of the bladder is estimated as 7 Gy.5 As this low α/β value of the prostatic cancer cells encourages the therapeutic gain factor in hypofractionated radiotherapy, hypofractionation has become a vital fractionation scheme for the treatment of localized prostate cancer. Hypofractionation in prostate has been implemented in many ways. Hypofractionated stereotactic body radiotherapy, hypofractionated high–dose rate (HDR) brachytherapy and hypofractionated intensity-modulated radiotherapy are the few forms of prostate hypofractionation treatment. Prostate brachytherapy has been performed either as low–dose rate, permanent implants with Iodine-125/Palladium-103 seeds8 or as HDR temporary implants9, 10, 11, 12, 13, 14 with Ir-192 sources. The fractionation regimen followed in HDR brachytherapy is an accelerated hypofractionation. Owing to the therapeutic gain advantage, hypofractionation has been implemented in IMRT as well.15 CyberKnife (Accuray Inc., Sunnyvale, CA) is a robotic radiosurgery unit in which most of the treatments are performed in hypofractionation alone. Prostate hypofractionation performed in CyberKnife is a stereotactic body radiotherapy technique. Though the hypofractionation is adopted in several modes, the fractionation regimen followed is not unique. The hypofractionation regimen for CyberKnife is still under research. CyberKnife clinical trials of localized prostate cancer by King et al.16 showed a positive outcome for a fractionation regimen of 36.25 Gy in 5 fractions (7.25 Gy per fraction). Clinical trials by Katz17 showed that the 35 Gy in 5 fractions (7 Gy per fraction) regimen gave better results than the 36.25 Gy in 5 fractions regimen. The fractionation regimens followed in HDR brachytherapy are also not unique. Murali et al.10 followed the protocol of 30 Gy in 3 fractions (10 Gy per fraction) for HDR brachytherapy of localized prostate whereas Martinez et al.11 and Grills et al.12 followed 38 Gy in 4 fractions (9.5 Gy per fraction). Demanes et al.13 followed 2 biologically equivalent fractionation regimens, 42 Gy in 6 fractions and 38 Gy in 4 fractions. The HDR brachytherapy hypofractionation has been accepted as a successful monotherapy mode for localized prostate.11, 12, 13 It is imperative to assess the degree of resemblance of a HDR brachytherapy dose distribution in a CyberKnife hypofractionated dose distribution as the CyberKnife stereotactic radiosurgery for prostate is in forward progression in the modern day radiotherapy. Fuller et al.18 made a trial to create a HDR-like distribution in CyberKnife treatment plan. However, the HDR equivalent dose distributions in CyberKnife treatment plans of prostate cancer have not yet been studied in depth. Sudahar et al.19 analyzed and reported the normalized total dose equivalents of CyberKnife hypofractionated doses of localized prostate cancer. 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 purpose, the HDR fractionation regimen followed by Murali et al.10 was taken as the reference.
Section snippets
Methods and Materials
Thirteen randomly selected localized prostate cancer cases were considered retrospectively for this study. The contouring was done in the dedicated CyberKnife treatment planning system, MultiPlan (Accuray Inc., Sunnyvale, CA). Computed tomography images of 1-mm slice thickness of the pelvic region were acquired and fused with magnetic resonance images to draw the planning target volume (PTV) of the prostate target and the OARs (rectum and bladder). The treatment plans were generated with
CyberKnife hypofractionated dose distribution
The analysis of the target dose distribution in the CyberKnife hypofractionated dose regimen is shown in Table 1. The relative percentages of the target doses are given in Table 2. D98% and D5% are the indicators of the minimum and maximum dose within the target, respectively. The mean values of D98% and D5% were 35.28 ± 1.09 Gy and 41.57 ± 2.41 Gy, respectively. The target conformity, homogeneity indices, and V100% are shown in Table 3. The conformity index was ranging between 1.15 and 1.73,
Discussion
Murali et al.10 compared the dose distribution between IMRT (conventional 2 Gy fractionation) and HDR brachytherapy (30 Gy in 3 fractions). According to that study, the D98% of the PTV is 91.4%. The mean D98% in the present study is 97.34%. 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.10 are 102.4%, 110.7%, 130.6%, 225.5%, and 306.7%, respectively. These volume doses in our present study are
Conclusions
The HDR equivalent doses of CyberKnife hypofractionation show a resemblance with the HDR dose distribution for up to 80% volume doses that are encompassing the prostate target. However, there is an appreciable difference observed in the high-dose regions. The rectum and bladder dose fall-off in CyberKnife are slightly inferior to those of the HDR dose distributions reported in the literature. This study must be extended for further analysis on the same set of patient data to ensure the results
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