Equivalent normalized total dose estimates in cyberknife radiotherapy dose delivery in prostate cancer hypofractionation regimens

Dosimetric and radiobiological comparison of CyberKnife M6™ InCise multileaf collimator over IRIS™ variable collimator in prostate stereotactic body radiation therapy

The impetus behind our study was to establish a quantitative comparison between the IRIS collimator and the InCise multileaf collimator (MLC) (Accuray Inc. Synnyvale, CA) for prostate stereotactic body radiation therapy (SBRT). Treatment plans for ten prostate cancer patients were performed on MultiPlan™ 5.1.2 treatment planning system utilizing MLC and IRIS for 36.25 Gy in five fractions. To reduce the magnitude of variations between cases, the planning tumor volume (PTV) was defined and outlined for treating prostate gland only, assuming no seminal vesicle or ex-capsule involvement. Evaluation indices of each plan include PTV coverage, conformity index (CI), Paddick's new CI, homogeneity index, and gradient index. Organ at risk (OAR) dose sparing was analyzed by the bladder wall D_max and V_37Gy, rectum D_max and V_36Gy. The radiobiological response was evaluated by tumor control probability and normal tissue complication probability based on equivalent uniform dose. The dose delivery efficiency was evaluated on the basis of planned monitor units (MUs) and the reported treatment time per fraction. Statistical significance was tested using the Wilcoxon signed rank test. The studies indicated that CyberKnife M6™ IRIS and InCise™ MLC produce equivalent SBRT prostate treatment plans in terms of dosimetry, radiobiology, and OAR sparing, except that the MLC plans offer improvement of the dose fall-off gradient by 29% over IRIS. The main advantage of replacing the IRIS collimator with MLC is the improved efficiency, determined from the reduction of MUs by 42%, and a 36% faster delivery time.

A study on rectal dose measurement in phantom and in vivo using Gafchromic EBT3 film in IMRT and CyberKnife treatments of carcinoma of prostate

The objective of this study is to check the feasibility of in vivo rectal dose measurement in intensity-modulated radiotherapy (IMRT) and CyberKnife treatments for carcinoma prostate. An in-house pelvis phantom made with bee's wax was used in this study. Two cylindrical bone equivalent materials were used to simulate the femur. Target and other critical structures associated with carcinoma prostate were delineated on the treatment planning images by the radiation oncologist. IMRT treatment plan was generated in Oncentra Master Plan treatment planning system and CyberKnife treatment plan was generated in Multiplan treatment planning system. Dose measurements were carried out in phantom and in patient using Gafchromic EBT3 films. RIT software was used to analyze the dose measured by EBT3 films. The measured doses using EBT3 films were compared with the TPS-calculated dose along the anterior rectal wall at multiple points. From the in-phantom measurements, it is observed that the difference between calculated and measured dose was mostly within 5%, except for a few measurement points. The difference between calculated and measured dose in the in-patient measurements was higher than 5% in regions which were away from the target. Gafchromic EBT3 film is a suitable detector for in vivo rectal dose measurements as it offers the possibility of analyzing the dose at multiple points. In addition, the method of extending this in vivo rectal dose measurement technique as a tool for patient-specific quality assurance check is also analyzed.

Analysis of high-dose rate brachytherapy dose distribution resemblance in CyberKnife hypofractionated treatment plans of localized prostate cancer

The present study is to analyze the CyberKnife hypofractionated dose distribution of localized prostate cancer in terms of high-dose rate (HDR) brachytherapy equivalent doses to assess the degree of HDR brachytherapy resemblance of CyberKnife dose distribution. Thirteen randomly selected localized prostate cancer cases treated using CyberKnife with a dose regimen of 36.25Gy in 5 fractions were considered. HDR equivalent doses were calculated for 30Gy in 3 fractions of HDR brachytherapy regimen. The D5% of the target in the CyberKnife hypofractionation was 41.57 ± 2.41Gy. The corresponding HDR fractionation (3 fractions) equivalent dose was 32.81 ± 1.86Gy. The mean HDR fractionation equivalent dose, D98%, was 27.93 ± 0.84Gy. The V100% of the prostate target was 95.57% ± 3.47%. The V100% of the bladder and the rectum were 717.16 and 79.6mm(3), respectively. Analysis of the HDR equivalent dose of CyberKnife dose distribution indicates a comparable resemblance to HDR dose distribution in the peripheral target doses (D98% to D80%) reported in the literature. However, there is a substantial difference observed in the core high-dose regions especially in D10% and D5%. The dose fall-off within the OAR is also superior in reported HDR dose distribution than the HDR equivalent doses of CyberKnife.