Journal of Medical Physics
: 2018  |  Volume : 43  |  Issue : 1  |  Page : 72--73

Dosimetric evaluation and optimization of fractionated stereotactic radio-surgery

Arun Chougule 
 Department of Radiological Physics, S.M.S. Medical College and Hospitals, Jaipur, Rajasthan, India

Correspondence Address:
Prof. Arun Chougule
Department of Radiological Physics, S.M.S. Medical College and Hospitals, Jaipur, Rajasthan

How to cite this article:
Chougule A. Dosimetric evaluation and optimization of fractionated stereotactic radio-surgery.J Med Phys 2018;43:72-73

How to cite this URL:
Chougule A. Dosimetric evaluation and optimization of fractionated stereotactic radio-surgery. J Med Phys [serial online] 2018 [cited 2021 May 14 ];43:72-73
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Full Text

Author: Raj Kishor Bisht, Department of Neuro-Surgery, AIIMS, New Delhi

Title: Dosimetric evaluation and optimization of fractionated stereotactic radio-surgery

Chief Guide: Prof. S. S. Kale, Department of Neuro-Surgery, AIIMS, New Delhi

Date of award: 29th July 2017

In recent years, stereotactic radiosurgery (SRS) has evolved as the major radiotherapy modality for brain tumors and requires very precise radiation dose delivery using state of the art treatment planning systems. SRS can be delivered by Gamma Knife, CyberKnife, or XKnife; however, Gamma Knife offers an efficient treatment to a number of neurological disorders in the brain, with relatively lower radiation dose fractions, have a radiobiological advantage on tumor and/or adjacent normal structures. In this study, RK Bisht under the guidance of neurosurgeon, medical physicist, and radiation oncologist has investigated and evaluated the accuracy of the system with the help of newly developed multipurpose human shaped phantom and physics support for the patients treated with custom-made Extend System (ES) on Gamma Knife Perfexion for various clinical indications.

The work is planned meticulously, executed accurately, the results obtained are analyzed wisely and presented in very elaborate and in a well-structured manner in the present thesis. I enjoyed the reading the thesis as the language used is simple, and the matter is discussed very extensively with relevant references. The thesis consists of eight main chapters.

The first chapter deals with historical development and physics approach of stereotactic irradiation using various stereotactic radiosurgical machines. Further, the Gamma Knife methodology for single and multiple fractions with ES is at the core of this thesis segment. Chapter 2 is devoted to review of literature, highlighting the background clinical and experimental work done so far using ES of Gamma Knife technology. The study believes that the single fraction SRS of small tumor volumes with cumulative higher radiation dose may carry additional risks of radiation damage to close proximal critical structures such as the brain stem, facial nerves or the optic nerve in the brain. In these cases, hypofractionated radiosurgery delivered in 3–5 fractions over consecutive days is an alternative treatment option. With appropriate references cited in the present study, it is understood that the multiple fraction SRS has a radiobiological advantage of effectively killing of distinguished rapidly dividing cells, whereas the single fraction radiosurgery do not explain the mitotic activity or inherent radiosensitivity. The repeated session of relatively lower dose fractions in hypofractions regimen significantly lowers the dose to adjacent normal tissue.

Chapter 3 of the thesis is dedicated to the description of research materials, equipment and methods followed by the candidate for this work. The use and preparation of individual ES is adequately described with associated tools such as frame, mouthpiece, patient control unit, the extend indicator, digital measurement probe, and repositioning tool in this chapter. Stereotactic imaging was acquired using latest X-ray computed tomography machine and the DICOM images were transferred through PACS system to the treatment planning system for preparation of precise treatment plan. Consistency in repeated reference positioning is an important factor during multi-fraction SRS which is described in an immobilization methodology section of this chapter.

Chapter 4 describes the initial study on evaluation of fractionated SRS with ES of Gamma Knife technology. The treatment preparation and clinical outcome of the patient treated with various clinical indications are discussed in this chapter. With the suggested approach on patient positioning technique, reference position recording, and treatment planning showed an improved clinical outcome. The study includes positional reproducibility check and dosimetric evaluation of ten patients treated with ES. In this study, RK Bisht has shown the improved planning indices, reduced neighboring structure doses and finally commendable clinical outcome demonstrating the efficacy of ES for fractionated SRS.

The most remarkable achievement of this work is the design and development of patient simulating phantom with capability for dosimetric verification of Gamma Knife ES based fractionated SRS. The candidate has meticulously and articulately fabricated a human upper body shaped phantom with thorax part to simulate fractionated SRS in a real patient. The phantom has been designed in such a way that it may also be used as a quality assurance tool for imaging modalities and verification mold with futuristic equipment and devices in addition to the traditional treatment verification gadgets such as ion chamber, thermoluminescent dosimeters/optically stimulated luminescent (TLDs/OSLs), and films. A patient treatment plan with fractionated regimen was delivered and identical fractions were compared using EBT3 films and in-house MATLAB codes. Gamma index analysis across fractions exhibited close agreement between LGP and film measured dose with >90% (max 93%) pixel pass rate at 1mm of spatial and 1% of dosimetric tolerances. The study finally concludes that the fractionated SRS with ES of Gamma Knife provided substantial treatment accuracy and the designed patient simulating phantom was highly versatile for the dosimetric verification of hypofractionated regimen.[1],[2],[3]

The chapter 6 of the thesis is focused on statistical optimization of SRS treatment. The purpose of this study was to estimate technical treatment accuracy in fractionated SRS using ES of Gamma Knife. The study concluded that the quality assurance of tools such as reposition check tool (RCT), digital probe, and vacuum supported patient head cushion integrity is essential for reliable patient positioning using ES. The study predicts the technical treatment uncertainty in the clinic, whereas combining the results with explicit medical uncertainties such as having knowledge of neurological abnormality and radiation response will be in the scope of future research for the determination of the treatment accuracy.

The work presented in the form of this thesis covers various practical approaches in the routine clinic to improvise the quality of highly conformal SRS. A newly designed head-neck phantom is a “comprehensive treatment setup simulation model” for SRS. The phantom is not just limited to performing film dosimetry, but the provision of placing various other dosimetric devices such as ion chamber, TLD/OSLs, chemical gels, and online dosimetry systems makes it an ideal dosimetric tool for treatment verification. The superior part of this phantom was designed to establish a link between Hounsfield unit and physical parameter of materials with varied density. The imaging information of this segment with various known/unknown materials with different physical density and/or dosimetric tools will be useful in modeling the “planning documents” for various TPS computation or model/factor based algorithms. In routine fractionated SRS treatments, the determination of positional shift with radial difference vector calculations is the only available “mathematical quality parameter” to quantify the treatment precision. In this study, the procedural uncertainty of the treatment was evaluated at calibration, imaging, and measurement stage. An “absence of medical uncertainties” in experiments, depends on imaging resolution (AAPM report no 54) and was believed to be within clinical tolerances.

In conclusion, the present study evaluates similar treatment regimen with comparable clinical results, however, a modified dose regimen like a dose per fraction, number of fractions and cumulative dose might be a clinical advancement in future research. Tangible future studies of the radiobiological effect in tumor volume and/or surrounding structures following routine or improvised hypofractionated regimen will be more realistic in determining the best possible clinical outcome. The research material and sequential methods, chosen in this thesis justify a procedural verification of fractionated treatment with ES. A statistical study with newly designed head-and-neck phantom certainly leads a mathematical optimization on the estimation of optimal uncertainty in routine fractionated treatment. Although the phantom was designed to investigate fractionated SRS procedures with ES of Gamma Knife, however, the model certainly helps in standardizing other immobilization techniques (using a thermoplastic mask, skeleton marker) and popular radiotherapy treatments.


1Natanasabapathi G, Bisht RK. Verification of gamma knife extend system based fractionated treatment planning using EBT2 film. Med Phys 2013;40:122104.
2Bisht RK, Kale SS, Gopishankar N, Kumar P, Singh MJ, Agarwal D, et al. Preliminary experience of fractionated stereotactic radiosurgery with extend system of Gamma Knife. Int J Cancer Ther Oncol 2016;4:1414.
3Bisht RK, Kale SS, Gopishankar N, Singh MJ, Agarwal D, Garg A, et al. Verification of Gamma Knife based fractionated radiosurgery with newly developed head-thorax phantom. Radiat Meas 2016;91:65-74.