Radiosurgery publications by our team

In this section we highlight recent publications for Gamma Knife based research undertaken by members of our team. You can click on the PMID which will provide links to access the full articles.

Adaptation and validation of a commercial head phantom for cranial radiosurgery dosimetry end-to-end audit. Dimitriadis, A. L. Palmer, R. A. S. Thomas, A. Nisbet, and C. H. Clark. “Br. J. Radiol., p. 20170053, Apr. 2017.
PMID: 28452563


To adapt and validate an anthropomorphic head phantom for use in a cranial radiosurgery audit.

Two bespoke inserts were produced for the phantom: one for providing the target and organ at risk for delineation and the other for performing dose measurements. The inserts were tested to assess their positional accuracy. A basic treatment plan dose verification with an ionization chamber was performed to establish a baseline accuracy for the phantom and beam model. The phantom and inserts were then used to perform dose verification measurements of a radiosurgery plan. The dose was measured with alanine pellets, EBT extended dose film and a plastic scintillation detector (PSD).

Both inserts showed reproducible positioning (±0.5 mm) and good positional agreement between them (±0.6 mm). The basic treatment plan measurements showed agreement to the treatment planning system (TPS) within 0.5%. Repeated film measurements showed consistent gamma passing rates with good agreement to the TPS. For 2%-2 mm global gamma, the mean passing rate was 96.7% and the variation in passing rates did not exceed 2.1%. The alanine pellets and PSD showed good agreement with the TPS (-0.1% and 0.3% dose difference in the target) and good agreement with each other (within 1%).

The adaptations to the phantom showed acceptable accuracies. The presence of alanine and PSD do not affect film measurements significantly, enabling simultaneous measurements by all three detectors. Advances in knowledge: A novel method for thorough end-to-end test of radiosurgery, with capability to incorporate all steps of the clinical pathway in a time-efficient and reproducible manner, suitable for a national audit.

Characterisation of a plastic scintillation detector to be used in a multicentre stereotactic radiosurgery audit. A. Dimitriadis, I. Patallo Silvestre, I. Billas, S. Duane, A. Nisbet, and C. H. Clark. Radiat. Phys. Chem., no. Special Issue: ICDA2 Conference Proceedings, Feb. 2017.


Current Status of Cranial Stereotactic Radiosurgery in the UK. Dimitriadis, K. J. Kirkby, A. Nisbet, and C. H. Clark. Br. J. Radiol., vol. 89, p. 20150452, Dec. 2016.
PMID: 26689091




To investigate and benchmark the current clinical and dosimetric practices in stereotactic radiosurgery (SRS) in the UK.


A detailed questionnaire was sent to 70 radiotherapy centres in the UK. 97% (68/70) of centres replied between June and December 2014.


21 centres stated that they are practising SRS, and a further 12 centres plan to start SRS by the end of 2016. The most commonly treated indications are brain metastases and acoustic neuromas. A large range of prescription isodoses that range from 45% to 100% between different radiotherapy centres was seen. Ionization chambers and solid-water phantoms are used by the majority of centres for patient-specific quality assurance, and thermoplastic masks for patient immobilization are more commonly used than fixed stereotactic frames. The majority of centres perform orthogonal kilovoltage X-rays for localization before and during delivery. The acceptable setup accuracy reported ranges from 0.1 to 2 mm with a mean of 0.8 mm.


SRS has been increasing in use in the UK and will continue to increase in the next 2 years. There is no current consensus between SRS centres as a whole, or even between SRS centres with the same equipment, on the practices followed. This indicates the need for benchmarking and standardization in SRS practices within the UK.


This article outlines the current practices in SRS and provides a benchmark for reference and comparison with future research in this technique.

Evaluation of Gafchromic EBT-XD film, with comparison to EBT3 film, and application in high dose radiotherapy verificationL. Palmer, A. Dimitriadis, A. Nisbet, and C. H. Clark.  Phys. Med. Biol., vol. 60, no. 22, pp. 8741–8752, 2015.
PMID: 26512917


There is renewed interest in film dosimetry for the verification of dose delivery of complex treatments, particularly small fields, compared to treatment planning system calculations. A new radiochromic film, Gafchromic EBT-XD, is available for high-dose treatment verification and we present the first published evaluation of its use. We evaluate the new film for MV photon dosimetry, including calibration curves, performance with single- and triple-channel dosimetry, and comparison to existing EBT3 film. In the verification of a typical 25 Gy stereotactic radiotherapy (SRS) treatment, compared to TPS planned dose distribution, excellent agreement was seen with EBT-XD using triple-channel dosimetry, in isodose overlay, maximum 1.0 mm difference over 200-2400 cGy, and gamma evaluation, mean passing rate 97% at 3% locally-normalised, 1.5 mm criteria. In comparison to EBT3, EBT-XD gave improved evaluation results for the SRS-plan, had improved calibration curve gradients at high doses, and had reduced lateral scanner effect. The dimensions of the two films are identical. The optical density of EBT-XD is lower than EBT3 for the same dose. The effective atomic number for both may be considered water-equivalent in MV radiotherapy. We have validated the use of EBT-XD for high-dose, small-field radiotherapy, for routine QC and a forthcoming multi-centre SRS dosimetry intercomparison.

Investigation of dosimetric differences between the TMR 10 and convolution algorithm for Gamma Knife stereotactic radiosurgery. Rojas-Villabona A, Kitchen N, Paddick I. J Appl Clin Med Phys.2016 Nov 8;17(6):6347.



Since its inception, doses applied using Gamma Knife Radiosurgery (GKR) have been calculated using a simple TMR algorithm, which assumes the patient’s head is of even density, the same as water. This results in a significant approximation of the dose delivered by the Gamma Knife. We investigated how GKR dose cal-culations varied when using a new convolution algorithm clinically available for GKR planning that takes into account density variations in the head compared with the established calculation algorithm. Fifty-five patients undergoing GKR and harboring 85 lesions were voluntarily and prospectively enrolled into the study. Their clinical treatment plans were created and delivered using TMR 10, but were then recalculated using the density correction algorithm. Dosimetric differences between the planning algorithms were noted. Beam on time (BOT), which is directly proportional to dose, was the main value investigated. Changes of mean and maximum dose to organs at risk (OAR) were also assessed. Phantom studies were performed to investigate the effect of frame and pin materials on dose calculation using the convolution algorithm. Convolution yielded a mean increase in BOT of 7.4% (3.6%-11.6%). However, approximately 1.5% of this amount was due to the head contour being derived from the CT scans, as opposed to measurements using the Skull Scaling Instrument with TMR. Dose to the cochlea calculated with the convolution algorithm was approximately 7% lower than with the TMR 10 algorithm. No significant difference in relative dose distribution was noted and CT artifact typically caused by the stereotactic frame, glue embolization material or different fixation pin materials did not systematically affect convolu-tion isodoses. Nonetheless, substantial error was introduced to the convolution calculation in one target located exactly in the area of major CT artifact caused by a fixation pin. Inhomogeneity correction using the convolution algorithm results in a considerable, but consistent, dose shift compared to the TMR 10 algorithm traditionally used for GKR. A reduction of the prescription dose may be neces-sary to obtain the same clinical effect with the convolution algorithm. Head shape definition using CT outlining can reduce treatment uncertainty from head shape approximations.

Evaluation of the stability of the stereotactic Leksell Frame G in Gamma Knife radiosurgery.Rojas-Villabona A, Miszkiel K, Kitchen N, Jäger R, Paddick I.J Appl Clin Med Phys. 2016 May 8;17(3):5944.


The purpose of this study was to evaluate the stability of the Leksell Frame G in Gamma Knife radiosurgery (GKR). Forty patients undergoing GKR underwent pretreatment stereotactic MRI for GKR planning and stereotactic CT immediately after GKR. The stereotactic coordinates of four anatomical landmarks (cochlear apertures and the summits of the anterior post of the superior semicircular canals, bilaterally) were measured by two evaluators on two separate occasions in the pre-treatment MRI and post-treatment CT scans and the absolute distance between the observations is reported. The measurement method was validated with an indepen-dent group of patients who underwent both stereotactic MRI and CT imaging before treatment (negative controls; n: 5). Patients undergoing GKR for arteriovenous malformations (AVM) also underwent digital subtraction angiography (DSA), which could result in extra stresses on the frame. The distance between landmark local-ization in the scans for the negative control group (0.63 mm; 95% CI: 0.57-0.70; SD: 0.29) represents the overall consistency of the evaluation method and provides an estimate of the minimum displacement that could be detected by the study. Two patients in the study group had the fiducial indicator box accidentally misplaced at post-treatment CT scanning. This simulated the scenario of a frame displacement, and these cases were used as positive controls to demonstrate that the evaluation method is capable of detecting a discrepancy between the MRI and CT scans, if there was one. The mean distance between the location of the landmarks in the pretreatment MRI and post-treatment CT scans for the study group was 0.71 mm (95% CI: 0.68-0.74; SD:0.32), which was not statistically different from the over-all uncertainty of the evaluation method observed in the negative control group (p = 0.06). The subgroup of patients with AVM (n: 9), who also underwent DSA, showed a statistically significant difference between the location of the landmarks compared to subjects with no additional imaging: 0.78 mm (95% CI: 0.72-0.84) vs. 0.69 mm (95% CI: 0.66-0.72), p = 0.016. This is however a minimal differ-ence (0.1 mm) and the mean difference in landmark location for each AVM patient remained submillimeter. This study demonstrates submillimeter stability of the Leksell Frame G in GKR throughout the treatment procedure.

The role of the concept of biologically effective does (BED) in treamtment planning in radiosurgery. Millar WT, Hopewell JW, Paddick I, Lindquist C, Nordstron H, Lidberg P, Garding J. Phys Med. 2015 Sep:31(6): 627-33. PMID: 25982304


Radiosurgery (RS) treatment times vary, even for the same prescription dose, due to variations in the collimator size, the number of iso-centres/beams/arcs used and the time gap between each of these exposures. The biologically effective dose (BED) concept, incorporating fast and slow components of repair, was used to show the likely influence of these variables for Gamma Knife patients with Vestibular Schwannomas. Two patients plans were selected, treated with the Model B Gamma Knife, these representing the widest range of treatment variables; iso-centre numbers 3 and 13, overall treatment times 25.4 and 129.6 min, prescription dose 14 Gy. These were compared with 3 cases treated with the Perfexion(®) Gamma Knife. The iso-centre number varied between 11 and 18, treatment time 35.7 – 74.4 min, prescription dose 13 Gy. In the longer Model B Gamma Knife treatment plan the 14 Gy iso-dose was best matched by the 58 Gy2.47 iso-BED line, although higher and lower BED values were associated with regions on the prescription iso-dose. The equivalent value for the shorter treatment was 85 Gy2.47. BED volume histograms showed that a BED of 85 Gy2.47 only covered ∼65% of the target in the plan with the longer overall treatment time. The corresponding BED values for the 3 cases, treated with the Perfexion(®) Gamma Knife, were 59.5, 68.5 and 71.5 Gy2.47. In conclusion BED calculations, taking account of the repair of sublethal damage, may indicate the importance of reporting overall time to reflect the biological effectiveness of the total physical dose applied.

Radiosurgery Nomenclature: A Confusion of Tongues.Chung C,Torrens M,Paddick I, Ryu S, Hanssens P, Jaffray D. Int J Radiat Oncol Biol Phys. 2015 Jul 1;92(3):512-3.
PMID: 26068485

Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams.Benmakhlouf H, Johansson J, Paddick I, Andreo P. Phys Med Biol. 2015 May 21;60(10):3959-73.
PMID: 25909660


The measurement of output factors (OF) for the small photon beams generated by Leksell Gamma Knife® (LGK) radiotherapy units is a challenge for the physicist due to the under or over estimation of these factors by a vast majority of the detectors commercially available. Output correction factors, introduced in the international formalism published by Alfonso (2008 Med. Phys. 35 5179-86), standardize the determination of OFs for small photon beams by correcting detector-reading ratios to yield OFs in terms of absorbed-dose ratios. In this work output correction factors for a number of detectors have been determined for LGK Perfexion™ (60)Co γ-ray beams by Monte Carlo (MC) calculations and measurements. The calculations were made with the MC system PENELOPE, scoring the energy deposited in the active volume of the detectors and in a small volume of water; the detectors simulated were two silicon diodes, one liquid ionization chamber (LIC), alanine and TLD. The calculated LIC output correction factors were within ± 0.4%, and this was selected as the reference detector for experimental determinations where output correction factors for twelve detectors were measured, normalizing their readings to those of the LIC. The MC-calculated and measured output correction factors for silicon diodes yielded corrections of up to 5% for the smallest LGK collimator size of 4 mm diameter. The air ionization chamber measurements led to extremely large output correction factors, caused by the well-known effect of partial volume averaging. The corrections were up to 7% for the natural diamond detector in the 4 mm collimator, also due to partial volume averaging, and decreased to within about ± 0.6% for the smaller synthetic diamond detector. The LIC, showing the smallest corrections, was used to investigate machine-to-machine output factor differences by performing measurements in four LGK units with different dose rates. These resulted in OFs within ± 0.6% and ± 0.2% for the 4 mm and 8 mm collimators, respectively, providing evidence for the use of generic OFs for these LGK beams. Using the experimentally derived output correction factors, OFs can be measured using a wide range of commercially available detectors.

Targeting and Conformality in Arteriovenous Malformation Radiosurgery: by Paddick, I.; Motti, E. Prog Neurol Surg. 2013;27:35-48. PMID: 23258507


The struggle to achieve a high degree of conformity around targets of complex morphology has been one of the driving forces in the development of ever more sophisticated radiosurgical devices and intricate treatment delivery. Rarely are radiosurgical targets more complex in shape than those associated with arteriovenous malformations (AVMs). In this report we examine theoretical and practical issues of target delineation and creation of conformal AVM treatment plans, and comment on the concepts of gradient and homogeneity.

‘Targeting and Conformality in Arteriovenous Malformation Radiosurgery” is a paper co-authored by Ian Paddick included in the new publication by Karger, Gamma Knife Radiosurgery for Brain Vascular Malformations, edited by Aray Niranjan, Hieyuki Kano and L Dade Lunsford. The book which can be previewed here is available for purchase from Karger.

Clinical outcomes of brain vascular malformation have been greatly improved by advances in microsurgery, endovascular techniques and stereotactic radiosurgery. Radiosurgery has proven to be the least invasive technique and is associated with documented long-term success. The papers in this volume present the experience of leading brain vascular malformation specialists who describe the outcome of radiosurgery for arteriovenous malformations, cavernous malformations and dural arteriovenous fistulas. These reviews expand the knowledge of the role of stereotactic radiosurgery alone or in combination with other treatment modalities. Evidence-based guidelines are included in each section to provide a summary of the current management strategies.

This unique publication includes additional data that will further define the long-term benefit and risks of radiosurgery for these often complex vascular disorders and makes valuable reading for neurosurgeons, neurologists and endovascular specialists interested in an excellent summary of more than 30 years of accumulated experience in the management of brain vascular malformations.

ISSN: 0079-6492
e-ISSN: 1662-3924

ISBN: 978-3-8055-9619-0
e-ISBN: 978-3-8055-9620-6