The role of the concept of biologically effective dose (BED) in treatment planning in radiosurgery

Abstract

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.

The impact of cobalt-60 source age on biologically effective dose in high-dose functional Gamma Knife radiosurgery

“…In several studies, results from variations in the LQ model have indicated that irradiation time is important in predicting the biological responses. 7,11,18,23 Fowler et al assumed DNA repair times similar to those used in the present study and predicted a significant correlation of BED with dose rate and irradiation time for fraction sizes commonly applied in clinical practice. 11 Furthermore, some authors have proposed to give particular attention to BED variation in radiation treatments lasting more than 30 minutes per fraction.…”

“…11 Millar et al, investigating the BED of GKRS in the treatment of vestibular schwannoma, reported that irradiation time was important to consider in determining the effect of a particular prescription dose. 18 Our study built on these previous studies by applying a similar model to functional GKRS procedures, and its results highlight the particular risk of neglecting source age, dose rate, and treatment time for these uniquely high-dose procedures.…”

“…Cobalt-60 source age directly correlates with dose rate and treatment time, factors that may independently influence the biologically effective dose (BED) of treatment depending on the propensity for the target tissue to undergo intrafraction, sublethal, doublestrand break (DSB) DNA-damage repair. 11,18 The target tissues for functional GKRS, normal CNS tissues, have a greater innate potential for DNA repair than tumor tissues do, a fact that may be more important to consider in functional GKRS than in GKRS-based tumor management. 11 Furthermore, a cobalt-60 source may age significantly over the course of its use before replacement, resulting in large variations in dose rate and treatment times in functional GKRS procedures.…”

SYRA3 COST Action – Microbeam radiation therapy: Roots and prospects

“…The wide range of topics presented in this collection, based on presentations held at the 1st SYRA3 COST Conference Radiation Therapy with Synchrotron Radiation: Achievements and Challenges, with support of the Institute of Nuclear Physics at the Polish Academy of Sciences in the beautiful city of Krakow in March 2014, reflects the multidisciplinary international activities of SYRA3. The topics covered in this focus issue range from medical physics aspects [6] over pre-clinical studies [7e11] towards clinical applications [12,13] including an industrial perspective [14], and finally an outlook towards future prospects of compact sources [15] and proton microbeams [16].…”

“…In the current clinical practice of radiosurgery, treatment is usually defined by the total physical dose to an iso-surface which is conformed as close as possible to the gross tumor volume. In the paper by Millar et al [13] the concept of biologically effective dose (BED) on treatment planning is discussed: BED allows a physical dose to be converted into a dose that describes the biological effect of the radiation on tumor or normal tissue and it takes into account also the repair of sublethal damage. This concept could be important to compare the effects of MRT versus other established techniques.…”