Copenhagen, Denmark

ESTRO 2022

Session Item

Intra-fraction motion management and real-time adaptive radiotherapy
Poster (digital)
Dosimetric effect of uncorrected rotational errors during prostate SBRT Cyberknife treatments.
David Sevillano, Spain


Dosimetric effect of uncorrected rotational errors during prostate SBRT Cyberknife treatments.

David Sevillano1, Juan David García-Fuentes2, Asunción Hervás3, Carmen Vallejo3, Fernando López-Campos3, Rafael Colmenares4, Rafael Morís4, Belén Capuz4, Miguel Cámara4, Pedro Retorta4, Pablo Galiano4, Sandra Williamson4, María José Béjar4, Daniel Prieto4, Feliciano García-Vicente4

1H. U. Ramón y Cajal. IRYCIS, Medical Physics, Madrid, Spain; 2H. U. Ramón y Cajal, IRYCIS, Medical Physics, Madrid, Spain; 3H. U. Ramón y Cajal, IRYCIS., Radiation Oncology, Madrid, Spain; 4H. U. Ramón y Cajal, IRYCIS., Medical Physics, Madrid, Spain

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Purpose or Objective

The Cyberknife system does only allow to perform rotational corrections if the prostate position is within certain tolerances (±5 degrees in pitch and ±3 degrees in yaw and ±2 degrees in roll). When it is not possible to setup the patient correctly, the user faces the decision of whether it is better to treat without rotational corrections or try to reposition or even replan that treatment. To ease this decision, this work studies the dosimetric effect of not applying rotational corrections in prostate SBRT treatments with Cyberknife.  

Material and Methods

17 prostate SBRT treatments with Cyberknife were studied. Treatment plans consist on delivering 36.25 Gy to the prostate+1/3 of seminal vesicles (SV) with an isotropic CTV-to-PTV margin of 5mm. For each plan, the dose delivered to the patient in one fraction was assessed by applying different rotational systematic errors to the dose matrices. Intrafractional variations of these rotations were also applied based on data from our patient population.

For each treatment, errors from -10 to 10 degrees in pitch and from -5 to 5 degrees in both roll and yaw were tested.  Thus, for each of these pitch/roll/yaw combinations, the D98 on both prostate and SV CTVs were obtained, while D1cc and Dmax of the urethra was also included in the analysis.

The mean values of the 17 treatments at each point were fitted to a second grade 3D polynomial.


A 2D representation of fitted values of prostate D98 and urethra D1cc at each plan are shown in Figures 1 and 2. All fits yielded R2 values of 0.97, 0.97, 0.99 and 0.93 for D98 of prostate, D98 of SV, D1cc of urethra and Dmax of urethra respectively.

Results depended on the volume analysed, prostate D98 was above the prescription dose of 36.26 Gy for all the rotations analysed, with dose being almost independent of roll rotation. In the case of SV D98, pitch and yaw rotations of ±5 degrees implied D98 less than 36.25 Gy but it was still over 34.4 Gy (90% of prescription dose) for rotations within ±7.5 degrees. Again, SV D98 showed less dependency with roll rotations for which ±10 degrees still assure a D98 of 36.25 Gy.

Regarding the urethra, Dmax showed little variation with rotational errors, with increases of up to 0.5 Gy for all rotations analysed, while D1cc of the urethra could be higher than its constraint (38.78 Gy) for pitch and yaw rotations of ±7.5 degrees. Nevertheless, for pitch values of more than 10 degrees D1cc was still below 39 Gy. 


We have developed a useful tool to decide whether it is safe to treat a prostate SBRT without rotational corrections for a given fraction with Cyberknife. Nevertheless, if large errors appear systematically for more than one fraction, an investigation of the reasons should be performed and replanning should be considered.