title: Radiobiological evaluation of the influence of dwell time modulation restriction in HIPO optimized HDR prostate brachytherapy implants.
abstract: OBJECTIVE
  One of the issues that a planner is often facing in HDR brachytherapy is the selective existence of high dose volumes around some few dominating dwell positions. If there is no information available about its necessity (e.g. location of a GTV), then it is reasonable to investigate whether this can be avoided. This effect can be eliminated by limiting the free modulation of the dwell times. HIPO, an inverse treatment plan optimization algorithm, offers this option. In treatment plan optimization there are various methods that try to regularize the variation of dose non-uniformity using purely dosimetric measures. However, although these methods can help in finding a good dose distribution they do not provide any information regarding the expected treatment outcome as described by radiobiology based indices.
  
  
  METHODS
  The quality of 12 clinical HDR brachytherapy implants for prostate utilizing HIPO and modulation restriction (MR) has been compared to alternative plans with HIPO and free modulation (without MR). All common dose-volume indices for the prostate and the organs at risk have been considered together with radiobiological measures. The clinical effectiveness of the different dose distributions was investigated by calculating the response probabilities of the tumors and organs-at-risk (OARs) involved in these prostate cancer cases. The radiobiological models used are the Poisson and the relative seriality models. Furthermore, the complication-free tumor control probability,P+and the biologically effective uniform dose ([Formula: see text]) were used for treatment plan evaluation and comparison.
  
  
  RESULTS
  Our results demonstrate that HIPO with a modulation restriction value of 0.1-0.2 delivers high quality plans which are practically equivalent to those achieved with free modulation regarding the clinically used dosimetric indices. In the comparison, many of the dosimetric and radiobiological indices showed significantly different results. The modulation restricted clinical plans demonstrated a lower total dwell time by a mean of 1.4% that was proved to be statistically significant (p= 0.002). The HIPO with MR treatment plans produced a higherP+by 0.5%, which stemmed from a better sparing of the OARs by 1.0%.
  
  
  CONCLUSIONS
  Both the dosimetric and radiobiological comparison shows that the modulation restricted optimization gives on average similar results with the optimization without modulation restriction in the examined clinical cases. Concluding, based on our results, it appears that the applied dwell time regularization technique is expected to introduce a minor improvement in the effectiveness of the optimized HDR dose distributions.
authors.affiliation: Department of Medical Radiation Physics, Karolinska Institutet and Stockholm University, Sweden.
authors.forename: Panayiotis
authors.initials: P
authors.lastname: Mavroidis
authors.affiliation: Department of Medical Physics & Engineering, Strahlenklinik, Klinikum Offenbach GmbH, Germany.
authors.forename: Zaira
authors.initials: Z
authors.lastname: Katsilieri
authors.affiliation: Department of Medical Physics & Engineering, Strahlenklinik, Klinikum Offenbach GmbH, Germany.
authors.forename: Vasiliki
authors.initials: V
authors.lastname: Kefala
authors.affiliation: Department of Medical Physics & Engineering, Strahlenklinik, Klinikum Offenbach GmbH, Germany.
authors.forename: Natasa
authors.initials: N
authors.lastname: Milickovic
authors.affiliation: Department of Radiological Sciences, University of Texas Health Sciences Center, San Antonio, Texas, USA.
authors.forename: Nikos
authors.initials: N
authors.lastname: Papanikolaou
authors.affiliation: Pi-Medical Ltd., Athens, Greece.
authors.forename: Andreas
authors.initials: A
authors.lastname: Karabis
authors.affiliation: Strahlenklinik, Klinikum Offenbach GmbH, Germany.
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Zamboglou
authors.affiliation: Department of Medical Physics & Engineering, Strahlenklinik, Klinikum Offenbach GmbH, Germany; Nuclear and Particle Physics Section, Physics Department, University of Athens, Greece.
authors.forename: Dimos
authors.initials: D
authors.lastname: Baltas
keywords: HDR; HIPO; optimization; radiobiology; treatment plan