Urethra-sparing stereotactic body radiotherapy for prostate cancer: how much can the rectal wall dose be reduced with or without an endorectal balloon?

This is a dosimetric comparative study intended to establish appropriate low-to-intermediate dose-constraints for the rectal wall (Rwall) in the context of a randomized phase-II trial on urethra-sparing stereotactic body radiotherapy (SBRT) for prostate cancer. The effect of plan optimization on low-to-intermediate Rwall dose and the potential benefit of an endorectal balloon (ERB) are investigated.

Ten prostate cancer patients, simulated with and without an ERB, were planned to receive 36.25Gy (7.25Gyx5) to the planning treatment volume (PTV) and 32.5Gy to the urethral planning risk volume (uPRV). Reference plans with and without the ERB, optimized with respect to PTV and uPRV coverage objectives and the organs at risk dose constraints, were further optimized using a standardized stepwise approach to push down dose constraints to the Rwall in the low to intermediate range in five sequential steps to obtain paired plans with and without ERB (Vm1 to Vm5). Homogeneity index for the PTV and the uPRV, and the Dice similarity coefficient (DSC) for the PTV were analyzed. Dosimetric parameters for Rwall including the median dose and the dose received by 10 to 60% of the Rwall, bladder wall (Bwall) and femoral heads (FHeads) were compared. The monitor units (MU) per plan were recorded.

Vm4 reduced by half D30%, D40%, D50%, and Dmed for Rwall and decreased by a third D60% while HIPTV, HIuPRV and DSC remained stable with and without ERB compared to Vmref. HIPTV worsened at Vm5 both with and without ERB. No statistical differences were observed between paired plans on Rwall, Bwall except a higher D2% for Fheads with and without an ERB.

Further optimization to the Rwall in the context of urethra sparing prostate SBRT is feasible without compromising the dose homogeneity to the target. Independent of the use or not of an ERB, low-to-intermediate doses to the Rwall can be significantly reduced using a four-step sequential optimization approach.

Radiation oncology (London, England). 2018 Jun 19*** epublish ***

Angèle Dubouloz, Michel Rouzaud, Lev Tsvang, Wilko Verbakel, Mikko Björkqvist, Nadine Linthout, Joana Lencart, Juan María Pérez-Moreno, Zeynep Ozen, Lluís Escude, Thomas Zilli, Raymond Miralbell

Geneva University Hospital, Rue Gabrielle-Perret-Gentil 4, 1205, Genève, Switzerland. ., Geneva University Hospital, Rue Gabrielle-Perret-Gentil 4, 1205, Genève, Switzerland., Department of Radiation Oncology, Chaim Sheba Medical Center, Tel-Hashomer, 52621, Ramat Gan, Israel., Department of Radiation Oncology, VU medical center, De Boelelaan 1117, P.O. Box 7057, 1007, MB, Amsterdam, The Netherlands., Department of Oncology and Radiotherapy, Turku University Hospital, PO Box 52, 20521, Turku, Finland., Onze-Lieve-Vrouw Ziekenhuis, Moorselbaan 164, 9300, Aalst, Belgium., Serviço de Radioterapia Externa, Instituto Portugues de Oncologia, Rua Dr Antonio Bernardino de Almeida, 4200-072, Porto, Portugal., Servicio de Oncología Radioterápica, Centro Integral Oncológico "Clara Campal", Hospital Universitario Madrid Sanchinarro, C/ Oña 10, 28050, Madrid, Spain., Neolife Medical Center, Nisbetiye Mah. Yucel Sokak, No: 6 Besiktas, 34340, Istanbul, Turkey., Servei de Radiooncologia, Institut Oncològic Teknon, C/ Vilana 12, 08022, Barcelona, Spain.