Abstract
BackgroundHypo-fractionated external beam radiotherapy with narrow CTV-PTV margins is increasingly applied for prostate cancer. This demands a precise target definition and knowledge on target location and extension during treatment. It is unclear how increase in fraction size affects changes in prostate volume during treatment. Our aim was to study prostate volume changes during extreme hypo-fractionation (7 × 6.1 Gy) by using sequential MRIs.MethodsTwenty patients treated with extreme hypo-fractionation were recruited from an on-going prospective randomized phase III trial. An MRI scan was done before start of treatment, at mid treatment and at the end of radiotherapy. The prostate was delineated at each MRI and the volume and maximum extension in left-right, anterior-posterior and cranial-caudal directions were measured.ResultsThere was a significant increase in mean prostate volume (14%) at mid treatment as compared to baseline. The prostate volume remained enlarged (9%) at the end of radiotherapy. Prostate swelling was most pronounced in the anterior-posterior and cranial-caudal directions.ConclusionsExtreme hypo-fractionation induced a significant prostate swelling during treatment that was still present at the time of last treatment fraction. Our results indicate that prostate swelling is an important factor to take into account when applying treatment margins during short extreme hypo-fractionation, and that tight margins should be applied with caution.
Highlights
Hypo-fractionated external beam radiotherapy with narrow CTV-PTV margins is increasingly applied for prostate cancer
Implementation of magnetic resonance imaging (MRI) for segmentation together with sophisticated image guided radiotherapy (IGRT) techniques based on implanted fiducials has resulted in improved accuracy and precision in RT for prostate cancer [1,2,3]
Our results indicate that a margin extension of similar magnitude could be needed to take prostate swelling into account during extreme hypo-fractionation
Summary
Hypo-fractionated external beam radiotherapy with narrow CTV-PTV margins is increasingly applied for prostate cancer. This demands a precise target definition and knowledge on target location and extension during treatment. It is unclear how increase in fraction size affects changes in prostate volume during treatment. Our aim was to study prostate volume changes during extreme hypo-fractionation (7 × 6.1 Gy) by using sequential MRIs. The field of radiotherapy (RT) is rapidly evolving with new advanced treatment techniques and improved imaging. Implementation of magnetic resonance imaging (MRI) for segmentation together with sophisticated image guided radiotherapy (IGRT) techniques based on implanted fiducials has resulted in improved accuracy and precision in RT for prostate cancer [1,2,3]. There is some evidence that prostate size increases slightly during the first week(s) after start of conventionally fractionated RT and decreases substantially during treatment and shrinks to below baseline by the end of treatment [19,20]
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