Abstract

Purpose

It is often difficult to maintain the consistency of bladder filling in prostate cancer radiation therapy. This work is to retrospectively study the dosimetric effect of the bladder volume variation between simulation and treatment and therefore to optimize the bladder preparation protocol.

Materials and Methods

The bladders of 37 patients treated for prostate cancer with the 36.25Gy/5fx VMAT SBRT technique were contoured on the simulation CT, MRI and CBCT images. The bladder volumes are in range of 55-800cc with differences of -82%~295% between simulation and treatment. The dose assessment metrics of V18.1Gy, V28Gy, V38Gy, V39.5Gy (bladder) and Dmax, V18.3Gy, V36.25Gy, V32.62Gy, V18.12Gy (bladder wall) were assessed against the bladder volumes in simulation and treatment.

Results

The absolute volume metrics and Dmax of bladder and bladder wall are not correlated to the bladder volume although the relative volume metrics do decrease with the bladder volume increase. If the bladder volume is >200cc, all the relative volume metrics are within the constraints. The absolute volume metrics of V38Gy of 7% and V39.5Gy of 14% of the patients exceed constraints when bladder filling is different from that in simulation while V38Gy of only one patient and V39.5Gy of only two patients exceed constraint if bladder volume >200cc.

Conclusions

The absolute volume dose assessment metrics of bladder and bladder wall are not correlated to the bladder volume. If bladder volume is >200cc, all the relative volume metrics of the patients in this study are within constraints. If the bladder dose of the initial plan is within constraints or deemed acceptable by a radiation oncologist and the bladder is >200cc on the treatment day, no further bladder preparation is needed for the treatment. A simple method is proposed to estimate the bladder volume based on CBCT image. The bladder preparation procedure can be potentially simplified and therefore reduce patient waiting time due to bladder filling variation.