Service evaluation project on the use of adaptive planning target volume (PTV) margins for prostate radiotherapy
by Anne McKenna
To determine whether an adaptive treatment margin allows for the reduction in planning target volume (PTV) margins. The dosimetric impact of an adaptive PTV margin, and adaptive PTV with reduced margins (from 10mm to 7mm for PTV1) were assessed using dose data acquired from Cone Beam Computerised Tomography (CBCT) and planning scans.
An adaptive PTV (10mm margin) and reduced margin adaptive (7mm margin) was derived from the first five fractions CBCTs and the planning CT. Coverage of the PTVs were assessed on subsequent weekly CBCTs. Dose Volume Histograms (DVHs) from the CBCTs were compared to the original planned DVHs to ascertain whether the delivered treatment varied from the original plan.
The mean prostate PTV1 D95 (in Gy) for the planned, adaptive and reduced volume margins were 62.3 (range 58.9-68.2), 60.7 (53.4-65.4) and 63.7 (57.2-68.1) respectively. No statistically significant difference was detected between the planned prostate PTV1 D95 and the adaptive prostate PTV1 (p=0.078). A statistically significant difference was detected between the planned prostate PTV1 D95 and the mean of the first five CBCTs (p=0.005). The mean centre of gravity of the first five CBCTs for all patients (in mm) were; in the lateral, anterior/posterior and superior/inferior directions -0.3, 0.1 and 1.6 respectively.
No statistical difference was found between the planned prostate PTV1 D95 and the adaptive prostate PTV1 (p=0.078). However the variations between patients for the adaptive PTV1 D95 suggest that the adaptive margin would not be an adequate class solution for this group of patients.