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interfractional vaginal motion in the LR and AP direction
and a moderate agreement in the CC direction (see figure
1), which we in all directions significant (p<0.00) .
Considering only interfractional vagina motion, applying
a BA based image guidance strategy requires CTV to PTV
margins of 0.3 cm, 0.8 cm and 1.0 cm in the LR, CC and
AP direction. When applying a FM or ST registration based
imaging strategy the residual LN variability (which move
with the BA) will be larger, and needs to be considered in
the CTV to PTV margins, leading to LN margins of 0.3, 1.1
and 1.3 cm in the LR, CC and AP direction.
Conclusion
FM registrations can be applied as an IGRT strategy to
measure and correct the vagina motion. However applying
FM registration increases the LN interfractional position
variability, subsequently increasing the CTV to PTV
margins for the LN regions even more in comparison to the
margins needed to encompass the interfractional vagina
motion. We are currently investigating an offline adaptive
workflow to address this.
PO-1017 Dose guided adaptive radiotherapy based on
cumulated dose in OAR for prostate cancer
M. Nassef
1
, A. Simon
1
, B. Rigaud
1
, L. Duvergé
2
, C.
Lafond
2
, J.Y. Giraud
3
, P. Haigron
1
, R. De Crevoisier
2
1
LTSI, INSERM U1099, Rennes, France
2
Centre Eugène Marquis, Radiothérapie, Rennes, France
3
CHU Grenoble, Radiothérapie, Grenoble, France
Purpose or Objective
Large dose differences between planned and delivered
doses may be observed in the rectum and in the bladder,
resulting from anatomical variation in the course of
prostate IMRT. The objective of this study was to compare
dosimetrically an original approach of Dose Guided
Adaptive Radiotherapy (DGART) to the standard IGRT
(CBCT daily repositioning).
Material and Methods
Based on a series of 24 patients with daily CBCT, planned
and delivered dose were compared in manually delineated
structures (prostate, rectum and bladder), using dose
accumulation process after estimation of the fraction dose
[Nassef et al, Radiother Oncol 2016]. The four patients
with the most important overdose in the rectum wall and
the bladder wall were selected to estimate the DGART
benefit compared to the standard IGRT.
The DGART strategy (Figure 1) was based on replanning(s)
triggered by monitoring the cumulated doses to the
prostate, the rectum wall and the bladder wall. Thereby,
the first step consisted in estimating the relative excess
of the cumulated dose compared to the planned dose after
every fraction for the prostate D
99
, the rectum wall V
72
and
the bladder wall V
70
. After an observation phase of 5
fractions, the adaptation was triggered (i.e. a replanning
was performed), if a 2 % underdose of D
99
for prostate or
an overdose of 10 % on V
72
for the rectum wall or V
70
for
the bladder wall occurred.
If a replanning was triggered at the fraction n, the CBCT
chosen for the replanning corresponded to the anatomy
leading to the highest dose drift compared to the planned
dose. For that, for every fraction x (x=1..n), an index (see
figure 2) was calculated to select the morphology leading
to the highest dose drift compared to the planned dose. If
the relative excess was compensated by the replanning,
no other adaption was needed and the new replanning was
used for the rest of the fractions. If the relative excess
was not compensated, the replanning process was
repeated in case of a new CBCT leading to a higher index
value. An example of DGART implementation is provided
in Figure 2, showing the benefit of DGART to decrease the
dose to the bladder.