ESTRO 36 Abstract Book

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S897 ESTRO 36 2017
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study are: (1) to determine the relation between the
treatment positioning uncertainty and the corresponding
workload, and (2) to obtain the optimal threshold for
couch shifts in prostate treatments.
Material and Methods
The quadratic sum of the uncertainties associated with
patient positioning is calculated. If the proposed shifts
remain below the threshold, the uncertainties are related
to the CBCT matching procedure and to the distribution of
residual errors. If the shifts are over the threshold, the
uncertainties are due to the couch movement accuracy
and, again, the CBCT matching procedure. The
relationship between treatment positioning uncertainty
and workload was optimized using the threshold for couch
shifts as an independent variable. Partial uncertainties
were computed based on 811 CBCT clinical cases, together
with the historical QA matching results from OBI’s
equipment and measurements from Varian’s couch
accuracy. The total positioning uncertainty with K = 2 was
calculated for VMAT treatments delivered in 28 sessions
with daily CBCT. The workload was estimated from the
probability of couch shifts, which was derived from the
statistics of the 811 clinical cases.
Results
The positioning uncertainty and the probability of couch
shifts as a function of the chosen threshold are shown in
Figure 1. As expected, if a high threshold is used (greater
than 12 mm) the workload is minimized but uncertainty is
stabilized at an excessively high value. On the contrary, if
a very low threshold is used, i.e. between 0 and 2 mm, the
probability of couch shifts is very high (between 97% and
100%). In this case, interestingly, the total uncertainty is
not significantly reduced due the contribution of the
remaining factors. Thus, the chosen threshold should be
between 2 and 12 mm. To facilitate the determination of
the optimal threshold, the derivations of both functions
are shown in Figure 2. It can be observed that uncertainty
has a maximum increase when the threshold is raised from
5 to 8 mm. However, if the same procedure is applied to
the probability distribution of couch shifts, the maximum
decrease takes place for a threshold between 4 and 5 mm.
Conclusion
A compromise between the patient uncertainty
positioning and the associated workload is needed. The
optimization of the threshold used for couch shifts is
subjective and depends on the importance given to both
factors. We showed that using a threshold <2 mm doesn’t
effectively reduce the total uncertainty. We believe that
a threshold of 3 or 4 mm is adequate, keeping the
positioning uncertainty below 1 mm and a reasonable
clinical workload.
EP-1671 Calculation of the skin dose-of- the-day during
Tomotherapy for head and neck cancer patients
M. Branchini1,2, C. Fiorino1, M. Mori1, I. Dell'Oca3, M.G.
Cattaneo1, L. Perna1, N.G. Di Muzio3, R. Calandrino1, S.
Broggi1
1San Raffaele Scientific Institute, Medical Physics, Milan,
Italy
2IRCCS Istituto Oncologico Veneto, Medical Physics,
Padova, Italy
3San Raffaele Scientific Institute, Radiotherapy, Milan,
Italy
Purpose or Objective
Late fibrosis is known to depend on the severity of acute
skin toxicity; an increase of skin dose during RT due to
anatomy deformation may translate into an increased risk
of acute toxicity, suggesting a potential benefit from
planning adaptation to counteract this effect. Within this
scenario, current study started to explore a previously
suggested method for dose-of-the-day calculation in
quantifying changes of the skin dose during Tomotherapy
(HT) for head and neck (HN) cancer.
Material and Methods
Planning CTs of 9 HN patients treated with HT (SIB:
54/66/69 Gy/30fr or sequential boost: 54/66.6-70.2Gy in
37-39 fr) were deformable registered to MVCT images
acquired at the 15th fraction (processed with anisotropic
diffusion filter) using a constrained intensity-based
algorithm (MIM software). At the same day, a diagnostic
kVCT was acquired with patient in treatment position
(CT15) and taken as reference. The original HT plans were
recalculated on both the resulting deformed images
(CTdef) and CT15 using the DQA (dose quality assurance)
HT module. In order to validate the method in computing
the dose-of-the-day of the skin, the superficial layers
(SL) of the body with thickness of 2, 3 and 5 mm (as a
surrogate of the skin dose distribution: SL2,SL3,SL5) were
considered in the body cranial-caudal extension
corresponding to the high-dose PTV. The SL V95%, V97%,
V98%, V100%, V102%, V105% and V107% of the prescribed
PTV dose (i.e: likely to correlate with skin toxicity) were
extracted for CT15 and CTdef and compared. In addition,
trendlines’ R2 of the graphs with Vd% of CT15 vs CTdef
were computed to assess correlation between the twos.
Then, as a first example of clinical application, skin dose
differences between fraction 15 and planning (V95%-
V107% of SL) were retrospectively analyzed for 8 patients
treated with SIB.
Results
The differences between SL2/SL3/SL5 V95%-V107% in CT15
and CTdef were very small (<1%/1cc Figure 1). The
correlation between SL DVHs parameters estimated on
CT15 and CTdef was high (mean R 2=0.91), with higher
correlation for lower doses (i.e.: V95%, R2: 0.97, 0.98 and
0.99 for SL2, SL3 and SL5, respectively). When looking to
the changes during HT, small average differences
between planned vs dose-of-the-day values of SL V95%-
V107% were found (< 2 cc), excepting one patient (out of
8) who showed a much more relevant difference between
the planned skin dose and the delivered dose at fr 15
(V102%=7cc for SL5, Figure 2).