ESTRO 36 Abstract Book

S184

ESTRO 36

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DP (1 VMAT and 1Tomo) were observed. Multivariate

analysis showed, for both cases, a significant correlation

(p<0.05) between Homogeneity Index (HI) and both OAR

dose sparing and PTV coverage. Irradiation techniques

correlated with spinal cord sparing; however institutions

using similar/same delivery/TPS techniques produced

quite different dose distributions, highlighting the

influence of the planner experience on the optimization

process (figure 1).

Fig1 Box plot relative to the single metastasis case. 18

plans were computed using VMAT, 8 VMAT FFF (linac Free

of Flattering Filter), 6 Ciberknife, 5 Tomotherapy, 7 IMRT,

1 3dCRT.

Conclusion

At our knowledge, this is the largest non-sponsored

multicentre planning comparison. Differences in DVH

binning among centres could explain minor violations. HI

is a key factor for planning optimization: prescribing to

lower isodose generally leads to better OAR sparing and

higher PTV coverage. Results have a dependence on the

irradiation technique, although the planner's experience

plays a not negligible role. A multicentre analysis as

proposed in this study can have an impact on the

standardization of plan quality for spinal SBRT.

OC-0348 Reducing the dosimetric impact of variable

gas volume in the abdomen during RT of esophageal

cancer

P. Jin

, J. Visser

, K.F. Crama

, N. Van Wieringen

, A.

Bel

, M.C.C.M. Hulshof

, T. Alderliesten

Academic Medical Center, Radiation Oncology,

Amsterdam, The Netherlands

Purpose or Objective

For middle/distal esophageal tumors, a varying gas volume

in the upper abdomen could induce changes in the

dosimetry of RT. In this study, we investigated the

dosimetric impact of abdominal gas pockets as well as a

density override (DO) strategy to mitigate dosimetric

effects.

Material and Methods

We retrospectively included 1 patient with middle and 8

patients with distal esophageal cancer. For these patients,

it was unclear whether re-planning was needed due to the

varying gas volume during treatment. For each patient, we

measured gas volumes in the planning CT (pCT) and 8–28

(median: 14) CBCTs to assess possible time trends.

Further, we made IMRT and VMAT plans with a prescription

of 41.4Gy (preoperative RT) or 50.4Gy (definitive RT). For

IMRT/VMAT, a DO strategy (i.e., assigning mass density to

gas pockets in the pCT) with three settings was used: no

DO (denoted as DO=0), DO=0.5, and DO=1 (equivalent to

an adipose-muscle mixture), resulting in 6 plans per

patient. Next, by copying the gas pockets derived from the

available CBCT to the pCT, a fractional CT was simulated

to calculate the fractional doses using all 6 plans. DVH

parameters of the CTV and organs at risk (OARs) were

compared between 1) the three DO settings, 2) IMRT and

VMAT, and 3) fractional and planned dose. Dose

distribution difference in the CTV between fractional and

planned dose was also compared.

Results

The range of initial gas volume measured in the pCT was

56–732ml. The gas volume fluctuated over the treatment

course with no time trend (range of mean: 33–519ml,

range of standard deviation: 20–162ml). For the fractional

dose, V

95%

of the CTV was always >98% for VMAT but not

for IMRT with DO=0 (Fig.1). For both IMRT and VMAT, DVH

parameters of the CTV were significantly larger for DO=1

than for DO=0 and 0.5 (

<0.05, Wilcoxon signed-rank test).

For an increasing gas volume, an overdose (>3.5% higher

than the planned dose) in the CTV was found in 72–88%/64–

77% cases for IMRT/VMAT with all three DO settings. The

amount of overdose increased as the gas volume increased

relative to the initial volume and was >5% when the

increase was >100ml (Fig.2). For a decreasing gas volume,

an underdose (>3.5% lower than the planned dose) in the

CTV was found for IMRT/VMAT in 34%/23% cases with

DO=0, 7%/0% cases with DO=0.5, and 0%/0% cases with

DO=1. The underdose became more severe as the gas

volume decreased for DO=0 and 0.5. An overdose (>3.5%)

still existed in up to 28% cases for DO=1 when the gas

volume decreased. DVH parameters of OARs in the

fractional dose were almost the same as in the planned

dose and below the clinical constraints for all scenarios.

Conclusion

For esophageal cancer RT, the use of VMAT with DO=0.5 in

treatment planning is preferable to avoid an

overdose/underdose in the CTV when the abdominal gas

volume decreases during treatment. However, when the

gas volume increases with >100ml, a DO strategy would

result in an overdose >5%. Therefore, in that case re-

planning may be a better solution.