Dose Differences in ITV Planning of Time-dependent Anatomy

Poster Viewing Abstract 3430; Table

Methods dV (%) dB (%) dQ(%)

Rigid 0.4+/-0.7 55.2 +/- 7.8 35.7 +/- 10.1Deformable 41.2 +/- 5.9 10.7 +/- 2.3 45.6 +/- 8.2Hybrid 0.8 +/- 0.6 9.8 +/- 2.8 9.2 +/- 3.8

International Journal of Radiation Oncology � Biology � PhysicsS756

Purpose/Objective(s): New imaging methods for quantifying physio-

logical characteristics of tumor sub-volumes require robust registration

techniques for the comparison of images acquired at multiple time points.

A significant obstacle to this is heterogeneous deformability among the

structures to be assessed. Applying a rigid transform only can result in

gross misalignment of the lung tissue, while a purely deformable algorithm

induces unphysical changes in the target volume. We propose a hybrid

rigid/deformable registration algorithm that enables a more physically

realistic assessment of response in both the target and the normal

structures.

Materials/Methods: FDG-PET/CT scans are obtained for 9 patients with

NSCLC approximately two weeks before radiation therapy (pre-RT) and

one month after therapy (post-RT). The planning CT scans and associated

treatment plans are also obtained. The CT images of pre-RT and post-RT

are individually registered to the planning CT. A hybrid multi-modal

registration was developed to accommodate the multiple registration

targets: the target volume (defined by the intensity profile of the planning

ITV contour) is modeled as a rigid structure, while the global lung bronchi

are modeled as non-rigid structures. Rigid-only and deformable-only

registration approaches are also used as comparison techniques. The

registration performance is quantified with three quality measures: dV

(target volume difference due to registration), dB (bronchi difference due

to registration) and dQ (quality index). dQ is defined by a comparison of

the overlapping registered pre/post-RT target volumes, as derived by an

algorithm versus a manual registration. Lower values of all indices are

associated with a better registration result.

Results: Twenty-seven sets of CT images (Pre-RT, Post-RT and Planning)

are acquired, and a total of 3780 DICOM images are processed. The hybrid

approach achieves the best overall combined performance in terms of

target rigidity (dV), bronchial deformability (dB) and fidelity of the target

geometry relative to manual matching. The results of quality indices are

listed in the Table. The CT-to-CT deformation maps are successfully

applied to the corresponding PET images to enable future evaluation of

response in sub-volumes of interest.

Conclusions: We have developed a novel hybrid multi-modal registration

for lung radiation therapy evaluation. Clinical results show that the

registration method achieves superior performance in terms of both target

fidelity and lung deformation.

Author Disclosure: Y. Yue: None. M. Aristophanous: None. A.B. Chen:

None. J. Killoran: None. J. Yap: None. R.I. Berbeco: None.

3431Dose Differences in ITV Planning of Time-dependent AnatomyW. Watkins,1 J.A. Moore,2 B. Cai,1 N.R. Anderson,1 C. Dial,1 G.D. Hugo,1

and J.V. Siebers1; 1Virginia Commonwealth University, Richmond, VA,2Johns Hopkins University, Baltimore, MD

Purpose/Objective(s): Tumor motion and deformation create ambiguity

in target dose coverage when prescribing to an internal target volume

(ITV). This work quantifies dose differences between single image dose

approximations and deformed, accumulated dose for five lung cancer

patients for several different measures of target coverage.

Materials/Methods: IMRT plans are developed to deliver 70 Gy to 95% of

the ITV volume on a mid-ventilation image (mid-vent) and on an average

CT image (aCT) for five lung cancer patients. Dose calculations are per-

formed for delivery to each of ten phase-sorted 4DCT images, and

deformable dose accumulation is carried out utilizing a fast-symmetric

Demons algorithm for registration. The accumulated dose distribution (or

4DD) is compared to the single image dose (or 3DD) through five metrics,

the relative target volume at prescription isodose, minimum dose, and

gEUD for a Z 1, -5, and -20 to represent mean dose, radiosensitive tumor

coverage, and aggressive tumor coverage, respectively. Dose measures are

considered for the gross tumor volume (GTV) and a 5 mm isotropic

expansion of the GTV to represent the clinical target volume (CTV) in

each of the plans.

Results: In 9 of 10 plans, CTV prescription isodose volume is over-

estimated in 3DD compared to 4DD (mean, 1 standard deviation of 9.9%

� 9.7%, range of -5.1% to 19.8%). Mean dose differences (gEUDa Z 1) in

the CTV are 1.55 Gy � 1.20 Gy and are overestimated in all ten 3DD

calculations (range of 0.2 Gy to 3.8 Gy); however mean GTV dose

differences are not significant (0.78 Gy � 1.32 Gy, p Z 0.0948).

Increasing the importance of cold-spots within the target, the differences in

3DD and 4DD become patient specific, with 3DD overestimating CTV-

gEUDa Z -5 and GTV-gEUDa Z -5 in five of ten plans. Measured differ-

ences between 3DD and 4DD for gEUDa Z -5 range from -6.5 Gy to 3.8

Gy for CTV, and from -1.1 Gy to 3.7 Gy for GTV. Measured gEUDa Z -20

is also patient-specific for both CTV (range -3.9 Gy to 9.6 Gy) and GTV

(range -9.9 Gy to 2.3 Gy) and the magnitude of observed differences

increased with decreasing a. As the value of a is decreased in gEUD

computations, the correlation with minimum target dose increases from

0.658 for a Z -5 to 0.900 for a Z -20. Planning on aCT decreases gEUD

differences compared to plans on mid-vent in 12 of 15 CTV measurements,

and in 10 of 15 GTV measurements.

Conclusions: Differences in 4D-accumulated dose and 3D dose on a single

image are evident from dose volume histogram levels. However, for mean

dose these differences are veiled, which is expected since this is

a summation of similar dose values, assuming only slight deviations of

conservation of mass and energy among different 4D images. When

magnifying the importance of low dose voxels through decreasing a in

gEUD, differences in 3DD and 4DD become patient and plan specific, and

should be considered in radiation therapy planning.

Author Disclosure: W. Watkins: None. J.A. Moore: E. Research Grant;

Phillips Medical Systems. B. Cai: None. N.R. Anderson: None. C. Dial:

None. G.D. Hugo: E. Research Grant; P01CA11602. J.V. Siebers: E.

Research Grant; P01CA11602, Philips Medical Systems.

3432Impact of Dose-Volume Parameters and Tumor Location on SevereAcute Radiation Pneumonitis for Lung Cancer Patients TreatedWith Concurrent Chemoradiation TherapyJ. Wang,1 Y. Bao,1 T. Zhuang,2 Z. He,1 L. Zhang,1 A. Tai,3 P. Prior,3

M. Chen,1 and X. Li3; 1Department of Radiation Oncology, Cancer Center,

Sun Yat-Sen University, Guangzhou, China, 2Cancer Hospital of Shantou

University Medical College, Shantou, China, 3Department of Radiation

Oncology, Medical College of Wisconsin, Milwaukee, WI

Purpose/Objective(s): The purpose of this study is to investigate the

impact of radiation dose volume parameters and tumor location on the

incidence of severe acute radiation pneumonitis (SARP) for non-small cell

lung cancer (NSCLC) patients treated with concurrent chemoradiation

therapy.

Materials/Methods: Dosimetric and outcome data collected for 147

patients with NSCLC of stage IIIa-IV treated between 2006 and 2010 were

analyzed. Three-dimensional conformal radiation therapy (3DCRT) with

prescribed doses in the range of 50 - 70 Gy was delivered. SARP was

defined as � grade 3 radiation pneumonitis based on acute radiation

morbidity scoring criteria from RTOG. Correlations of SARP with a series

of dose-volume parameters and tumor location were investigated with

various statistical tools and were modeled with the Lyman-Kutcher-Bur-

man (LKB) model.

Results: The total incidence of SARP was 9.5% (14/147). In the whole

patient group, univariate logistic regression analysis showed that MLD

(mean lung dose), V20 (percentage volume receiving at least 20 Gy), V30,

V40, and V50 were determining factors for SARP with P Z 0.017, 0.025,

0.010, 0.009, and 0.027, respectively. The SARP incidence rates were

15.7% for the patient group with tumor in the middle or either of the lower