Physica Medica: European Journal of Medical Physics - Volume 32 - Supplement 1

Purpose:

To assess the clinical implications of the Collapsed Cone algo-

rithm implemented in the Masterplan Oncentra TPS in VMAT treatments

of nasopharyngeal tumors (NPC) versus the Pencil Beam algorithm (PBC).

Material and Methods:

Ten plans initially produced with the PBC algo-

rithm were recalculated using the CCC algorithm. Doses of 70.5, 60.0 and

55.5 Gy were prescribed in 30 fractions to the primary tumour (PTV70.5),

high-risk (PTV60.0) and low-risk nodal area (PTV55.5). The PTV70.5 was

also separated into tissue (PTVtiss) and air (PTVair) components. Differ-

ences in dose distribution for PTVs and critical organs were assessed using

different metrics. CCC plans were renormalized in order to obtain the same

D95% coverage of PBC calculation.

Results:

PBC algorithm overestimated the dose to PTVs for all considered

metrics. For PTV70.5: Dmean and D95% calculated by CCC decreased by 1.8%

(range:0.9–2.8%) and 3.1% (range:1.5–5.3%), respectively (1.5% and 2.8% lower

for PTVtiss, and 5.5% and 8.6% lower for PTVair). D98% decreased by 3.4%

(2.4% in tissue and 9.4% in air). V95% decreased from 96.0% to 90.2% (from

96.1% to 91.2% for PTVtiss, and from 96.0% to 70.9% for PTVair). The mag-

nitude of dose differences was strongly correlated with the amount of air

cavities in PTV70.5. Doses to spine and brainstem were found to be 1 Gy

lower with CCC. No differences were observed for parotids and mandible.

When the dose calculation was performed in water, the two algorithms pro-

vided differences in dose distributions

0.5%.

Conclusion:

CCC is recommended instead of PBC in order to avoid serious

overestimation in target doses. A key question remains open: should the

prescription dose be adjusted to the actually delivered dose, more accu-

rately predicted by CCC algorithm? If radiation oncologists would keep the

PBC original dose prescription and the same accepting criteria for target

coverage, when switching from PBC to CCC, up to 5% more radiation doses

would be given.

http://dx.doi.org/10.1016/j.ejmp.2016.01.114

A.111

BLADDER DOSE-SURFACE MAPS CORRELATE WITH ACUTE URINARY

TOXICITY AFTER RADIOTHERAPY FOR PROSTATE CANCER: FINAL RESULTS

OF A MULTICENTRIC STUDY

I. Improta

* , a ,

F. Palorini

a ,

C. Cozzarini

b ,

T. Rancat

i c ,

B. Avuzzi

d ,

P. Franc

o e ,

C. Degli Espositi

f ,

E. Garibaldi

g ,

G. Girelli

, C. Iotti

i ,

V. Vavassori

j ,

V. Valdagni

c , d ,

C. Fiorino

a .

San Raffaele Scientific Institute, Milano, Italy;

Radiotherapy, San Raffaele Scientific Institute, Milano, Italy;

Prostate Cancer

Program, Fondazione IRCCS Istituto Nazionale dei Tumori, Milano, Italy;

Radiation Oncology 1, Fondazione IRCCS Istituto Nazionale dei Tumori, Milano,

Italy;

Radiotherapy, Ospedale Regionale U.Parini, Aosta, Italy;

Radiotherapy,

Ospedale Bellaria, Bologna, Italy;

Radiotherapy, IRCCS, Candiolo, Italy;

Radiotherapy, Ospedale ASL9, Ivrea, Italy;

Radiation Therapy Unit, Department

of Oncology and Advanced Technology, ASMN Hospital IRCCS, Reggio Emilia,

Italy;

Radiotherapy, Cliniche Gavazzeni-Humanitas, Bergamo, Italy

Purpose:

To assess local dose predictors of acute urinary (GU) toxicity (tox)

in patients (pts) treated with radical RT for prostate cancer (PCa) by a pixel-

wise method for analysis of bladder dose-surface maps (DSMs).

Materials and Methods:

The final cohort of a prospective study was con-

sidered. It included 539 pts with PCa treated with conventional (1.8–2 Gy/

fx, CONV) or moderately hypo-fractionated RT (2.2–2.7 Gy/fx, HYPO).

GU tox was evaluated by the International Prostate Symptoms Score (IPSS)

comprising 7 questions relating to different symptoms: incomplete emp-

tying, frequency, intermittency, urgency, weak stream, straining and nocturia.

Acute tox was evaluated for each symp by considering an IPSS score

≥

4 at

RT end excluding pts with symps before RT.

DSMs of all pts were generated by cutting bladder contour anteriorly at

the points intersecting the sagittal plane passing through its centre of mass,

normalized in the axial direction and aligned at the posterior central point

of bladder base, generating a common frame. Average DSMs of pts with/

without tox were compared pixel by-pixel, for HYPO and CONV pts: the

resulting p-value maps were used for identifying the regions better dis-

criminating between pts with tox.

Results:

For HYPO pts, all endpoints (excepting weak stream) showed areas

significantly correlated with GU tox localized in the bladder trigone, with

evidence of a threshold effect around 85 Gy (2 Gy equivalent, α/β

10 and

recovery γ-factor

0.7 Gy/day). For CONV pts, only frequency and urgency

showed significantly predictive areas, summarized in the % surface receiv-

ing

50–70 Gy at 5 mm from the base and the vertical extension of 50–

70 Gy isodoses along the bladder central axis.

Conclusion:

The method of DSMs allowed identifying specific regions of

bladder whose sparing seems to be correlated with a decreased risk of

patient-reported acute urinary tox and promises to find important appli-

cations to be potentially applied outside the bladder case.

http://dx.doi.org/10.1016/j.ejmp.2016.01.115

A.112

DOSIMETRIC COMPARISON OF 3D CONFORMAL VS VOLUMETRIC ARC

RADIATION THERAPY VS HYBRID 3D/B-VMAT FOR MEDIASTINAL TUMOR

E. Infusino

, L. Bellesi, A. Carnevale, C. Di Venanzio, G. Gaudino, A. Mameli,

P. Matteucci, E. Molfese, G. Stimato, L. Trodella.

Universita’ Campus Biomedico

Di Roma, Roma, Italy

Introduction:

The study is to investigate a Hybrid 3D-CRT/B-VMAT (but-

terfly, multiple arcs) technique which 3D conformal radiation and volumetric

modulated arc therapy (VMAT) for the treatment of mediastinal tumor.

Material and Methods:

Three different treatment techniques were created

for 10 patients: 3DCRT, hybrid 3DCRT/B-VMAT and VMAT plans. The hybrid

plans were a combination of 3 partial arcs VMAT (2 coplanar arcs of 60°

gantry starting angles of 150° and 330°) and 1 noncoplanar arc of 60° gantry

(starting angle of 330° and couch angle of 90°) and non-coplanar field 3DCRT.

The dose distribution of planning target volume (PTV) and organs at risk

(OARs) for hybrid technique was compared with 3D-CRT and VMAT. Beam

energy was 6 MV, and all planning solutions were optimized by inverse

planning with specific dose-volume constraints on OAR (lungs, heart, esoph-

agus, spinal cord). Dose–Volume Histograms (DVHs), conformity number

(CN) and homogeneity index (HI) were calculated and then compared, both

for target and OAR by a statistical analysis (Wilcoxon’s Test).

Results:

Hybrid 3D/VMAT improved the target conformity compared with

3DCRT and VMAT. The mean CN was 0.75, 0.82, and 0.84 for 3DCRT, VMAT,

and hybrid plans, respectively. Hybrid 3D/VMAT also improved the PTV dose

homogeneity compared with 3D-CRT (1.01 vs 1.06; p

0.05) and VMAT (1.01

vs 0.89; p

0.05). Hybrid technique reduced V5, V10, V20, and mean lung

dose (MLD) of normal lung compared with 3DCRT and spared the OARs

better with fewer MUs with the cost of a little higher V5, V10, and MLD

of normal lung compared with VMAT. The maximum doses of spinal cord

for hybrid plans were 5.3 Gy lower than those for 3DCRT plans (p

0.05).

The mean doses of esophagus and heart for hybrid plans were 2.7% and

12.1% lower than 3DCRT plans (p

0.05).

Conclusions:

Hybrid technique improved the target conformity and ho-

mogeneity compared with 3DCRT and VMAT. Hybrid 3DCRT/B-VMAT can

be a viable radiotherapy technique with better plan quality.

http://dx.doi.org/10.1016/j.ejmp.2016.01.116

A.113

DOSE PAINTING BY NUMBERS (DPBN) GUIDED BY DIFFUSION-WEIGHTED

MRI APPLIED TO RECURRENT GLIOBLASTOMA (GBM): AN EXPERIMENTAL

PROTOCOL OF PHASE I/II

M. Iori

, a ,

P. Ciammella

a ,

M. Orlandi

a ,

D.E. Chiari

b ,

M. Campioli

c ,

L. Orsingher

a ,

A. Bott

i a ,

R. Sghedoni

a ,

E. Cagni

a ,

S. Cavuto

a ,

R. Pascarella

a ,

A. Pisanello

a ,

C. Iotti

a .

Arcispedale S. Maria Nuova – IRCCS, Reggio Emilia,

Italy;

Università di Trento, Trento, Italy;

Università di Modena e Reggio Emilia,

Reggio Emilia, Italy

Introduction:

Standard treatment for GBM is surgery followed by radio &

chemotherapy. However, disease recurs in almost all patients and after

failure, therapeutic re-irradiation is an option to be considered. The topic

of re-irradiation is generally controversial because of the risk of toxicity.

However, re-irradiation has been shown to be of value after local relapse

and the literature provides consistent data supporting both the feasibility

and the survival-lengthening capability of radiation compared with sup-

portive care only. To plan similar re-treatments, voxel-based re-irradiations

guided by magnetic resonance (MR) imaging were simulated with appar-

ent diffusion coefficient (ADC) used as biomarker for tumor cell density.

Materials and Methods:

The relapse areas of 6 GBM patients treated with

STUPP protocol and followed with MR images were analyzed to seek how

to define a possible signal to dose transfer function based on imaging

e33

Abstracts/Physica Medica 32 (2016) e1–e70