Physica Medica: European Journal of Medical Physics - Volume 32 - Supplement 1 - February 2016

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A.65
EVALUATION OF DELIVERY ACCURACY OF HEAD AND NECK
RADIOTHERAPY WITH CONSTANT DOSE RATE VMAT
A. Didona *, C. Zucchetti, A.C. Dipilato, C. Fulcheri, R. Tarducci. Medical Physics
Department, Santa Maria della Misericordia Hospital, Perugia, Italy
Purpose: The aim of this work was to evaluate the delivery accuracy of a
volumetric modulated arc therapy that uses a constant dose rate (CDR-
VMAT) for head and neck cancer.
Materials and methods: For deliverability checks and dosimetric verifi-
cation purposes, we delivered 15 plans to the Octavius 4D® phantom (PTW,
Germany) that can rotate synchronously with the gantry, taking time- and
gantry angle-resolved dose measurements; the phantom contains a matrix
with 1405 ionization chambers arranged in a checkerboard pattern on
27 cm × 27 cm. The data were collected and evaluated by means of the
VeriSoft software package that is part of the Octavius® 4D system. All the
verified plans were generated by Pinnacle3 TPS (Philips); each plan con-
sisted of two 6 MV arcs (182°–178°; clockwise and counterclockwise
directions; final arc spacing 4°, collimator angle 45°). All the measures were
conducted with the use of a Varian Clinac 2100 DHX-S accelerator.
All the dose distributions acquired were compared to calculated ones by
Pinnacle TPS. A gamma evaluation, both local and global, was performed
with a dose threshold of 10% of the maximum dose. We used a 3%/3 mm
criteria and we evaluated gamma distribution for both the two arcs indi-
vidually and for the entire plan. The analysis was then conducted with a
2%/2 mm criteria. It was considered acceptable a passing score of 95% for
global gamma with 3%/3 mm criteria.
Results: 13 plans passed the global gamma analysis using the 3%/3 mm cri-
teria with an average passing score of 98.0% (96.1%–99.2%); for the individual
arcs we found an average passing rate of 98.4% (96.7%–99.1%) and 98.2%
(96.9%–99.1%). 2 plans scored less than 95% (89.2%, 93.7%), but a deeper
analysis showed that acquisition software started late for one of the
two arcs.
Conclusion: Our study showed a more than acceptable agreement between
delivered plans and calculated ones by TPS and it leads us to conclude that
CDR-VMAT plans can be accurately delivered.
A.66
VOLUMETRIC MODULATED ARC THERAPY FOR DELIVERY OF HEAD AND
NECK RADIOTHERAPY: COMPARISON BETWEEN CONSTANT AND
VARIABLE DOSE RATE
A. Didona C. Zucchetti C. Aristei R. Tarducci a Medical Physics
Department, Santa Maria della Misericordia Hospital, Perugia, Italy; b Radition
Oncology Department, Santa Maria della Misericordia Hospital, Perugia, Italy
Purpose: Volumetric modulated arc therapy (VMAT) is a form of intensi-
ty modulated radiotherapy that allows the radiation dose to be delivered
during continuous gantry rotation, using either a constant dose rate (CDR)
or variable dose rate (VDR). The aim of this work was to compare these
two techniques for delivery of head and neck (HN) radiotherapy.
Materials and methods: VDR and CDR plans were created for 15 patients
with various types of HN cancer. For each patient, 3 PTVs were defined with
prescribed doses of 66 Gy, 60 Gy and 54 Gy for 30 fractions. OARs in-
cluded mandible (MA), spinal cord (SC), brain stem (BS), parotids (PA),
salivary glands (SG), esophagus (ES), larynx (LA) and thyroid (TH). For both
CDR and VDR two 6 MV arcs (182°–178°; clockwise and counterclock-
wise directions; final arc spacing 4°, collimator angle 45°) were used. All
plans were performed with Pinnacle3 TPS (v 9.8) according to an internal
protocol; it was required that PTVs D95% be 95% of the prescribed dose
and OARs be spared as more as possible.
Dose distributions were compared by evaluating PTVs’ Dmean, D2%, D50%,
D98% and homogeneity index (HI). For MA, SC, BS the analysis included
D2%; for PA, SG, ES, LA and TH Dmean and a number of different dose-
volume data in the range V20Gy–V50Gy were compared.
To compare CDR and VDR, monitor unit (MU) and treatment delivery time
(TDT) were also evaluated.
Results: About PTV coverage, no statistically significant differences were
found. For OARs, VDR showed a reduction in LA and ES Dmean, V20Gy and
V30Gy, in TH Dmean, V45Gy and V50Gy and Dmean for PA and SG, but dif-
ferences were always within 1.5 Gy and all constraints were respected. No
significant differences were found in D2% for MA, SC and BS. TDT and MU
were lower for CDR, on average 20 sec and 50 MU respectively.
Conclusion: Our study showed that CDR offers levels of quality compara-
ble to VDR and a valid option of rotational arc radiotherapy for linacs without
variable dose rate.
A.67
A COMPARATIVE PLAN ANALYSIS: CONSTANT DOSE RATE VMAT AND
STEP-AND-SHOOT IMRT IN HEAD AND NECK CANCER
A. Didona * C. Zucchetti C. Aristei R. Tarducci a Medical Physics
Department, Santa Maria della Misericordia Hospital, Perugia, Italy;b Radiation
Oncology Department, Santa Maria della Misericordia Hospital, Perugia, Italy
Purpose: Constant dose rate VMAT (CDR-VMAT) introduces rotational arc
radiotherapy for linacs incapable of dose rate variation. The goal of this study
was to evaluate CDR-VMAT adequacy for the treatment of head and neck
(H&N) cancer compared to step-and-shoot IMRT.
Materials and methods: IMRT and CDR-VMAT plans were created for 15
patients. For each plan, 3 PTVs were defined (prescribed doses of 66 Gy,
60 Gy and 54 Gy, 30 fractions). OARs included mandible (MA), spinal cord
(SC), brain stem (BS), parotids (PA), salivary glands (SG), esophagus (ES),
larynx (LA) and thyroid (TH). For IMRT seven 6 MV coplanar beams were
used; CDR-VMAT plans consisted of two 6 MV arcs (182°–178°; clockwise
and counterclockwise directions; final arc spacing 4°, collimator angle 45°).
All plans were performed with Pinnacle3 TPS (v 9.8) according to an in-
ternal protocol; it was required that PTVs D95% be 95% of the prescribed
dose and OARs be spared as more as possible.
Dose distributions were compared by evaluating PTVs’ Dmean, D2%, D50%,
D98% and homogeneity index (HI). For MA, SC, BS the analysis included
D2%; for PA, SG, ES, LA and TH Dmean and a number of different dose-
volume data in the range V20Gy–V50Gy were compared.
For comparison of the efficiency of IMRT and CDR-VMAT, MUs and treat-
ment delivery time were also evaluated.
Results: Concerning PTV coverage, all the techniques were equivalent al-
though CDR-VMAT significantly improved HI and decreased D2%. For OARs,
all the techniques were able to satisfy all hard constraints; significantly better
results were found for CDR-VMAT that showed a reduction in the volume
of LA, TH and ES receiving medium and high doses and medium doses to
LA, TH, PA and SG. No significant differences were found in D2% for MA,
SC and BS. Compared with IMRT, CDR-VMAT reduced delivery times al-
though MUs were higher.
Conclusion: Our study showed that CDR-VMAT offers an additional option
of rotational arc radiotherapy for linacs without variable dose rate with a
lower cost.
A.68
EVALUATION OF SUPERFICIAL DOSE IN TOMOTHERAPY: A PHANTOM
STUDY
A. Dipilato C. Zucchetti M. Iacco R. Tarducci a Radiotherapy and
Oncology Department, Santa Maria della Misericordia Hospital, Perugia, Italy;
b Medical Physics Department, Santa Maria della Misericordia Hospital, Perugia,
Italy
Introduction: In helical tomotherapy treatment technique the dose dis-
tribution near the surface of the patient may be considerably different from
other forms of intensity modulated delivery and the calculated superfi-
cial doses are overestimated between 3% and 20% when the target extend
to the surface. The aim of this study is to evaluate the variation of skin dose
in helical tomotherapy treatment as a function of different contractions of
the planning target volume (PTV).
Materials and methods: A PTV was defined on a CT scan of a phantom
made of certified solid water slabs (16 × 16 × 12 cm 2) to simulate a 2 Gy
per fraction IMRT treatment. Four target volumes were created contract-
ing the PTV of 1, 3, 5 and 7 mm from the phantom surface (PTV eval). Four
identical helical tomotherapy treatment plans were inverse planned pre-
scribing the dose to the corresponding PTV eval and delivered on a
e20 Abstracts/Physica Medica 32 (2016) e1–e70