ESTRO 36 Abstract Book

S812

ESTRO 36 2017

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Purpose or Objective

To demonstrate with end-to-end tests the ability of

RayStation v5.02 (RaySearch Laboratories AB, Stockholm,

Sweden) fallback planning module (RFP) to perform an

accurate Helical Tomotherapy (HT) to volumetric

modulated arc therapy (VMAT) plan conversion by

validating the dose-mimicking algorithm used during the

automatic optimization of the fallback plans.

Material and Methods

Thirty patient plans of various treatment sites previously

treated with HT were switched to 6 MV dual-arc VMAT

plans using RFP and default dose-mimicking algorithm

parameters. For the purpose of this study no further

optimizations were performed and delivery quality

assurance (DQA) were designed for each fallback plan.

DQA were delivered on a TrueBeam linear accelerator

(Varian Medical Systems, Palo Alto, CA) and

planar/absolute dose measurements were acquired using

the ArcCHECK diode array (Sun Nuclear Corporation,

Melbourne, FL) with an insert containing an Exradin A1SL

ionization chamber (Standard Imaging, Middleton, WI). 3D

dose distributions in the patient geometry were

reconstructed within 3DVH software (Sun Nuclear

Corporation, Melbourne, FL) by using ArcCHECK Planned

Dose Perturbation (ACPDP). Agreement between planned

and delivered dose was eventually evaluated with global

and local 2D/3D gamma-index analysis (3%/3mm and

2%/2mm criteria) and DHV-based comparisons were

performed using the following dosimetric parameters:

quality of coverage (Q=D98%/Dref), mean dose to target

(MDT=Dmean/Dref) and integral dose to organs at risks

(ID_OAR=∑·Di·Vi).

Results

Results of point dose measurements, gamma-index

analysis and HDV-based comparisons are listed in table 1.

Absolute dose differences were all <1% with an average

value of 0.4±0.4%. Average differences of gamma passing

rate (%GP) with a low-dose threshold of 10% of the

maximum dose were 99.9±0.2% and 99.2±1.2% (2D global

3%/3mm and 2%/2mm criteria), 95.9±3.4% and 89.4±6.5%

(2D local 3%/3mm and 2%/2mm criteria), 99.5±0.9% and

97.0±2.9% (3D global 3%/3mm and 2%/2mm criteria),

96.9±2.8% and 89.2±6.0% (3D local 3%/3mm and 2%/2mm

criteria) respectively. Finally, DVH-based comparisons

between calculated and delivered fallback plans showed

differences of respectively -0.5±0.8% for the quality of

coverage (Q), -1.0±0.7% for the mean dose to target (MDT)

and 0.3±0.9% for the integral dose to organs at risks

(ID_OAR).

Conclusion

Fallback planning is an advanced RayStation feature that

uses a dose-mimicking function to automatically

replicate the DVH and the dose per voxel of a given plan,

but for an alternative treatment machine or technique.

Results presented here through a Helical Tomotherapy to

VMAT plan conversion show a good agreement between

planned and delivered dose for point dose

measurements, gamma-index analysis and DVH-based

comparisons, hence validating the dose-mimicking

algorithm used during the automatic optimization of the

fallback plans.

EP-1531 Collimator angle influence on dose coverage

for VMAT SRS treatment of four brain metastases

C. Ferrer

, C. Huertas

, A. Castaño

, A. Colmenar

, R.

Plaza

, R. Morera

, A. Serrada

Hospital universitaria La Paz, Radiofísica y

Radioprotección, Madrid, Spain

Hospital universitaria La Paz, Oncología Radioterápica,

Madrid, Spain

Purpose or Objective

To evaluate the collimator angle influence on the dose

coverage of 4 brain metastases treated with volumetric

modulated arc therapy (VMAT) stereotactic radiosurgery

(SRS).

Material and Methods

Three brain metastases were prescribed to 18Gy, and a

fourth one located in the cerebellar tonsil to 16Gy.

Treatment was planned with Elekta Monaco treatment

planning system (v. 5.00.00), and optimized using

biological and physical based cost functions for mono-

isocentric VMAT SRS treatment on an Elekta Synergy linear

accelerator equipped with a 160-leaf Agility MLC. Five non

coplanar partial arcs were used, plus a full clockwise-

counterclockwise arc with 0° couch rotation to modulate

only the fourth lesion with different prescription and away

from the other three. Planning target volume (PTV)

coverage and dose to organs at risk (OAR) have been

evaluated for three different collimator angle positions,

5°, 45° and 95°. Treatment constraints were the same for

the three plans, one treatment plan for each collimator

angle.

Results

The best plan in terms of target coverage and number of

monitor units was achieved with collimator angle set to

95°, with the 95% of the PTV volume receiving more than

95% of the prescription dose for the 4 lesions, with 35.8%

less total MU compared with the 5° collimator angle plan

(5176 MU versus 8061 MU). The target coverage for the 45°

collimator angle plan was lower than for the other two

plans. OAR maximal doses were similar for the brainstem,

optic nerves and eye lens, but maximum dose to the optic

chiasm was 42% and 49.1% lower for the 5° collimator

angle plan compared with the 95° and 45° angle plan

respectively.

Conclusion

The choice of collimator angle influences the target

coverage as well as the total MU and the doses to OAR.

The optimal choice of this angle in VMAT SRS treatments

improves the optimization outcome.

EP-1532 ITV optimization for SBRT lung treatment

planning accounting for respiratory dose blurring