Session 9 radiation oncology

Master in Medical Physics 2015 to 2016

Practical Report of Radiation Oncology

Francisco J. Hernandez Flores

International Centre for Theoretical Physics

[email protected]

December 23, 2015

Abstract

In this practical session were evaluated two important point for the safety ofthe patient during the complex plan like VMAT or IMRT in radiation therapy: thefirst part evaluated was plan verification of the prostate patient and second part wasthe quality control in multileaf using gafchromic film. Using the octavius phantomwith the appropriated software was evaluated the plan of VMAT technique whente plan of patient is made with one of the advanced technique in radiotherapy isrequires a dedicated QA procedure for dosimetric verification of a planned dosedistribution to check for the agreement between a dose distribution calculatedby the Treatment Planning System (TPS) and the corresponding measured dosedistribution before to treat the patient. In this plan the acceptance criteria γ

was 98.7% taking in to account the strict analysis 3mm and 3% of dose pixelby pixel. The second point was inspection of the static leaf positions of MultileafCollimator (MLC) devices is essential for safe radiotherapy deliveries in bothstatic and dynamic modes. The purpose of this quality control in multileaf is toprovide accurate delivered dose when the patient is treated with the most advancedtechniques in radiotherapy (3D conformal, IMRT, IGRT and VMAT), which useirregular fields using multileaf collimators in a linear accelerator.

I. Introduction

Intensity-modulated radiation therapy (IMRT) and Volumetric Arc Therapy(VMAT) these techniques requires detailed verification of the multileaf collima-tor (MLC) leaves position, because geometric discrepancies between plannedand actual beam segments affect the treatment quality (increase the dose in thepatient), as demonstrated in literature both in terms of spatial and dosimetricerrors. For this reason, controls should be performed to verify leaves and jawssettings when using several small fields, both during acceptance and subsequentcontrols. [1]

The basic definition of the MLC leaf positions in the commercially availabletreatment distance between the leaf tips and the central line of the radiationsystems is the radiation field. Accordingly, the MLC leaf positions inspection

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should ideally be performed by measuring these distances practically and com-paring it with the MLC file coming from planning system. Besides, the centralline of the radiation field should ideally intersect the mechanical isocentre ofthe linac at the radiation center point.

II. Theory

Multi-Leaf collimators (MLC).Is a machine collimation system that incorporates a set of computer controlledleaves that allow the creation of user defined beam apertures.

Dynamic Multileaf CollimatorWhen MLC moves during treatment different parts of the field are shieldedresulting in different overall radiation levels delivered in different parts of thebeam: Intensity modulated radiotherapy.

• The transmission through the collimators should be less than 2% of theprimary (open) beam.

• The transmission between the leaves should be checked to ensure that itis less than the manufacturer’s specification.

MLC configurations: Comparison of the leaf travel configurations of commer-cially available MLCs fig.1. The maximum leaf extensions are compared to a40cm by 40cm maximum field size. [2]

Figure 1: Leaf travel in total field 40 by 40 cm

Transmission Requirements: Interleaf Transmission.Two important factors determine the cross-sectional shapes of the leaves

1. the leaves are focusing in the plane orthogonal to their travel and thereforehave to incorporate divergence

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2. have to overlap their neighbors to minimize interleaf transmission.

The fig.2 illustration of different leakage paths between leaves and the effectof leaf cross-section shape on penumbra along the size of an MLC leaf. Theoverall pitch of the leaf pattern may be 1 cm, the profile of a strip irradiatedby the retraction of a single leaf is somewhat wider (W’) and has a penumbrabroadened by the leaf side design, being governed by (W ′ −W)/2. [2]

In practice, the depths of the leaf side steps are only fractions of a millimeter,and so the broadening is quite small.

Figure 2: leakage paths between MLC

III. Materials and Methods

Linac accelerator synergy Elekta, Phantom octavius 4D with the appropriatedsoftware, Gafchromic film, red mosaiq and treatment planing system monaco.

The (common) VMAT quality control for verification in specific patientprocess the VMAT patient plan would be approved before to do the qualitycontrol of plan verification. Beams can remain as planed for the verificationplan, During verification process the planned treatment is transferred from thepatient to the phantom geometry, the treatment plan is recalculated on a CTscan of the phantom and then the dose distribution is measured using octaviusdevice and compared the fluence of dose measure with the fluence dose of thetreatment plan and the acceptance criteria is take in to account if the plan isapproved or not depending of pass criteria.

In case of quality control of multileaf colimator there are, several form toprove if the good movement of the multileaf, there is a test based in alphabeticletter, in other case used software for to check the good movement of themultileaf, but in our case we use the The analysis of the film irradiated with apicket fence pattern, and visual behavior of the leaf in the film.

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IV. Results and discussion

All plans of the patient made with the VMAT technique were analyzed usingthe three parameter %DA (limit 3%), DTA (limit 3%), (limit 3 mm), and γ-indexwith the 3% dose tolerance and 3 mm distance to agreement in relation to thetreatment planning system. The gamma criterion was considered fulfilled if γ

<1 in at least 95% of the points. Results confirmed a good agreement betweenthe two distribution with high and conformed dose to the target and low doseto the organ at risk in our practice the plan was evaluated with strict analysis3mm and 3% and obtaining a 98.7% of acceptance and pass criteria.

Qualitative examination of garden fence field. The analysis of the filmirradiated with a picket fence pattern shows that in both irradiated films theshifts are easy detectable at visual inspection. Fig.3 displays the differentpatterns used to perform the QA protocol.

Figure 3: Different fields pattern used in test of good positioning in multileaf collimator.

V. Conclusion

• An important benefit of this technique is the determination of a qualitymeasurement, termed the γ index, that indicates the difference betweenthe calculation and measurement relative to the acceptance tolerances.

• A γ-index distribution can be generated and displayed, providing a quan-titative assessment of the quality of the calculation, both in regions thatpass and fail the acceptance criteria.

• The accuracy of the delivered dose in the patient with advanced techniquesdepends on dosimetric characteristics of the multileaf collimators. Thereis an option for optimizing the jaws to the irregular MLC field to reducethe scattered radiation and interleaf radiation leakage beyond the field.

References

[1] Sharpe MB et al., Monitor unit settings for intensity modulated beams deliveredusing a step-and-shoot approach., Med Phys. 2000 Dec;27(12):2719-25.

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[2] Rossela Vidimari, Lecture of exercise in radiotherapy, ICTP Trieste Italy, 2nd

trimester 2015.

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Health & Medicine

Session 9 radiation oncology