Medical Imaging and Radiotherapy Journal (MIRTJ) 39 (Suppl. 1) 37
Medical Imaging and Radiotherapy Journal (MIRTJ) 39 (Suppl. 1)
GRADIENTNA TEHNIKA PRI OBSEVANJU KRANIOSPINALNEGA 
PODROČJA Z UPORABO VOLUMETRIČNE LOČNE TERAPIJE
SEGMENT GRADIENT BASED TECHNIQUE FOR CRANIOSPINAL IRRADIATION WITH VMAT
Matevž Mlekuž
Onkološki inštitut Ljubljana, Zaloška cesta 2, 1000 Ljubljana, Slovenija / 
Institute of Oncology Ljubljana, Zaloška cesta 2, 1000 Ljubljana, Slovenia
Korespondenca / Corresponding author: mmlekuz@onko-i.si
Prejeto/Recived: 25. 11. 2019
Sprejeto/Accepted: 20. 2. 2020
IZVLEČEK
Uvod in namen: Namen prispevka je predstaviti prednosti 
volumetrično modulirane ločne terapije (VMAT, angl. 
volumetric modulated arc therapy) in uporabo gradientne 
tehnike (tj. tehnika, pri kateri ustvarimo postopno 
padajoč dozni profi l žarkovnega snopa) pri obsevanju 
kraniospinalnega področja (CSI, angl. craniospinal irradiation). 
V nadaljevanju prispevka je predstavljeno tudi preverjanje 
kakovosti obsevalnega načrta pred obsevanjem (QA, angl. 
quality assurance). 
Metode: Za namen raziskave sem izdelal osnoven obsevalni 
načrt VMAT s tremi izocentri. Vsakemu izmed izocentrov sem 
pripisal dva para prekrivajočih se ločnih žarkovnih snopov. 
Žarkovni snopi so zajeli področje glave in dve spinalni 
področji. Z namenom pridobitve postopno padajočega 
profi la doze, sem v področju prekrivanja žarkovnih snopov 
ustvaril deset dodatnih segmentov oz. kontur znotraj planirno 
tarčnega volumna (angl. planning target volume). Vsa 
področja z ustreznimi žarkovnimi snopi sem ločeno dozno 
optimiziral. Najprej sem optimiziral področje glave in spodnji 
spinalni predel, naknadno pa še zgornje spinalno področje 
in pri tem upošteval dozno porazdelitev že optimiziranih 
predelov. Za ovrednotenje občutljivosti oz. robustnosti (angl. 
plan robustness) obsevalne tehnike na premike pri nastavitvi 
pacienta (angl. set up errors) sem simuliral ± 3,5 in 10 mm 
premike v longitudinalni smeri. Normalizirane dozne profi le (% 
PDD) obsevalnih načrtov s simulacijo premika sem primerjal z 
osnovnim obsevalnim načrtom. 
Ustreznost prvotnega obsevalnega načrta sem preveril s 
primerjavo načrtovane in izmerjene doze. Za analizo sem 
uporabil gamma kriterij (γ), (ang. Gamma index), z nastavitvami 
3 % v dozi (DD, angl. dose diff erence) in 3 mm v oddaljenosti 
(DTA, angl. distance to agreement) med točkama primerjave.
Rezultati: Simulacija premikov pri nastavitvi pacienta ± 3, 
5 in 10 mm v longitudinalni smeri rezultira v ≈ 6,7 in 16 % 
razliko v % PPD glede na osnoven obsevalni načrt. Pri analizi 
načrtovane in izmerjene doze je 98,8 % merjenih točk manjših 
od γ vrednosti 1.  
Zaključek: Uporaba tehnike VMAT z gradientnim pristopom 
pri obsevanju kraniospinalnega področja omili dozimetričen 
učinek, ki nastane kot posledica napake pri nastavitvi pacienta. 
Analiza dozne porazdelitve kaže na ustrezno ujemanje 
izračunane in izmerjene doze. 
Ključne besede: obsevanje kraniospinalnega področja, 
volumetrično modulirana ločna terapija, tehnika gradientnega 
pristopa, napake pri nastavitvi pacienta, robustnost 
38 Medical Imaging and Radiotherapy Journal (MIRTJ) 39 (Suppl. 1)
Medical Imaging and Radiotherapy Journal (MIRTJ) 39 (Suppl. 1)
ABSTRACT
Introduction and purpose: To present advantages of 
volumetric modulated arc therapy technique (VMAT) 
with “segment gradient based approach” for craniospinal 
irradiation (CSI) and associated pre-treatment quality 
assurance procedure.
Methods: A three isocenter VMAT plan was designed. 
Each isocenter contained two pairs of overlapping partial 
arc fi elds that covered the cranial and two spinal parts. In 
the overlapping regions, an additional 10 segments were 
contoured in order to obtain a gradually decreasing dose 
profi le. All parts (cranial and two spinal) with associated partial 
arc fi elds were optimized separately. Cranial and lower spinal 
parts were optimized fi rst. Subsequently, upper spinal part was 
optimized by taking into account the dose contribution from 
previously optimized parts. To investigate plan insensitivity 
against the set up errors ± 3, 5, and 10 mm, longitudinal shifts 
were simulated. Normalized dose diff erence profi les (%PDD) 
with original plan were evaluated.
For plan verifi cation phantom dose was calculated and 
compared with the measured dose. Analysis was performed 
using gamma index (γ) criteria with settings 3% of dose 
diff erence (DD) and 3mm of distance to agreement (DTA).
Results: Simulating set up errors ± 3, 5 and 10 mm in 
longitudinal direction % PPD versus the original plan, were 
≈ 6,7 and 16%, respectively. Plan verifi cation dose analysis 
revealed that 98.8% of measured points were within γ index<1.
Conclusion: The VMAT with “segment gradient based 
approach” for CSI has turned out to be a favorable technique 
in terms of its robustness to set up errors. Dose distribution 
analysis showed an appropriate calculated and measured 
dose matching. 
Keywords: craniospinal irradiation, gradient based approach, 
set up errors, robustness 
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