Commissioning and optimization of a total skin electron therapy technique using a high rate electron facility

Abstract

English: Total skin electron therapy (TSET) is the treatment of choice for several malignant diseases of the skin (Kaposi sarcoma, mycosis fungoides). Several different techniques have been developed in various centers, in order to achieve homogeneous dose distribution over a large irradiation field (200 x 80 ern"). However, to implement a TSET technique one has to account for a variety of parameters, from geometric (room design, space constrains) to physical (number, angle and energy of the beams). To obtain the most acceptable dose distribution an extensive set of measurements and a large number of calculations have to be performed. Therefore Monte Carlo simulation of TSET can facilitate optimization of this technique. In this study we implemented and optimized a TSET technique using 4 and 6 MeVelectron beams. The dosimetrie procedure intended to obtain adequate dose uniformity over the entire surface of the patient, and to reduce the patient treatment time using a high dose rate facility on the Elekta Precise accelerator. The EGS4/BEAM code package running on a Windows based platform was used for the MC simulation. Percentage depth-dose curves and beam profiles were calculated and measured experimentally for the 40x40 cm2 nominal field at both 100 cm SSD and at the patient surface at the treatment plane (SSD 350 cm) for a single beam. The accuracy of the simulated beam was validated by the good correspondence (within less than 2%) between measured beam characteristic parameters (Rso, dmax, Rp) and Monte Carlo calculated results. To obtain a uniform profile vertically, two vertical angles of incidence were used. The angle between the two beams that gave best uniformity was considered the optimum angle. The patient is to be placed on a rotating platform perpendicular to the beam and rotated through 60 degree increments to obtain six horizontal directions of beam incidence. The doses expected in the patient were measured with Kodak EDR2 films positioned at different levels between slices of a Rando phantom. TLDs were placed on the surface to relate the film measurements to dose. The delivered doses in the treatment plane were compared to simulated data that was obtained from the MC simulation. The penetration depth of the dose distribution varied over various scanning directions between 2-3 mm and 3- 4 mm for 4 and 6 MeV respectively. This information is useful when treatment of lesions of different thickness are being considered. The composite percentage depth dose of all six dual fields for both 4 and 6 MeV yielded an 80 % dose at - 7 mm and - 9 mm depth, respectively. Good dose uniformity was achieved for both energies and it was about ± 5% for 4 MeVand about ± 3% for 6 MeVover a range of - 100 to +100 cm. The bremsstrahlung contamination was 0.9 and 1.3 %. Generally there was good agreement between the dose distribution calculated with MC and measured with films, thus validating our MC calculations. The dose distributions in phantom were found to comply with the guidelines described in the AAPM TG-23 protocol, showing the suitability of this technique for treatments of the skin diseases. The HDRE is a useful operational mode providing reasonable output, field size, and Xray contamination. Use of a dual field technique produces reasonable beam uniformity over an area large enough to allow total skin electron therapy in a conventional treatment room. Monte Carlo techniques provided a guiding principle to assist the verification of the beam characterization of a TSET technique. The absolute calibration of dose to the patient required the measurement of the ratio "skin dose to calibration point dose"; thiswas achieved by measurements with a parallel plate ionization chamber and TLDs.

Afrikaans: Heelliggaam elektron terapie (TSET) is die behandeling van keuse vir verskeie maligne siektes van die vel (Kaposi sarkoom, mycosis fungoides). Verskeie tegnieke is ontwikkel deur verskillende sentra om 'n homogene dosisverspreiding oor 'n groot stralingsveld (200 x 80 crrr') te bewerkstellig. Om egter die TSET tegniek te implementeer moet daar 'n verskeidenheid parameters in ag geneem word, van geometries (kamer ontwerp, ruimtelike beperkings) tot fisies (aantal, invalshoeke en energieë van die velde). Om die mees aanvaarbare dosisverspreiding te verkry moet 'n omvattende stel metings sowel as berekenings gedoen word. Om hierdie rede kan Monte Carlo (MC) simulasie van TSET die optimisering van die tegniek vergemaklik. In hierdie studie is 'n TSET tegniek geïmplementeer en ge-optimiseer vir 4 en 6 MeVelektronbundels. Die doel van die dosimetriese prosedure was om aanvaarbare uniformiteit van dosis oor die hele oppervlakte van 'n pasiënt te verkry en om die behandelingstyd te verkort deur van 'n hoë dosistempo opsie (HDRE) op die Elekta Precise versneller gebruik te maak. Die EGS4/BEAM program wat op 'n Windows gebaseerde platform geïnstalleer is, is vir die MC simulasie gebruik. Persentasie dieptedosis krommes en bundelprofiele is bereken en ook gemeet vir die 40x40 cm2 nominale veld by beide 100 cm SSD en by die pasiënt oppervlak by die behandelingsvlak (350 cm SSD) vir 'n enkelveld. Die akkuraatheid van die gesimuleerde veld is geverifieer deur die goeie ooreenkoms (binne 2% ) tussen gemete karakteristieke bundelparameters (R5o, dmaks , R, ) en MC berekende resultate. Om 'n uniforme vertikale profiel te verkry is twee vertikale invalshoeke gebruik. Die hoek tussen die twee velde wat die beste uniformiteit gegee het is as die optimale hoek aanvaar. Die pasiënt salop 'n roterende platform staan, loodreg op die invalsrigting van die veld en sal roteer word deur 60 grade intervalle om ses horisontale invalshoeke te bewerkstellig. Die verwagte dosis in die pasiënt is gemeet met Kodak EDR2 film wat op verskillende vlakke tussen die snitte van 'n Rando fantoom geplaas is. TLDs is op die oppervlakte geplaas om die filmrnetings te koppel aan dosis. Die gelewerde dosisse in die behandelingsvlak is vergelyk met data verkry van die MC simulasies. Die penetrasiediepte van die dosisverspreiding oor verskillende skandeerrigtings het gevarieer tussen 2-3 mm en 3-4 mm vir die 4 en 6 MeVonderskeidelik. Hierdie inligting is waardevol wanneer behandeling van letsels met verskillende diktes oorweeg word. Die saamgestelde persentasie dieptedosis van al ses dubbelvelde vir beide 4 en 6 MeV het 'n 80% diepte van - 7 mm en - 9 mm opgelewer. Goeie dosisuniformiteit is vir beide energieë verkry en dit was ± 5% vir 4 MeV en ± 3% vir 6 MeV. Die bremsstrahlung kontaminasie was tussen 0.9 en 1.3 %, In die algemeen was daar goeie ooreenkoms tussen die dosisverspreidings wat bereken is met MC en die wat gemeet is met film, wat dus die geldigheid van die MC berekenings bevestig, Die dosisverspreidings in die fantoom het voldoen aan die riglyne beskryf in die AAPM TG-23 protokol, wat die toepaslikheid van hierdie tegniek vir behandeling van velsiektes bevestig.Die HDRE opsie is 'n nuttige operasionele tegniek wat 'n redelike bundelopbrengs, veldgrootte en x-straal kontaminasie lewer. Die gebruik van die dubbelveld tegniek lewer redelike bundeluniformiteit oor 'n oppervlakte wat groot genoeg is vir heelliggaam elektronbestraling in 'n konvensionele behandelingskamer. MC tegnieke verskaf 'n riglyn om te help met die verifikasie van die bundel eienskappe vir TSET. Die absolute kalibrasie van die dosis aan die pasiënt vereis die meting van die verhouding "veldosis tot kalibrasiepunt dosis"; dit is verbry of bereek of gaedaen deur metings met 'n parallelplaat ionisasiekamer en met TLDs,