Characterization of small megavoltage photon beams for radiography

Abstract

English: Introduction The landscape of radiation treatment techniques is ever evolving in pursuit of improved target coverage. The latest techniques such as IMRT, SBRT, SRS and VMAT, provide improved target coverage by controlling the intensity of the given dose through the use of multiple small fields in contrast to large fields in conventional treatments. The advantage of using these large fields is that, their characteristics are fully understood. The introduction of small fields leads to improved coverage, but the physics of these fields are not fully understood. So, when used in patient treatment, it resulted in unaccounted radiation exposure due to inaccurate commissioning and inaccurate absolute dose calibration at these field sizes. The errors were due to incorrect detectors used for data collection, and incorrect application of factors when performing absolute dose calibration. This report investigated the characteristics of these small fields using different detectors whilst varying the SSD and the incident photon beam energy. The measurements included beam profiles, percentage depth dose (PDD) curves as well as the relative output factors (ROF). Materials and Methods The photon energies, 6 MV, 10 MV and 15 MV were delivered using the Synergy LINAC, which is equipped with Agility multileaf collimators (MLCs). The detectors that were investigated were the CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film and the EDR2 radiographic film. Measurements were carried out using water as a medium for the CC01 ion chamber, EFD-3G diode and the PTW60019. Films were placed in between water equivalent RW3 phantom slabs. These measurements were carried out at 90 cm, 95 cm, 100 cm and 110 cm source to surface distances (SSD). The field sizes that were investigated were 1×1 cm², 2×2 cm², 3×3 cm², 4×4 cm², 5×5 cm² and 10×10 cm², these fields sizes were set using Jaws and MLCs. The 10×10 cm² field size was included as a reference field. Results and Discussion The results showed that the beam profiles were insignificantly different at the various SSDs for the detectors. The EBT2 film showed the sharpest penumbra, with the EDR2 and the CC01 showing broad penumbrae, but the difference was negligible. The PDD measurements showed that the difference between the detectors after Depth of maximum dose (Dmax) were insignificant. The films differed significantly at shallower depths, and this can be attributed to setup, as well as the artefacts that showed up when the films were being analyzed. The PDD measurements indicated that the setup used for the films was not adequate for measuring the 1 cm square field sizes and below. Dmax was used to compare the detectors, though it did not vary greatly for the detectors, it was shown that there is a change in the manner in which this factor changes with field size. Below a certain field size, 2 cm for the 6 MV and 10 MV and 3 cm for the 15 MV, the Dmax would start shifting back to the surface instead of moving deeper as expected. The relative output factor (ROF) increased with energy, and this is true for all the fields which had lateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due to the onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gave the highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs. This showed that the density of the detector had an effect on the output factor measured. Conclusion The fields were characterized with the different detectors, barring the artefacts experienced with film measurements in some instances, these detectors can be used safely for the small fields. The ROFs can be measured at longer SSDs as they showed little variation due to increased SSDs.

Afrikaans: Inleiding Die aantal moderne beskikbare bestralingstegnieke is konstant besig om te vermeerder ter wille van beter teiken dekking. Die nuutste bestralingstegnieke soos IMRT, SBRT, SRS en VMAT bied beter teiken dekking deur die intensiteit van die gegewe dosis te verdeel in veelvuldige kleiner bestralingsvelde in plaas van die groot bestralings velde wat tydens konvensionele radioterapie gebruik word. Die voordeel van konvensionele radioterapie is dat die eienskappe van groot bestralingsvelde ten volle verstaan word. Die bekendstelling van klein bestralingsvelde kan lei tot beter teiken dekking, maar die fisiese wette van klein veld bestraling word nog nie ten volle verstaan nie. Wanneer klein veld radioterapie dus in pasiënt behandeling toegepas word kan onbeplande bestralingsblootstelling plaasvind as `n resultaat van onakkurate bundle karakterisering en die onakkuraatheid van absolute dosis kalibrasie vir klein velde. Hierdie foute is as gevolg van die feit dat die verkeerde bestralingsdetektore gebruik word en omdat faktore verkeerdelik toegepas word tydens absolute kalibrasie van bestralingsdosis. Hierdie verslag ondersoek die eienskappe van hierdie klein velde met behulp van verskillende bestralingsdetektore terwyl die SSD en die intree foton bundel energie verander word. Die metings sluit bundel profiele, persentasie diepte dosis (PDD) kurwes en relatiewe opbrengs faktore (ROF) in. Materiale en metodes Foton energieë, 6 MV, 10 MV en 15 MV was gelewer met behulp van die Synergy lineer versneller, wat toegerus is met Agility multipleet kollimators (MLCs). Die toerusting wat ondersoek was die CC01 ionisasie kamer, EFD-3G diode, PTW60019 mikro diamant detektor, EBT2 radiochromiese film en die EDR2 radiografiese film. Metings is geneem met water as medium vir die CC01 ionisasie kamer, EFD-3G diode en die PTW60019 mikro diamant detektor. Die films was geplaas tussen water ekwivalente RW3 fantoom vlakke. Metings is gemaak met ` bron-oppervlak afstande (SSD) van 90 cm, 95 cm, 100 cm en 110 cm. Die groottes van die velde wat ondersoek was, was 1 × 1 cm², 2 × 2 cm², 3 × 3 cm², 4 × 4 cm², 5 × 5 cm² en 10 × 10 cm ². Die veldgrootte van die verwysingsveld was 10 × 10 cm ². Resultate en bespreking Die resultate het getoon dat die bundel profiele nie beduidend verander het tussen die onderskeie SSDs vir die detektore nie. Die EBT2 film het die skerpste penumbra getoon. Die EDR2 en die CC01 het breë penumbrae getoon, maar die verskil was nie so beduidend nie. Die PDD metings het getoon dat die verskil tussen die detektore in die meting van diepte van maksimum dosis (Dmax) nie beduidend was nie. Die films het aansienlik verskil by vlakker dieptes, en dit kan toegeskryf word aan die opstelling, asook die artefakte wat gepresenteer het toe die films geskandeer was. Die PDD metings dui daarop dat die opstelling wat gebruik was vir die films nie voldoende was vir die metings vir 1 vierkante cm en kleiner veld groottes. Dmax was gebruik om die toerusting te vergelyk, al was die intertoerusting variasie min, was daar getoon dat verandering was in die manier hoe die faktore verander met veldgrootte. Onder 'n sekere veld grootte het Dmax vlakker begin beweeg in plaas daarvan om dieper te beweeg soos verwag word. Die veldgrootte waarteen die verskuiwing begin het verskil met die invallende foton energie, 2 cm vir die 6 MV en 10 MV en 3 cm vir die 15 MV. Die ROF het toegeneem met foton energie, en dit is waar vir al die veldgroottes wat laterale elektroniese balans gehad het (LEE). Die verhouding het verval soos die groottes van die velde afgeneem het as gevolg van die ontstaan van laterale elektroniese onewewigtigheid (LED). Die hoë-digtheid detector, PTW60019 het die hoogste ROF gegee vir die verskillende energieë, met die minder digte CC01 wat die laagste ROFs getoon het. Dit het getoon dat die digtheid van die detector 'n uitwerking op die gemete opbrengs faktor het. Gevolgtrekking Die velde was gekaraktariseer met die verskillende detektore, behalwe die artefakte wat ondervind was met film metings in sekere gevalle, kan hierdie toerusting met veiligheid gebruik word vir die kleinveld metings. Die ROFs kan gemeet word by langer SSDs omdat hulle min variasie getoon het as ‘n gevolg van verhoogde SSDs.