SUM MARYSkin-sparing properties of megavoltage photon beams are compromised by electron contamination. Higher energy beams do not necessarily produce lower surface and basal cell layer doses due to this electron contamination. For a 5xScm field size the surface doses for 6 MVp and 18MVp X-ray beams are 10% and 7 % of their respective maxima. However, a t a field size of 4Ox40cm t h e percentage surface dose is 42% for both 6 M V p and 18 M V p beams. The introduction of beam modifying devices such as block trays can further reduce the skin-sparing advantages of high energy photon beams. Using a 10mm perspex block tray, t h e surface doses for 6 M V p and 18MVp beams with a Sx5cm field size are 10% and 8%, respectively. A t 4Ox40cm, surface doses are 61 % and 63% for 6 M V p and 18MVp beams, respectively. This trend is followed a t the basal cell layer depth. A t a depth of 1 mm, 18MVp beam doses are always a t least 5% smaller than 6 M V p doses for t h e same depth a t all field sizes when normalized to their respective Dmax values. Results have shown that higher energy photon beams produce a negligible reduction of t h e delivered dose to the basal cell layer (0.1 mm). Only a small increase in skin sparing is seen a t t h e dermal layer (1 mm), which can be negated by the increased exit dose from an opposing field.
Purpose: Total Body Irradiation (TBI) treatments are mainly used in a preparative regimen for haematopoietic stem cell (or bone marrow) transplantation. Our standard regimen is a 12 Gy / 6 fraction bi‐daily technique. To evaluate the delivered dose homogeneity to the patient, EBT3 Gafchromic film is positioned at the head, neck, chest, pelvis and groin for all fractions. This work investigates and quantifies the build‐up dose characteristics at TBI distances and requirements for in‐vivo dosimetry bolusing. Methods: Percentage dose build up characteristics of photon beams have been investigated at large extended SSD's using parallel plate ionisations chambers (Attix) and EBT3 Gafchromic film. Measurements were made to open fields at different field sizes as well as large 40cm × 40cm fields with differing scatter conditions such as the introduction of standard Perspex scattering plates at different distances to the measurement point. Results: Percentage surface dose measured values for open fields at 300 cm SSD were found to range from 20 % up to 65.5 % for fields of 5 cm × 5 cm to 40 cm × 40 cm. With the introduction of 1cm Perspex scattering plates used in TBI treatments the surface dose values increased up to 83% to 90%, depending on the position of the Perspex scattering plate compared to the measurement point. Our work showed that at least 3mm water equivalent bolus / scatter material should be placed over the EBT3 for accurate dose assessment for TBI treatments. Conclusion: Build up dose characteristics exist at long (300cm) SSD's including treatments using Perspex scattering plates placed at various distances form the patient during TBI treatment. Top accurately assess the applied dose during treatment, in‐vivo dosimeters such as Gafchromic EBT3 should have at least 3mm bolus / scatter material placed over them to measure actual applied doses.
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