Room: AAPM ePoster Library
Purpose: of electron treatments are setup clinically with physicians palpating the tumor site and drawing a PTV that encompasses ~1cm margin for the electron block. Prescription is written based on percent depth dose (PDD) curves and desired isodose line for the adequate depth coverage. We evaluate variations in dose fall-off at the field edge as a function of energy, block size and SSD to ensure adequate target coverage in the treatment of small superficial lesions.
Methods: at reference depth were acquired from a Varian TrueBeam Linac for 6, 9, 12, 16 and 20MeV electron beams using a 3D water tank (IBA Blue phantom2). Open field sizes of 6, 10, and 25cm2 were used for SSDs of 100, 105 and 110cm. Then profiles were acquired for blocked fields of 4, 8, and 20cm2 respectively. Gafchromic EBT3 films were also used to get planar dose distributions for both open and blocked fields at reference depth in solid water phantom.
Results: the distance between 50% (geometric field edge) and 95% dose lines for each profile, marked differences were observed with addition of blocks as a function of field size, energy, and SSD. For example, for the 6x6 applicator, the 95% isodose line located 1.33cm from the field edge, shifted to 1.1cm when a 2cm block was inserted, resulting in 18.93% difference in target margin. Larger SSD’s dictate larger distances from the edge to achieve 95% coverage.
Conclusion: is advised in determining margins for blocked fields in treatment of small legions. Following the rule of thumb of 1cm margin around GTV to define the block size, which is prevalent in clinical electron setups, may not result in 95% PTV coverage of the distal edge. In most cases 1.5cm margin ensures better PTV coverage, and 2cm margin is more appropriate for extended SSD’s.