Abstract
Verification of dose to the anterior rectal wall in helical tomotherapy to the prostate is important due to the close proximity of the rectal wall to the treatment field. The steep dose gradient makes these measurements challenging. A phantom‐based study was completed, aimed at developing a system for measurement of anterior rectal wall doses during hypofractionated prostate stereotactic body radiotherapy (SBRT) utilizing tomotherapy delivery. An array of four dual MOSkin™ dosimeters, spaced 1 cm apart, was placed on a replica Rectafix® immobilization spacer device. This Perspex probe is a more rigid alternative to rectal balloons, to improve geometric reproducibility. The doses at each point were measured in real time and compared to doses calculated by the treatment planning system (TPS). Additionally, distance‐to‐agreement (DTA) measurements were acquired to assist in the comparison of measured and predicted doses. All dual MOSkin detectors measured dose to within ±5% of the TPS at the anterior rectal wall. Whilst several points were outside of experimental error, the largest deviation from the TPS predicted dose represented a DTA of only 1.3 mm, within the acceptable DTA tolerance of 3 mm. Larger deviations of up to −11.9% were observed for the posterior and side walls; however, if acceptable DTA measurements are accounted for, then an agreement of 75% was observed. Although larger differences were observed at the other rectal wall locations, the overall effect of dose at these points was not as significant, given the lower doses. Despite the very high‐dose gradient region, real‐time measurements of the anterior rectal wall doses were within acceptable limits of TPS‐predicted doses. The differences between measured and planned data were due to difficulties in precisely locating each detector on the TPS dose grid, which presented large variations in dose between CT voxels in regions of steep dose gradients. The dual MOSkin system would, therefore, be a useful device for detecting errors in real time, such as patient shifts or incorrect setup, during tomotherapy of the prostate.PACS numbers: 87.53.Ly, 87.55.km, 87.55.N‐
Highlights
108 Alnaghy et al.: In vivo dosimetry for tomotherapy boosts of 19 Gy in 2 fractions
MOSkin detectors were chosen for this method due to their superior measurement capabilities of absorbed dose to the anterior rectal wall, such as their ability for measurements in high-dose gradient areas.[5,9] The MOSkin (University of Wollongong, NSW, Australia), a MOSFET-based dosimeter, is a silicon detector designed to measure doses at air/skin interfaces.[5]. These detectors have a reproducible water-equivalent depth (WED) of 70 μm, which allows for measurements at this depth,(5) consistent with ICRU recommendations for skin dosimetry.[10,11,12] This is the depth that corresponds to the basal layer, which is the first radiosensitive layer of the epidermis.[9,13,14] To achieve this depth measurement, a thin Kapton film (DuPont, Willmington, DE) overlays the gate, which acts as a buildup layer
The results demonstrate that, even with a dose grid that is smaller than that utilized clinically for tomotherapy prostate boost treatments, the reliance on positional accuracy in a high-dose gradient region still has a significant effect on the results
Summary
108 Alnaghy et al.: In vivo dosimetry for tomotherapy boosts of 19 Gy in 2 fractions. SBRT has been shown to produce low rates of acute low urinary and rectal toxicity (Grade II).(1-3) Jabbari et al[2] completed a study with 20 patients treated with prostate SBRT as a monotherapy, with a prescription of 9.5 Gy in 4 fractions Another 18 patients were given a boost with SBRT of 9.5 Gy in 2 fractions, after their treatment with external-beam radiation therapy (EBRT) and androgen deprivation therapy (ADT). They found that 42% had acute Grade II gastrourinary and 11% of patients had acute Grade II gastrointestinal toxicity. Thirteen percent of patients experienced Grade II gastrourinary toxicity and 4% of patients experienced Grade II gastrointestinal toxicity, with no Grade III observed.[3]
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