SU‐F‐T‐56: Dosimetric Characterization of the INTRABEAM 50 KV X‐Ray System with a Needle Applicator in Heterogeneous Tissues

Abstract

Purpose: We report the depth dose measurements in air, in solid water, and in bone materials for the Intrabeam 50 kV x‐rays with a needle applicator. Methods: The absolute dose was measured using a PTW TN34013W soft x‐ray ion chamber. Gammex tissue equivalent materials of solid water, inner bone, and cortical bone slabs (minimum thickness of 2 mm) were used. In addition, the PTW solid water slabs with a minimum thickness of 1 mm were used. The manufactory calibrated depth dose data in water were compared. The x‐ray source together with a needle applicator was secured on an Intrabeam stand. The slabs lay on a 6 degrees of freedom treatment couch with a digitally controlled minimum step size of 0.1 mm. The depth of the source to the ion chamber was accurately and reproducibly adjusted by moving the couch up and down. Results: The depth dose measurements for the Intrabeam 50 kV x‐rays with a needle applicator were conducted up to 20 mm in depth. The values for the PTW solid water were close to those for water. The Gammex solid water demonstrated lower values compared to water, consistent with the observation of its positive CT number. At a depth of 10 mm, the dose rates of the system are 29.6, 3.6, 1.2, and 0.24 Gy/min in air, in water, in inner bone, and in cortical bone, respectively. The 10 mm water equivalent depths in inner and cortical bone are about 6.4 and 4.1 mm. A function of power law combining exponential was used to fit and interpolate data well. Conclusion: Direct depth dose measurements in different materials provide a basis for treatment calculation and planning taking into account the heterogeneous effect. The results can be used for verification of analytical and/or Monte Carlo dose calculation methods as well.

Feasibility of dose escalation using intraoperative radiotherapy following resection of large brain metastases compared to post-operative stereotactic radiosurgery

“…Other critical organs were not compared in this study such as the scalp, ocular structures (such as lens or retina), or cochlea. Due to the low penetration through high density material such as the skull of the 50 kV X-rays used in IORT, scalp, ocular structures, or cochlea dose is always assumed to be lower than SRS [20, 24]. Furthermore, the relative biologic effectiveness of IORT versus SRS beam profiles were not accounted for, wherein at distances greater than 1–1.5 cm (such as was the case for most of the critical organs studied) the decreased relative biologic effectiveness of IORT 50 kV X-rays may further reduced the effective dose to critical organs and volume of normal brain receiving 12 Gy relative to that measured here-in [21, 25].…”