Surface dosimetry for breast radiotherapy in the presence of immobilization cast material

Abstract

Curative breast radiotherapy typically leaves patients with varying degrees of cosmetic damage. One problem interfering with cosmetically acceptable breast radiotherapy is the external contour for large pendulous breasts which often results in high doses to skin folds. Thermoplastic casts are often employed to secure the breasts to maintain setup reproducibility and limit the presence of skin folds. This paper aims to determine changes in surface dose that can be attributed to the use of thermoplastic immobilization casts. Skin dose for a clinical hybrid conformal/IMRT breast plan was measured using radiochromic film and MOSFET detectors at a range of water equivalent depths representative of the different skin layers. The radiochromic film was used as an integrating dosimeter, while the MOSFETs were used for real-time dosimetry to isolate the contribution of skin dose from individual IMRT segments. Strips of film were placed at various locations on the breast and the MOSFETs were used to measure skin dose at 16 positions spaced along the film strips for comparison of data. The results showed an increase in skin dose in the presence of the immobilization cast of up to 45.7% and 62.3% of the skin dose without the immobilization cast present as measured with Gafchromic EBT film and MOSFETs, respectively. The increase in skin dose due to the immobilization cast varied with the angle of beam incidence and was greatest when the beam was normally incident on the phantom. The increase in surface dose with the immobilization cast was greater under entrance dose conditions compared to exit dose conditions.

Comparative evaluation of modern dosimetry techniques near low‐ and high‐density heterogeneities

“…A special design of MOSFET detector known as MOS kin for its dosimetry capabilities at skin surface and interfaces 26 , 27 , 52 , 53 , 54 , 55 , 56 , 57 , 58 was used in this study. MOS kins are real‐time detectors offering water‐equivalent effective depth (WED) of measurement of 0.02 or 0.07 mm, depending on type, developed at the Centre for Medical Radiation Physics (CMRP), University of Wollongong, Australia.…”

Dosimetric effects caused by couch tops and immobilization devices: Report of AAPM Task Group 176

“…Several groups have examined the effect of thermoplastic immobilization shells on surface dose for both photons and electrons and have concluded that they can significantly increase skin dose. 29,31,[50][51][52][53][54] Surface dose was found to decrease as the mask material was increasingly stretched which reduced the areal density and mask thickness (and reproducibility). For 6 MV photons, without stretching, the surface dose was 61% compared to 16% without a mask, but with stretching that increased the area of the mask by 125% or 525%, the surface dose changed to 48% or 29%, respectively.…”

Megavoltage cone beam CT near surface dose measurements: potential implications for breast radiotherapy

“…The MOSFETs surface dose response increases with increasing beam obliquity. 19 Previous studies have utilized this MOSFET for in-field skin and surface dosimetry. 19,26 Also for rectal wall interface dosimetry 27 and for brachytherapy applications.…”

“…19 Previous studies have utilized this MOSFET for in-field skin and surface dosimetry. 19,26 Also for rectal wall interface dosimetry 27 and for brachytherapy applications. 24 The accuracy of the MOSFET, utilized for this study, for out-of-field dosimetry was assessed.…”

“…16 The skin dose to various regions such as the contralateral breast from different breast radiotherapy techniques has been well reported. [17][18][19] Kilovoltage (kV) CBCT is the most common imaging modality provided by several vendors (XVIV R , Elekta Oncology Systems; OBI, Varian Medical Systems; Artiste, Siemens); however, MV CBCT (Ref. 1) still makes up a substantial proportion of the imaging systems integrated with linear accelerators.…”