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Determination of exit skin dose for I 192 r intracavitary accelerated partial breast irradiation with thermoluminescent dosimeters
Author(s) -
Raffi Julie A.,
Davis Stephen D.,
Hammer Cliff G.,
Micka John A.,
Kunugi Keith A.,
Musgrove Jana E.,
Winston John W.,
RicciOtt Terresa J.,
DeWerd Larry A.
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3429089
Subject(s) - dosimeter , thermoluminescent dosimeter , brachytherapy , imaging phantom , nuclear medicine , dose profile , radiation treatment planning , medicine , dosimetry , irradiation , materials science , radiation therapy , radiology , physics , nuclear physics
Purpose Intracavitary accelerated partial breast irradiation (APBI) has become a popular treatment for early stage breast cancer in recent years due to its shortened course of treatment and simplified treatment planning compared to traditional external beam breast conservation therapy. However, the exit dose to the skin is a major concern and can be a limiting factor for these treatments. Most treatment planning systems (TPSs) currently used for high dose‐rate (HDR)I192 r brachytherapy overestimate the exit skin dose because they assume a homogeneous water medium and do not account for finite patient dimensions. The purpose of this work was to quantify the TPS overestimation of the exit skin dose for a group of patients and several phantom configurations. Methods The TPS calculated skin dose for 59 HDRI192 r APBI patients was compared to the skin dose measured with LiF:Mg,Ti thermoluminescent dosimeters (TLDs). Additionally, the TPS calculated dose was compared to the TLD measured dose and the Monte Carlo (MC) calculated dose for eight phantom configurations. Four of the phantom configurations simulated treatment conditions with no scattering material beyond the point of measurement and the other four configurations simulated the homogeneous scattering conditions assumed by the TPS. Since the calibration TLDs for this work were irradiated withC137 s and the experimental irradiations were performed withI192 r , experiments were performed to determine the intrinsic energy dependence of the TLDs. Correction factors that relate the dose at the point of measurement (center of TLD) to the dose at the point of interest (basal skin layer) were also determined and applied for each irradiation geometry. Results The TLD intrinsic energy dependence forI192 r relative toC137 s was1 . 041 ± 1 . 78 % . The TPS overestimated the exit skin dose by an average of 16% for the group of 59 patients studied, and by 9%–15% for the four phantom setups simulating treatment conditions. For the four phantom setups simulating the conditions assumed by the TPS, the TPS calculated dose agreed well with the TLD and MC results (within 3% and 1%, respectively). The inverse square geometry correction factor ranged from 1.023 to 1.042, and an additional correction factor of 0.978 was applied to account for the lack of charged particle equilibrium in the TLD and basal skin layer. Conclusions TPS calculations that assume a homogeneous water medium overestimate the exit skin dose for intracavitary APBI treatments. It is important to determine the actual skin dose received during intracavitary APBI to determine the skin dose‐response relationship and establish dose limits for optimal skin sparing. This study has demonstrated that TLDs can measure the skin dose with an expanded uncertainty( k = 2 )of 5.6% when the proper corrections are applied.