In March 2004, the recommendations of the American Association of Physicists in Medicine (AAPM) on the interstitial brachytherapy dosimetry using and were reported in Medical Physics [TG‐43 Update: Rivard et al., 31, 633–674 (2004)]. These recommendations include some minor changes in the dose‐calculation formalism and a major update of the dosimetry parameters for eight widely used interstitial brachytherapy sources. A full implementation of these recommendations could result in unintended changes in delivered dose without corresponding revisions in the prescribed dose. Because most published clinical experience with permanent brachytherapy is based upon two widely used source models, the Model 6711 and Model 200 sources, in this report we present an analysis of the dosimetric impact of the 2004 TG‐43 dosimetry parameters on the history of dose delivery for these two source models. Our analysis indicates that the currently recommended prescribed dose of 125 Gy for Model 200 implants planned using previously recommended dosimetry parameters [AAPM dose prescription: Williamson et al., Med. Phys. 27, 634–642 (2000)] results in a delivered dose of 120 Gy according to dose calculations based on the 2004 TG‐43 update. Further, delivered doses prior to October 1997 varied from 113 to 119 Gy for a prescribed dose of 115 Gy compared to 124 Gy estimated by the AAPM 2000 report. For implants using Model 6711 seeds, there are no significant changes (less than 2%). Practicing physicians should take these results into account when selecting the clinically appropriate prescribed dose for interstitial implant patients following implementation of the 2004 TG‐43 update dose‐calculation recommendations. The AAPM recommends that the radiation oncology community review this report and consider whether the currently recommended dose level (125 Gy) needs to be revised.

To identify prognostic factors for grade 3 radiation dermatitis following passive-scattering proton therapy for breast cancer. This retrospective study included data on 23 (11 post-mastectomy and 12 post-lumpectomy) breast cancer patients who underwent proton therapy with the passive scattering technique in our institute from 2012 to 2016. Each patient received 50–50.4 cobalt Gy equivalent (CGE) at 1.8 or 2 CGE per daily fraction. Logistic regression analysis was performed to identify prognostic factors for grade 3 skin toxicity. Receiver operating characteristic (ROC) curve analysis and the area under the curve (AUC) were used to evaluate the performance of the models. 43% of the studied patients developed grade 3 radiation dermatitis. The dose-volume histogram (DVH) parameters of V52.5CGE and D10cm3 to skin5mm were correlated with grade 3 radiation dermatitis in both univariate and multivariate logistic regression analyses. Univariate logistic regression analysis suggested that D10cm3 to skin5mm (AUC = 0.69) and V52.5CGE to skin5mm (AUC = 0.70) were prognostic for grade 3 skin toxicity. The models using the combination of D10cm3 to skin5mm or V52.5CGE to skin5mm with breast volume marginally increased the AUC to 0.72 and 0.73, respectively. Models using the combination of D10cm3 to skin5mm or V52.5CGE to skin5mm with history of smoking increased the AUC to 0.75 and 0.83, respectively. In the current study, we identified prognostic factors for grade 3 radiation dermatitis in patients treated with passive-scattering proton therapy for breast cancer. This study provides promising tool for identifying high risk patients for whom treatment plan adjustment could be done to reduce the risk of radiation-induced grade 3 skin toxicity.

Background/Objectives: Traditionally, proton therapy assumes a fixed relative biological effectiveness (RBE) value of 1.1, which can lead to inaccurate dose calculations in treatment planning. This study examines the effect of dynamic arc delivery and pencil beam scanning (PBS) linear energy transfer (LET) optimization on LET-dependent RBE dose escalation to nervous tissue structures in head and neck (H&N) cancer patients undergoing proton beam therapy, utilizing two RBE dose models. Methods: Fifteen head and neck cancer patients previously treated with PBS proton therapy at high risk of nervous tissue toxicity were retrospectively analyzed. Three plans were developed for each patient: PBS, dynamic arc, and PBS with LET optimization. RBE-weighted dose distributions were calculated and compared for all patient plans using a linear LET-weighted model and a tissue-specific α/β-dependent model. LET-dose (LET_d) constraints were systematically tested to determine optimal values for plan quality and efficacy in altering LET spatial distribution in regions of concern. LET-dependent RBE enhancement was calculated for the three different planning methods. Results: Dynamic arc plans increased RBE enhancement when compared to PBS, while LET optimization for PBS consistently reduced RBE-enhanced dose to nervous tissue compared to non-optimized plans for both RBE models. Patient-specific variability in optimization benefit was observed, with the most significant improvements in cases with greater initial RBE enhancement. A maximum LET_d constraint of 2.5 µm/keV above 80% of the maximum structure dose threshold was found to balance plan quality and RBE mitigation. Conclusions: Dynamic arc delivery increased RBE enhancement relative to static PBS. LET optimization successfully modified LET_d spatial distributions to minimize RBE enhancement to nervous tissue structures when compared to non-LET optimized PBS and dynamic arc plans. Patient-specific risk stratification should be used when clinically deploying LET optimization.