Nuclear magnetic resonance analysis of a chemically cross-linked ferrous-methylthymol blue-polyvinyl alcohol radiochromic gel dosimeter.

A new formulation of hydrogel dosimeter consisting of ferrous-methylthymol blue (MTB)-polyvinyl alcohol (PVA) cross-linked chemically with glutaraldehyde (GTA) was studied and evaluated by nuclear magnetic resonance analysis by means of the R2 spin-spin relaxation rate. Previous optical studies of this transparent solid chemically cross-linked gel showed important dosimetric features in terms of sensitivity, auto-oxidation rate, and diffusion. This study shows that the MTB-PVA-GTA dosimeter has a reproducible linear dose response up to 40 Gy. For the optimum formulation of 0.1 mM MTB, 2.5% PVA, and 26.6 mM GTA, the measured R2 sensitivity was higher than that of traditional natural matrix-containing gels (MTB-gelatin) and all other reported PVA gel-based radiochromic dosimeters with MTB, xylenol orange (XO), or GTA (MTB-PVA, XO-PVA, XO-PVA-GTA). Additionally, the auto-oxidation rate was approximately ten times lower than that of the Fricke-MTB-gelatin system, which is consistent with the spectrophotometry results. The results of the independent experimental spectrophotometry and nuclear magnetic resonance analyses indicate that the transparent cross-linked dosimeter has good and consistent dosimetric features.

Adaptive radiotherapy in patients receiving neoadjuvant radiation for soft tissue sarcoma.

AIM: The aim of this study is to evaluate tumor volume changes during preoperative radiotherapy and to assess the role of adaptive radiation. BACKGROUND: Contemporary neoadjuvant radiotherapy utilizes image guidance for precise treatment delivery. Moreover, it may depict changes in tumor size and shape. MATERIALS AND METHODS: Between 2016 and 2018, 23 patients aged ≥18 years with soft tissue sarcoma were treated with neoadjuvant radiation followed by surgical resection. The tumor volumes (cc) were measured using the Pinnacle planning system prior to starting radiotherapy and during treatment, the changes in volume and absolute differences were estimated. Moreover, patient's position on the machine was evaluated to assess setup offsets. The triggers for plan adaptation were >1 cm expansion or unacceptable setup offsets. RESULTS: The mean tumors volume at presentation was 810 cc (range, 55-4000). At last cone beam CT the tumor volume had changed in 14 patients (61%); it was stable in nine patients (39%). Disease regression was documented in eight patients (35%), with median shrinkage of -20.5% (range, -2 to -29%), while tumor progression was observed in six cases (26%), the median change was 12.5% (range, +10 to +25%).Adaptive radiation was required in four patients (17%). For the remaining 19 cases (83%), the dose distribution was adequate to cover target volumes. CONCLUSIONS: Change in soft tissue sarcoma volume during radiation is not uncommon. Image guidance should be used to reduce setup errors and to detect differences in tumor volume. Image guidance and adaptive radiation are paramount to ensure optimal radiation delivery.