Potential for reduced radiation‐induced toxicity using intensity‐modulated arc therapy for whole‐brain radiotherapy with hippocampal sparing

Abstract

The purpose of this study was to retrospectively investigate the accuracy, plan quality, and efficiency of using intensity‐modulated arc therapy (IMAT) for whole brain radiotherapy (WBRT) patients with sparing not only the hippocampus (following RTOG 0933 compliance criteria) but also other organs at risk (OARs). A total of 10 patients previously treated with nonconformal opposed laterals whole‐brain radiotherapy (NC‐WBRT) were retrospectively replanned for hippocampal sparing using IMAT treatment planning. The hippocampus was volumetrically contoured on fused diagnostic T1‐weighted MRI with planning CT images and hippocampus avoidance zone (HAZ) was generated using a 5 mm uniform margin around the hippocampus. Both hippocampi were defined as one paired organ. Whole brain tissue minus HAZ was defined as the whole‐brain planning target volume (WB‐PTV). Highly conformal IMAT plans were generated in the Eclipse treatment planning system for Novalis TX linear accelerator consisting of high‐definition multileaf collimators (HD‐MLCs: 2.5 mm leaf width at isocenter) and 6 MV beam for a prescription dose of 30 Gy in 10 fractions following RTOG 0933 dosimetric criteria. Two full coplanar arcs with orbits avoidance sectors were used. In addition to RTOG criteria, doses to other organs at risk (OARs), such as parotid glands, cochlea, external/middle ear canals, skin, scalp, optic pathways, brainstem, and eyes/lens, were also evaluated. Subsequently, dose delivery efficiency and accuracy of each IMAT plan was assessed by delivering quality assurance (QA) plans with a MapCHECK device, recording actual beam‐on time and measuring planed vs. measured dose agreement using a gamma index. On IMAT plans, following RTOG 0933 dosimetric criteria, the maximum dose to WB‐PTV, mean WB‐PTV D2%, and mean WB‐PTV D98% were 34.9±0.3 Gy,33.2±0.4 Gy, and 26.0±0.4 Gy, respectively. Accordingly, WB‐PTV received the prescription dose of 30 Gy and mean V30 was 90.5%±0.5%. The D100%, and mean and maximum doses to hippocampus were 8.4±0.3 Gy,11.2±0.3 Gy, and 15.6±0.4 Gy, on average, respectively. The mean values of homogeneity index (HI) and conformity index (CI) were 0.23×0.02 and 0.96×0.02, respectively. The maximum point dose to WB‐PTV was 35.3 Gy, well below the optic pathway tolerance of 37.5 Gy. In addition, compared to NC‐WBRT, dose reduction of mean and maximum of parotid glands from IMAT were 65% and 50%, respectively. Ear canals mean and maximum doses were reduced by 26% and 12%, and mean and maximum scalp doses were reduced by 9 Gy (32%) and 2 Gy (6%), on average, respectively. The mean dose to skin was 9.7 Gy with IMAT plans compared to 16 Gy with conventional NC‐WBRT, demonstrating that absolute reduction of skin dose by a factor of 2. The mean values of the total number of monitor units (MUs) and actual beam on time were 719×44 and 2.34×0.14 min, respectively. The accuracy of IMAT QA plan delivery was (98.1±0.8) %, on average, with a 3%/3 mm gamma index passing rate criteria. All of these plans were considered clinically acceptable per ...

Volumetric modulated arc therapy (VMAT) for hippocampal-avoidance whole brain radiation therapy: planning comparison with Dual-arc and Split-arc partial-field techniques

“…In the present study, dac-VMAT technique comprises 2 coplanar full arcs with large field size covering the whole brain with reference to previous published studies [19][20][21]. The large irradiation field of whole brain PTV requires a larger jaw opening.…”

“…The dac-VMAT plans comprised 2 coplanar arcs of 359.8°each. Collimator rotation of 30°and 330°were used with reference to previous studies [19][20][21]. The isocentre was placed at the center point equidistant from both hippocampi.…”

“…Radiation Therapy Oncology Group (RTOG) 0933 is a single-arm phase II clinical trial that studies the effectiveness of hippocampal sparing in WBRT and has demonstrated promising results in terms of memory preservation using the dose criteria (Table 1) in the protocol [16]. In the meantime, dosimetric characteristics of dual-arc conventional volumetric modulated arc therapy (dac-VMAT) in WBRT with hippocampal sparing have been reported in previous studies following RTOG 0933 criteria [19][20][21]. The large irradiation field of dac-VMAT for whole brain planned target volume (PTV) required a wide jaw opening which may result in suboptimal multi-leaf collimator (MLC) movements as described in previous publication [22]: (1) Hardware restrictions for MLC movements; (2) Restricted MLC velocity from one gantry angle to another; (3) MLC may reach their limit of travelling distance when they are attempting to move to the distal part of the PTV.…”

Dosimetric Predictors of Acute and Chronic Alopecia in Primary Brain Cancer Patients Treated With Volumetric Modulated Arc Therapy

“…Data regarding volumes in cc were collected both for the whole scalp and for the areas of acute alopecia. The following dosimetric parameters were collected: dose received by 0.1 cc of the ROI (D 0.1cc ), mean dose (D mean ), absolute volumes that received 16,20,25,30,35,40, and 43 Gy (V 16Gy , V 20Gy , V 25Gy , V 30Gy , V 35Gy , V 40Gy , and V 43Gy ).…”

“…These studies differ from the present study for several reasons: firstly, the number of patients included was significantly lower (range 6(12)−17 (27) patients) compared to our experience; secondly, the prescription dose for WBRT (EQD2 28-36 Gy) is significantly different than the one used for primary tumors (EQD2 50, 4-60 Gy). Thirdly, most of them did not include clinical data on alopecia: plans of patients who had been previously treated with conventional opposed lateral fields were simply replanned with IMRT to confirm the potential advantage of IMRT techniques in reducing scalp dose (10,11,(28)(29)(30). Lastly, although some series including clinical evaluation of alopecia exist (12,27,28), their authors did not generate hypotheses about dose/permanent hair loss relationship and they did not provide clear dose constraints to minimize the risk of chronic alopecia.…”