Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_8_3193 - Dosimetric Analysis of Flat versus Unflat beams for Frameless Image guided Trigeminal Radiosurgery

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Dosimetric Analysis of Flat versus Unflat beams for Frameless Image guided Trigeminal Radiosurgery
V. N. Bhaskar1, V. Shankar1, C. Haritha2, A. Karuppusamy1, A. Bhange1, S. Maurya1, B. ajai Kumar3, D. Sen1, S. Mutha4, R. Purohit5, K. Chigurupalli5, H. Vyas5, and P. Deepanjali5; 1HCG Cancer Center, Mumbai, India, 2CR Reddy Cancer Center, Nellore, Andhra Pradesh, India, 3HCG Cancer Center, Bengaluru, India, 4Inlaks & Budhrani Hospital, Pune, India, 5Geetanjali Cancer Center, Udaipur, India

Purpose/Objective(s): Radiosurgery is proven option in trigeminal neuralgia. Tough 5mm small collimators gives reasonable dose distributions in the planning systems, the dosimetric uncertainties arise due to beam dependent issues (focal size, llateral disequilibrium) or detector issues (Volume averaging and fluence perturbation effects, Corrections which depend on beam spot size). Given the subtle dosimetric uncertainties for a tiny 5mm target the present study focussed on dosimetric comparison between flat and unflat (FFF) beams wrt target coverage, brainstem doses, cochlear and temporal lobe doses used for frameless image guided trigeminal radiosurgery.

Materials/Methods: 20 consecutive patients aged between 33-88yrs with Trigeminal neuralgia (Typical -17, Atypical-3) underwent double reinforced patient- specific mask immobilisation, CT (0.75mm post contrast axial), MR simulation (0.75mm 3D space, 3-D TOF sequences) & treatment on C-Arm Linac using 6-DOF robotics and intrafraction imaging using flat beams. Following rigid fusion, Radiation doses of 75–80 Gy prescribed to the 90% isodose line were used with a single 5-mm isocenter positioned at the RGZ or the DREZ along the symptomatic trigeminal nerve. For purpose of study, plans were generated for flat and unflat beams using Monte-Carlo photon algorithm with 0.1mm grid and 0.5% statistical uncertainty and analysed for target dose conformity, homogeneity and dose coverage. In brainstem the volume doses D1%, D2% were taken for analysing the higher doses in the brain stem. The dose fall off was analysed in terms of D5% and D10%. In addition, max temporal doses and mean cochlear doses analysed. The p-values were calculated using two tailed Student's T-test and tabulated in appropriate tables.

Results: For flat versus unflat beams - the maximum dose within the trigeminal nerve target was 80±1.6Gy & 80±1.68 (P=0.3); marginal target dose – 54.7 ±1.3 & 55.7± 1.8 (p=0.4); The mean conformity index & Homogeneity index – 2.3±0.5 & 3.0±0.6 (p=0.4); 1.07±0.02 & 1.07±0.02(p=0.4). The mean D1%, D2% of the brainstem was 11.43±4.4Gy , 11.63± 4.3 (P=0.5316); 8.8±3.7, 9.0±3.7 (p=0.4) and the mean value of the maximum point dose within the brainstem was 25.0±8.6; 25.19±8.9Gy (p=0.3). Similarly there are no statistically significant difference between the brainstem D5% & D10%, temporal lobes (p=0.4)and cochlear doses (p=0.3). Statistically significant differences are noticed in the MU and the Beam on time for unflat and flat beams – (17,306± 1082, 17860±1060, p=0.02) & 13± 0.9, 35±1.7,p=0.01).

Conclusion: Ability to decrease the overall treatment time is a major trend in stereotactic radiation.With not much significant differences in the target dose coverage and OAR doses between 2 types of beams, FFF beams would be preferred for trigeminal radiosurgery due to drastic reduction in the beam on time and consequentially the overall treatment time, which hypothetically will increase the delivery accuracy due to decreased patient mobility.

Author Disclosure: V.N. Bhaskar: None. V. Shankar: None. A. Karuppusamy: None. A. Bhange: None. B. ajai Kumar: Partner; Health Care Global Enterprises. Stock; Health Care Global Enterprises. CEO; Health Care Global Enterprise. D. Sen: None. S. Mutha: None. K. Chigurupalli: None. H. Vyas: None. P. Deepanjali: None.

Valuvil Bhaskar, MD

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TU_8_3193 - Dosimetric Analysis of Flat versus Unflat beams for Frameless Image guided Trigeminal Radiosurgery



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