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Effect of Pitch and Modulation Factor in Plan Quality for Helical Tomotherapy by Using AAPM TG119 Test Suites

Venkatesan Kaliyaperumal*, T Kataria, Maragatha Veni, TamilSelvan, D Manigandan, Thushant Nair, Asif Ahmed, Rajesh S, Medanta The Medicity, Gurgaon, Haryana, India,


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Pitch and modulation factor (MF) are the key parameters for planning in tomotherapy. Task group (TG)–119 test suites namely mock head/Neck, C-shaped and mock prostate provided for validating the intensity modulated radiation therapy. This study, analyze the impact on plan quality, beam on time and dose delivery accuracy by varying the pitch and modulation based on TG–119 test.

Based on TG 119 test contours, head/neck, prostate and C-shaped target were contoured in anthropomorphic phantom and plastic water phantom. Treatment plans were generated with varying pitch 0.1–0.3 and modulation factor ranging from 1.1–2.8 with dynamic field width of 2.5cm and fixed 1cm for all the targets as per TG–119 dose constraints. Homogeneity Index (HI) and conformity index (paddock CI) were calculated as per ICRU–83. Target coverage and normal structures dose (parotids, spinal cord, bladder and rectum) were compared between the plans. Treatment beam on time and dose delivery accuracy was analyzed (point dose verification, gamma passing percentage by using gafchromic fims).

Beam on time significantly decreases with increasing pitch and MF for all targets (9–2.8min). In all targets, HI and CI were improved with increasing pitch and MF and saturated beyond certain values. The similar pattern was observed in normal structures as well. Increasing pitch and MF reduced the normal structure doses. In pre–treatment plan quality assurance, all plans were within the acceptable limit (2%, >95% gamma passing criteria).

For head and neck cases, optimal plan quality was obtained with pitch values 0.28–0.3 with MF 2.3–2.7. In prostate cases, plan quality is higher when pitch value is between 0.25–0.3 with MF 2.3–2.7. In C-Shaped targets, pitch value is 0.2–0.3 with MF 2.3–2.7 provides the optimal plans.


Tomotherapy, Dosimetry


TH- External Beam- Photons: tomotherapy

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