Effects of organ motion on IMRT treatments with segments of few monitor units

Abstract

Interplay between organ (breathing) motion and leaf motion has been shown in the literature to have a small dosimetric impact for clinical conditions (over a 30 fraction treatment). However, previous studies did not consider the case of treatment beams made up of many few-monitor-unit (MU) segments, where the segment delivery time (1-2 s) is of the order of the breathing period (3-5 s). In this study we assess if breathing compromises the radiotherapy treatment with IMRT segments of low number of MUs. We assess (i) how delivered dose varies, from patient to patient, with the number of MU per segment, (ii) if this delivered dose is identical to the average dose calculated without motion over the path of the motion, and (iii) the impact of the daily variation of the delivered dose as a function of MU per segment. The organ motion was studied along two orthogonal directions, representing the left-right and cranial-caudal directions of organ movement for a patient setup in the supine position. Breathing motion was modeled as sin(x), sin 4 (x), and sin 6 (x), based on functions used in the literature to represent organ motion. Measurements were performed with an ionization chamber and films. For a systematic study of motion effects, a MATLAB simulation was written to model organ movement and dose delivery. In the case of a single beam made up of one single segment, the dose delivered to point in a moving target over 30 fractions can vary up to 20% and 10% for segments of 10 MU and 20 MU, respectively. This dose error occurs because the tumor spends most of the time near the edges of the radiation beam. In the case of a single beam made of multiple segments with low MU, we observed 2.4%, 3.3%, and 4.3% differences, respectively, for sin(x), sin 4 (x), and sin 6 (x) motion, between delivered dose and motion-averaged dose for points in the penumbra region of the beam and over 30 fractions. In approximately 5-10% of the cases, differences between the motion-averaged dose and the delivered 30-fraction dose could reach 6%, 8% and 10-12%, respectively for sin(x), sin 4 (x), and sin 6 (x) motion. To analyze a clinical IMRT beam, two patient plans were randomly selected. For one of the patients, the beams showed a likelihood of up to 25.6% that the delivered dose would deviate from the motion-averaged dose by more than 1%. For the second patient, there was a likelihood of up to 62.8% of delivering a dose that differs by more than 1% from the motion-averaged dose and a likelihood of up to ~30% for a 2% dose error. For the entire five-beam IMRT plan, statistical averaging over the beams reduces the overall dose error between the delivered dose and the motion-averaged dose. For both patients there was a likelihood of up to 7.0% and 33.9% that the dose error was greater than 1%, respectively. For one of the patients, there was a 12.6% likelihood of a 2% dose error. Daily intrafraction variation of the delivered dose of more than 10% is non-negligible and can potentially lead to biological effects. We observed [for sin(x),...

Use of reduced dose rate when treating moving tumors using dynamic IMRT

“…( 4 , 5 ) We also found that the average of the doses calculated for each individual starting point in the respiratory phase gave a dose distribution that agreed with the expected dose distribution within 1%. However, Seco et al ( 1 ) showed that there are situations where although the average dose discrepancy will be small, the dose discrepancy can still be large for a minority of patients. This may happen when there is not an even distribution in the starting point of the respiratory cycle where the beam is turned on.…”

“…It has been shown that certain combinations of MLC sequence and target motion can give large dose discrepancies ( 1 , 2 ) for individual fractions. These effects generally average out when multiple fractions are used, ( 3 – 9 ) but Seco et al ( 1 ) found that this is not always the case (e.g., for step‐and‐shoot IMRT) and, for some situations, a dose error could still remain after multiple fractions.…”

Practical methods for improving dose distributions in Monte Carlo‐based IMRT planning of lung wall‐seated tumors treated with SBRT

“…Due to the motion of some lung tumors and issues such as the interplay effect with the leaf motion of the multileaf collimator (MLC), it has been noted that IMRT for hypofractionated lung tumors could be problematic, an issue which is minimized with a 3D approach 32 , 34 . The iterative reoptimization technique could increase the modulation factor, which could have an impact on the interplay effect, although the degree of this impact is unknown.…”

Does gated beam delivery impact delivery accuracy on an Elekta linac?

“…Using the Response kit, respiratory motion was simulated with a breathing period of 4 seconds in free‐breathing (FB) mode with a number of beam‐on to beam‐off combinations (1:3), (1.5:2.5), (2:2), and (3:1) (Table 4) and beam‐on/off times of 6 and 12 seconds to simulate a breath‐hold (BH) scenario (Table 4). 16 …”

“…The use of a tight gating window combined with beam‐on delays can result in a low number of monitor units (MUs) per deliverable segment. Previous studies have demonstrated the need to characterize beam stability for short irradiation times 14, 15, 16. Additionally, for breath‐hold‐based gating, the beam is held for an extended period between each delivery segment.…”