Investigation of pulsed IMRT and VMAT for re-irradiation treatments: dosimetric and delivery feasibilities

Abstract

Many tumor cells demonstrate hyperradiosensitivity at doses below ~50 cGy. Together with the increased normal tissue repair under low dose rate, the pulsed low dose rate radiotherapy (PLDR), which separates a daily fractional dose of 200 cGy into 10 pulses with 3 min interval between pulses (~20 cGy/pulse and effective dose rate 6.7 cGy min−1), potentially reduces late normal tissue toxicity while still providing significant tumor control for re-irradiation treatments. This work investigates the dosimetric and technical feasibilities of intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT)-based PLDR treatments using Varian Linacs. Twenty one cases (12 real re-irradiation cases) including treatment sites of pancreas, prostate, pelvis, lung, head-and-neck, and breast were recruited for this study. The lowest machine operation dose rate (100 MU min−1) was employed in the plan delivery. Ten-field step-and-shoot IMRT and dual-arc VMAT plans were generated using the Eclipse TPS with routine planning strategies. The dual-arc plans were delivered five times to achieve a 200 cGy daily dose (~20 cGy arc−1). The resulting plan quality was evaluated according to the heterogeneity and conformity indexes (HI and CI) of the planning target volume (PTV). The dosimetric feasibility of retaining the hyperradiosensitivity for PLDR was assessed based on the minimum and maximum dose in the target volume from each pulse. The delivery accuracy of VMAT and IMRT at the 100 MU min−1 machine operation dose rate was verified using a 2D diode array and ion chamber measurements. The delivery reproducibility was further investigated by analyzing the Dynalog files of repeated deliveries. A comparable plan quality was achieved by the IMRT (CI 1.10–1.38; HI 1.04–1.10) and the VMAT (CI 1.08–1.26; HI 1.05–1.10) techniques. The minimum/maximum PTV dose per pulse is 7.9 ± 5.1 cGy/33.7 ± 6.9 cGy for the IMRT and 12.3 ± 4.1 cGy/29.2 ± 4.7 cGy for the VMAT. Six out of the 186 IMRT pulses (fields) were found to exceed 50 cGy maximum PTV dose per pulse while the maximum PTV dose per pulse was within 40 cGy for all the VMAT pulses (arcs). However, for VMAT plans, the dosimetric quality of the entire treatment plan was less superior for the breast cases and large irregular targets. The gamma passing rates for both techniques at the 100 MU min−1 dose rate were at least 94.1% (3%/3 mm) and the point dose measurements agreed with the planned values to within 2.2%. The average root mean square error of the leaf position was 0.93 ± 0.83 mm for IMRT and 0.53 ± 0.48 mm for VMAT based on the Dynalog file analysis. The RMS error of the leaf position was nearly identical for the repeated deliveries of the same plans. In general, both techniques are feasible for PLDR treatments. VMAT was more advantageous for PLDR with more uniform target dose per pulse, especially for centrally located tumors. However, for large, irregular and/or peripheral tumors, IMRT could produce more favorable PLDR plans. By taking the biological benefit of PLDR de...

Volumetric-modulated arc therapy for oropharyngeal carcinoma: A dosimetric and delivery efficiency comparison with static-field IMRT

“…The dosimetric quality and treatment delivery characteristics have been evaluated by previous investigators [5–9,12–18] . Intrinsically, VMAT uses more beam angles and therefore spreads the incident/exit beams more evenly in surrounding normal tissues leading to more uniform surface doses.…”

“…In our institution, a systematic investigation of static field IMRT and single- and double-arc VMAT plans led to a clinical decision to allow for both IMRT (typically 7 to 9 fields) and VMAT with at least two arcs [12] . Lin et al demonstrated that VMAT (with two arcs) was more suitable for centrally located targets while IMRT was more advantageous for large, irregular-shaped or peripheral targets although accurate treatment delivery could be achieved with both modalities [18] . In a busy clinic with limited planning capabilities it would be impractical to perform comparisons of different treatment techniques (e.g., even between Arc1 and Arc2) for every patient.…”

Reirradiation of Prostate Cancer Local Failures After Previous Curative Radiation Therapy: Long-Term Outcome and Tolerance

“…Focal SBRT to the recurrent intraprostatic tumor (12) with internal organ immobilization (13) and rectal sparing (14), as well as combination of hyperthermia and RT (15) or delivery of pulsed low-dose-rate schedules (16), should be explored in very selected cases as alternative reirradiation modalities in accurately designed prospective trials.…”

Tissue TGF-β expression following conventional radiotherapy and pulsed low-dose-rate radiation

“…[1][2][3][4] This involves spreading out a conventional radiation therapy dose into short pulses of dose with breaks in between. By example, a PLDR regimen may involve delivering a dose of 2 Gy as 10 pulses of 0.2 Gy, each 3 minutes apart.…”

“…3,4 While cell survival data from cell lines and in vivo animal experiments have been published, no mechanistic data are available to explain the phenomenon of improved clinical toxicity with this treatment technique. Therefore, we performed in vivo experiments in mice to investigate the effect of PLDR and conventional radiotherapy (CRT) on normal tissues.…”