Use of Pulsed Low-Dose Rate Radiotherapy in Refractory Malignancies

Pulsed low-dose rate radiotherapy for recurrent bone sarcomas: case reports and brief review

PURPOSE

Re-irradiation for bulky recurrent sarcoma carries significant risks. Pulsed low-dose rate radiotherapy (PLDR) is an attractive option for re-irradiation due to inherent radiobiological advantages.

MATERIALS AND METHODS

We present two patients who underwent reirradiation using PLDR technique, followed by a literature review.

RESULTS

The first case is that of a 76-year-old male who developed an in-field recurrence of a bulky pelvic bone high-grade chondrosarcoma after he was treated with definitive radiotherapy using helical TomoTherapy with a total dose of 66 Gy. The patient was re-irradiated using PLDR with a shrinking field technique; 50 Gy in 2 Gy fractions followed by a boost of 20 Gy in 2 Gy fractions. The patient remains disease-free without significant toxicity 60 months post-irradiation. The second case is that of an 82-year-old female who was treated with a definitive irradiation of 66 Gy in 33 fractions for a right shoulder grade II chondrosarcoma. She developed an in-field recurrence 28 months later and presented with bulky disease causing brachial plexopathy and lymphedema. The patient was re-irradiated with a palliative intent to a total dose of 50 Gy in 2 Gy fractions over 5 weeks using PLDR. Brachial plexopathy resolved shortly after re-irradiation, but local progression near the surface was evident 8 months later. She passed away from unrelated causes 11 months later.

CONCLUSION

We present two cases highlighting our early experience with PLDR, which was effective in the reirradiation of recurrent bony sarcoma. Our study highlights PLDR as an option for reirradiation in recurrent unresectable tumors.

Pulsed low dose-rate radiotherapy: radiobiology and dosimetry

Pulsed low dose-rate radiotherapy (PLDR) relies on two radiobiological findings, the hyper-radiosensitivity of tumor cells at small doses and the reduced normal tissue toxicity at low dose rates. This is achieved by delivering the daily radiation dose of 2 Gy in 10 sub-fractions (pulses) with a 3 min time interval, resulting in an effective low dose rate of 0.067 Gy min−1. In vitro cell studies and in vivo animal experiments demonstrated the therapeutic potential of PLDR treatments and provided useful preclinical data. Various treatment optimization strategies and delivery techniques have been developed for PLDR on existing linear accelerators. Preliminary results from early clinical studies have shown favorable outcomes for various treatment sites especially for recurrent cancers. This paper reviews the experimental findings of PLDR and dosimetric requirements for PLDR treatment planning and delivery, and summarizes major clinical studies on PLDR cancer treatments.

The Effect of High-Dose-Rate Pulsed Radiation on the Survival of Clinically Relevant Radioresistant Cells

We demonstrated that low dose pulsed radiation (0.25 Gy) at a high-dose-rate, even for very short intervals (10 s), decreases cell survival to a greater extent than single exposure to a similar total dose and dose rate. The objective of this study was to clarify whether high-dose-rate pulsed radiation is effective against SAS-R, a clinically relevant radioresistant cell line. Cell survival following high-dose-rate pulsed radiation was evaluated via a colony assay. Flow cytometry was utilized to evaluate γH2AX, a molecular marker of DNA double-strand breaks and delayed reactive oxygen species (ROS) associated with radiation-induced apoptosis. Increased cytotoxicity was observed in SAS-R and parent SAS cells in response to high dose rate pulsed radiation compared to single dose, as determined by colony assays. Residual γH2AX in both cells subjected to high-dose-rate pulsed radiation showed a tendency to increase, with a significant increase observed in SAS cells at 72 h. In addition, high-dose-rate pulsed radiation increased delayed ROS more than the single exposure did. These results indicate that high-dose-rate pulsed radiation was associated with residual γH2AX and delayed ROS, and high-dose-rate pulsed radiation may be used as an effective radiotherapy procedure against radioresistant cells.

Pulsed radiation therapy for the treatment of newly diagnosed glioblastoma

BACKGROUND

Pulsed radiation therapy (PRT) has shown effective tumor control and superior normal-tissue sparing ability compared with standard radiotherapy (SRT) in preclinical models and retrospective clinical series. This is the first prospective trial to investigate PRT in the treatment of patients with newly diagnosed glioblastoma (GBM).

METHODS

This is a single-arm, prospective study. Patients with newly diagnosed GBM underwent surgery, followed by 60 Gy of PRT with concurrent temozolomide (TMZ). Each day, a 2-Gy fraction was divided into ten 0.2-Gy pulses, separated by 3-minute intervals. Patients received maintenance TMZ. Neurocognitive function (NCF) and quality of life (QoL) were monitored for 2 years using the Hopkins Verbal Learning Test‒Revised and the European Organisation for Research and Treatment of Cancer QLQ-C30 QoL questionnaire. Change in NCF was evaluated based on a minimal clinically important difference (MCID) threshold of 0.5 standard deviation.

RESULTS

Twenty patients were enrolled with a median follow-up of 21 months. Median age was 60 years. Forty percent underwent subtotal resection, and 60% underwent gross total resection. One patient had an isocitrate dehydrogenase (IDH)-mutated tumor. Median progression-free survival (PFS) and overall survival (OS) were 10.7 and 20.9 months, respectively. In a post-hoc comparison, median OS for the prospective cohort was longer, compared with a matched cohort receiving SRT (20.9 vs 14 mo, P = 0.042). There was no decline in QoL, and changes in NCF scores did not meet the threshold of an MCID.

CONCLUSIONS

Treatment of newly diagnosed GBM with PRT is feasible and produces promising effectiveness while maintaining neurocognitive function and QoL. Validation of our results in a larger prospective trial warrants consideration.

Imaging prior to radiotherapy impacts in-vitro survival

Background and purpose

Cone Beam Computed Tomography (CBCT) is routinely used in radiotherapy to identify the position of the target volume. The aim of this study was to determine whether the CBCT dose, when followed by the treatment, influences the therapeutic outcomes as determined by in-vitro clonogenic cell survival in a radiobiological experiment.

Materials and methods

Human cell lines, four cancer and one normal, were exposed to a 6 MV photon beam, produced by a linear accelerator. For half of each sample, a prior imaging dose was delivered using the on-board CBCT. A sample size of n = 103 was used to achieve statistical power.

Results

The experimental group of cell lines exposed to CBCT imaging prior to treatment exhibited a reduction in mean cancer cell survival of ~17 times (p = 0.02) greater than predicted from the average dose response and equivalent to more than 5% of the therapeutic dose, compared to 11 times greater than predicted for normal cells (n.s.).

Conclusion

The greater than predicted reduction in survival resulting from the additional CBCT dose is consistent with radiation-induced bystander effects.