Dosimetric Analysis and Acute Toxicity in Craniospinal Irradiation Using Helical Tomotherapy: Experience from a Tertiary Cancer Center

doi:10.21203/rs.3.rs-4365851/v1

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Dosimetric Analysis and Acute Toxicity in Craniospinal Irradiation Using Helical Tomotherapy: Experience from a Tertiary Cancer Center

https://doi.org/10.21203/rs.3.rs-4365851/v1

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Purpose

The purpose of this study was to assess the feasibility and efficacy of helical intensity-modulated radiation therapy (IMRT) for Craniospinal Irradiation (CSI) on tomotherapy, aiming to enhance target coverage while minimizing radiation dose to nearby organs at risk (OARs) and to assess acute toxicities.

Methods

It was a prospective observational single-arm study, 24 patients meeting inclusion criteria were enrolled, with most common histology being medulloblastoma. Planning was executed using the Accuray Precision system, adhering to European Society of Paediatric Oncology guidelines for contouring. Dosimetric analysis encompassing target coverage, conformity, homogeneity, and OAR sparing was performed. Acute side effects were assessed using CTCAE 4.0.

Results

Tomotherapy showed acceptable target coverage and spared organs at risk (OARs), although some patients exceeded QUANTEC dose limits due to factors like target volume, prescribed dose and proximity to OARs. Hematological and skin-related adverse events were observed, mostly mild to moderate. These toxicities were generally acceptable. Plans with 2.5 cm field width had higher toxicity rates compared to 5 cm width, attributed to increased monitor units and beam-on times.

Conclusion

Tomotherapy demonstrated effectiveness in target coverage and OAR sparing, careful consideration of dose limits and field width is crucial to minimize toxicity risks. These findings contribute to the understanding of the feasibility and safety profile of Tomotherapy in Craniospinal Irradiation. Larger cohorts and extended follow-up periods are necessary to evaluate late toxicities comprehensively.

Craniospinal Irradiation

Tomotherapy

Helical IMRT

Dosimetric analysis

Toxicity

Craniospinal irradiation (CSI) plays an important role in the comprehensive treatment approach for central nervous system (CNS) tumors that tend to spread along the neuroaxis through cerebrospinal fluid pathway. Such tumors include Medulloblastoma (most common), Pinealoblastoma, Supratentorial Primitive Neuroectodermal Tumor (PNET), Anaplastic ependymoma, Ependymoma, Germinoma or Intracranial Germ Cell Tumor, Atypical Rhabdoid/Teratoid Tumor, Intracranial Rhabdomyosarcoma etc.^1–9

General management of these tumors typically involves surgery, aiming for maximum safe resection, owing to advancements in microsurgery techniques. This is followed by more effective chemotherapy regimens and postoperative radiotherapy, utilizing modern technologies such as CSI with posterior fossa or primary surgical bed boost, with or without chemotherapy, except for germ cell tumors. Germ cell tumors, or germinomas, are usually treated with radiotherapy alone, often in the form of CSI with a primary boost. CSI also serves as a salvage treatment for patients with disseminated neuraxial disease.

Planning, verification, and delivery of CSI remain technically challenging due to the goal of achieving uniform dosage throughout the CSF spaces, including the subarachnoid space, intracranial vault, spinal canal, and ventricles. This complexity arises from the need for uniform and homogeneous radiation dose distribution in a lengthy and complex-shaped target volume.¹⁰

Fundamental treatment planning for CSI entails using lateral parallel opposed fields for the cranium and upper cervical spinal canal, along with a matching posterior spinal field to cover the entire spinal subarachnoid space.¹¹ For larger patients, employing separate upper and lower posterior spinal fields is advised. Regularly shifting junctions between fields, including cranio-spinal and spinal-spinal junctions, ensures a smooth transition of dose, minimizing hotspots and cold spots. This practice, known as feathering, involves adjusting fields by 5 mm on each side during irradiation. Feathering, typically done after 5 to 7 fractions, promotes a more uniform dose distribution along the spinal cord, reducing the risk of overdose or underdose.¹²

Challenges associated with administering anesthesia to young children, along with the relative discomfort experienced in the prone position, have contributed to the transition to CSI being performed in the supine position. The field planning for CSI has advanced from traditional reliance on bony landmarks using 2D radiographs to the latest computed tomography (CT) simulation techniques. This shift has been further facilitated by the widespread availability of CT simulation in radiation oncology departments. CT-based conformal treatment planning in a supine position is now widely adopted and recommended.^13–16

Conformal planning for CSI can be achieved through various techniques with photon beams such as 3-dimensional Conformal Radiotherapy (3DCRT), Volumetric Arc Therapy (VMAT), Intensity Modulated Radiation Therapy (IMRT) using conventional linear accelerators, or Helical tomotherapy. Apart from these techniques, electron and proton beams can also be used for treatment delivery.^17,18 These highly conformal techniques can reduce the dose to structures anterior to the vertebrae, albeit at the cost of exposing a larger volume of the body to low-dose irradiation.

Helical tomotherapy offers a novel approach to cancer radiotherapy, especially beneficial for CSI due to its continuous beam delivery along the patient's entire length, eliminating the need for junctions or beam matching found in conventional techniques.^19,20 With a 6 MV Linear Accelerator mounted on a ring gantry, tomotherapy delivers radiation in a helical fashion, treating large cylindrical volumes of up to 40 × 160 cm^2 effectively. Dose calculations, typically performed using Anisotropic Analytical Algorithms (AAA), prioritize optimizing Organ at Risk (OARs) doses while maintaining target coverage. Plan optimization parameters include fan beam thicknesses, pitch values, and modulation factors tailored to individual patient needs.

CSI is associated with various acute toxicities, including nausea, vomiting, and fatigue, which are usually manageable. However, irradiation of red bone marrow may lead to moderate hematologic toxicity, sometimes necessitating treatment adjustments and supportive care. Long-term effects, especially in growing children, encompass neurocognitive impairment, growth issues, hormonal imbalances, and increased risks of secondary malignancies.¹⁰

The unique features of tomotherapy have been explored by several research groups for CSI, yielding promising dosimetric results. This analysis aims to conduct a dosimetric analysis of tomotherapy in CSI to assess improvements in target coverage, homogeneity, and conformity, as well as the sparing of normal tissue in terms of maximum and mean doses while studying acute toxicities from a cohort of patients treated in a prospective feasibility study of tomotherapy-based CSI at a tertiary cancer center.

This was a single-arm, prospective analytical study done at our institute. A total of 24 patients with brain tumors necessitating treatment of the entire neuro-axial axis were enrolled in the study. Patients underwent appropriate neurosurgical procedures or diagnostic imaging modalities, including biopsy, surgical resection, Cerebrospinal fluid (CSF) cytology, Magnetic resonance imaging (MRI) of the brain, and whole spine screening. Patients were enrolled after obtaining ethical committee approval and written informed consent from those aged 18 years or older, while parents provided consent for children under 18 years of age. Patients had Karnofsky Performance Scale Index (KPS) scores between 60 and 100. Patients receiving financial support from government beneficiary schemes or willing to pay were included.

Pre-treatment evaluations, including complete blood profiles, renal and liver function tests, coagulation profiles, serum, and CSF hormone levels, as well as radiological imaging such as CT or MRI, were conducted. Post-operative MRI of the brain with spine screening, histopathological reports, CSF cytology, and baseline visual and auditory assessments were also obtained for each patient.

The planning CT images were acquired of a “Somaton Confidence” model of “Siemens Healthineers” CT simulator with a 5 mm thickness of slice from vertex to upper 1/3rd thigh with proper immobilization using devices like thermoplastic casts and vacloks. Pediatric patients were immobilized under general anesthesia. IV contrast was given to all the patients unless medically contraindicated at the rate of 1ml/kg.

The planning was done in “Accuray Precision system version 2.0.1.1 (5)”. For the contouring of the target an Organ-at-risk (OARs), SIOPE – brain tumor group consensus guidelines of craniospinal target volume delineation for high-precision radiotherapy were used.²¹ For pediatric patients, whole spinal vertebras were contoured in Planning target volume (PTV) of the spine to evaluate homogenous dose distribution to all vertebras due to the growing age of the patient.

The dose prescription was given as per disease protocols.²² The median CSI dose was 36 Gy in 20 fractions (range: 19.5–36 Gy) and the mean dose was 33.4 Gy. The median dose per fraction was 1.8 Gy (range: 1.5–1.8 Gy). 21 (88%) patients were given boost to brain after completion of CSI while 2 (8%) patients received boost to spine after completion of CSI who were having disseminated gross spinal cord metastatic disease. 15 (63%) patients were given concurrent chemotherapy during radiation which included weekly Vincristine 1-2mg/m^2 or daily carboplatin 15mg/m^2 for the first 15 fractions of CSI.

Helical Intensity-modulated Radiation therapy (Helical IMRT), and Direct-IMRT (Tomo-Direct) planning were done. All the CSI and brain boost plans were made of Helical IMRT with inverse planning technique. Patients with drop metastasis in the spine who required additional spine boost were planned with tomo-Direct IMRT planning with 0, 160, 180, and 200 degrees 4 beam plan. The parameters like Field width, pitch, and modulation factor were noted for each plan for comparison and dosimetric evaluation purposes. The Median Width, Pitch, and Modulation factor for CSI plans were 5 cm, 0.3, and 3 respectively. For brain boost plan, the values were 1 cm, 0.3, and 3 respectively and for spine boost plan, the values were 2.5 cm, 0.25, and 2.2 respectively.²³ Anisotropic Analytical Algorithm (AAA) with 2.5 mm spatial resolution was used as dose calculation algorithm. Planning was done with prescription to 70 to 80% isodose line which covers 100% of PTV. ¹⁹

Mean doses and Mean Integral doses to PTVs, OARs, Patient’s body, and Body minus PTV, Conformality index, and Homogeneity index of PTVs were evaluated for each plan. Conformality Index and Homogeneity index were calculated by the precision planning system according to radiation therapy oncology group (RTOG) guideline formula.^24–27 Dosimetric evaluation for the target was done in terms of V100% of GTV, V98% of CTV, and V95% of PTV. Hotspot inside PTV was evaluated as V107% and cold spot inside PTV was evaluated in terms of volume (cc) receiving dose less than 90%. Low doses of Body minus PTV were evaluated in terms of V5Gy and V1Gy. OAR dosimetry was evaluated according to Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) guidelines. The plan approval was done by the clinical criteria which includes that 95% of the planning target volume (PTV) should receive at least the prescription dose and restriction the maximum dose limit to 107% as recommended by the International Commission of Radiation Unit and Measurements report 50 (ICRU-50).²⁸ The dose to the surrounding critical organs were accepted as much as possible within their tolerance limits as defined in the QUANTEC report. ²⁹

After the plan approval, QA plan in the treatment planning system (TPS) is then delivered on the cheese phantom and the dose is measured and compared with approved plan dose. Dose deviation less than 3% between TPS plan and measured dose on phantom was approved for treatment delivery.

The treatment was delivered on “Tomotherapy Radixant-X9” Linear accelerator. MVCT scan provided by accuracy for the tomotherapy model was used for daily matching of setup errors. MVCT scan of three regions including whole brain including upper cervical vertebras, dorsal vertebras (D1 to D5) and Lumbo-sacral vertebras (L3 to S2) were taken. The images are registered default with respect to the spinal bony anatomy and the skull by the console.^19,30,31 Manual registration was done by the Radiation Oncologist and Technologist with respect to all the targets and OARs. After obtaining 3 shifting values of 3 different MVCT regions, an average is applied for all 6 areas of movements and the patients were treated on the average values. As tomotherapy cannot correct pitch and yaw shifting they were kept at zero during treatment.

Acute side effects were mainly bone marrow and skin related which were assessed during the course of radiation. Assessment was done with Common toxicity criteria (CTC) grading system.^32,33

The study cohort consisted 24 patients, reported to the Radiation Oncology OPD between August 2020 and January 2023 with notable variations in histology, risk stratification, prior therapy, and prognosis.

Table 1: Patient characteristics, treatment details, and clinical outcomes of the study cohort

The age range of patients was between 2 and 70 years, with a median age of 15.5 years (range: 2–51 years). Among these, 14 patients were pediatric with a median age of 10.35 years (range: 2–18 years), and 10 were adults with a median age of 30 years (range: 20–51 years). Most patients had medulloblastoma (15 out of 24).

The mean fractional MU (Monitor unit) for CSI was 1190.77 (Range: 5624.9–29160.68). The mean beam-on time was 666.3 seconds (Range: 323–1685.6). The beam-on time is typically between 5 to 28 minutes which is relatively longer compared to conventional techniques. The MVCT imaging before treatment was also quite lengthy to acquire, which took up to 10 to 15 minutes of imaging time per fraction daily. Therefore, the total time the patient is on the table from setup to end of the treatment typically varied between 20 minutes up to 45 minutes for an adult and from 15 to 30 minutes for children.

The mean coverage of GTV V100% of brain was 99.77% and for spine was 96.4%. The dosimetric parameters for PTV combined coverage showed a Dmax of 35.5 Gy (106%) and a Dmean of 33.02 Gy (98.86%), with no instances of V107% observed and only 0.45 cc (0.02% of PTV volume) of the volume receiving dose < V90%. Additionally, the Conformality Index (CI) was measured at 1.08, and the Homogeneity Index (HI) at 1.1.

Evaluation of OARs dosimetry suggested acceptable dosimetry according to QUANTEC guidelines excepts V5Gy of lung which was > 42% in 17 patients out of 24. However, V20Gy < 31% and Dmean were within acceptable limits.^34–36 8 patients out of 24 received mean dose to cochlea > 45Gy.^37,38 Further analysis suggested than those 8 patients received brain boost RT in posterior fossa (high-risk medulloblastoma cases) in which cochlea was covered in the PTV target of boost. 16 patients out of 24 received a Dmax of > 54Gy for brainstem, maximum value being 56.06%, however D1-10cc received by brainstem was < 59Gy in all of the patients.²⁹ Constraints of optic chiasma were not achievable in 3 patients, they received Dmax of > 55Gy, all 3 of them being pineal tumors which was justifiable due to close proximity between target and chiasma.²⁹ Dmean, Integral dose, V80%, V50%, V20%, V10%, V5% to OARs were comparable with standard published data.^18,39–43

Bone marrow toxicity during treatment included hematological parameters like leukopenia Grades 1–4 (18.75%, 50%, 18.75% and 12.5%, respectively), anemia Grades 0–4 (25%%, 37.5%, 25%, 12.5% and 0%, respectively) and thrombocytopenia Grades 0–4 (25%, 37.5%, 31.25%, 6.25% and 0%, respectively). The most common acute GI toxicities were vomiting, nausea, loss of appetite, weight loss and. Skin toxicities were between grades 1–4 (56.25%, 37.5%, 6.25% and 0%, respectively). The study analyzed field width, fractional MU, and beam-on time in relation to acute toxicities. A 2.5 cm field width exhibited higher fraction MU and beam-on time (15553.43 and 894.32 sec) compared to 5 cm width (9158.66 and 525.86 sec), resulting in shorter treatment duration and less patient radiation exposure. Incidence of > = grade 2 toxicities was 73.33% in 2.5 cm width plans versus 42.42% in 5 cm plans. Nine out of 24 patients required radiation breaks due to acute toxicities.

At a median follow-up of 20 months (range 5.8–34.5), the 2-year progression-free survival (PFS) and overall survival (OS) in months (with standard deviation) and percentage for the entire cohort were 12.29 months (± 7.16) (75%) and 18.58 months (± 7.96) (79.2%), respectively. Outcomes within the cohort varied significantly depending on disease biology. Outcomes were excellent for medulloblastoma, classified as a disease with favorable biology, with a 2-year OS of 86.7%, compared to miscellaneous histologies with an OS of 66.7% (P-value = 0.337) (Fig. 2).

Figure 2: Kaplan-Meier Survival Curves for (2A): Progression-free survival (PFS) for entire cohort and (2B): overall survival (OS) (in months) estimates for entire cohort and (2C): comparative analysis between medulloblastoma and other histologies.

CSI poses challenges due to the length of the target area, requiring the matching of radiation fields and leading to dose inhomogeneity at junctions with conventional 2D techniques. With advancement in conformal radiotherapeutic techniques which includes 3DCRT, IMRT and VMAT, dose homogeneity was achieved better compared to 2D technique but the issue of field matching remains constant. Field matching is a difficult technique and it may result in cold or hotspot throughout the target which makes patient prone to recurrence at the site of field matching in case of cold spot or spinal cord toxicity in case of hotspot. A hotspot can lead to serious complications like radiation myelopathy which has been reported in the literature.^10,13,19,44

Nearly all published data of late toxicity after CSI includes neuro-cognitive decline, endocrinopathies, or growth retardation which are mainly inherent due to the treatment of target volume itself.⁴⁵ In contrast, better results have been reported in literatures regarding late toxicities of the organs outside the planning target volumes when using conformal techniques. It is of uttermost importance to spare the healthy orans like thyroid, heart, lungs, kidneys, larynx even for relatively low dose levels to most evident reported late toxicities of these organs.

Recent advancement for CSI is the use of tomotherapy machine. This approach overcomes challenges of field matching, junctions, and beam gaps encountered with conventional techniques, thereby reducing complexity in treatment planning and delivery.

We have evaluated the following parameters and compared it with different international studies for evaluation of our observed data.

Table 3: Comparison of PTV-Combine coverage with various studies

The low doses of radiation to the non-target body (Body-PTV), measured in terms of V5Gy (%), were 38.95% in our study compared to 22.9% in the study by Seravalli et al. Similarly, V1Gy (%) was 71.33% in our study versus 52.6% in theirs.¹⁸ Although our study demonstrates higher percentages of low doses (V5Gy and V1Gy) compared to Seravalli et al., these dose constraints are justifiable. This is because the patients enrolled in Seravalli et al.'s study were predominantly adult patients from 15 different institutes, whereas our study included pediatric patients with lower body surface area and larger target areas (whole vertebra as PTV) compared to adults (14 pediatric vs 10 adults).

Table 4: Comparison of Dmean (Gy) of various OARs

Table 5: Comparison of various dose constraints of OARs: Mean (SD)

The incidence of acute hematologic toxicity in our study was moderate, transient, and amenable to intervention through the administration of blood components or growth factors. Among our cohort of 24 patients, we observed Grade 3 leukopenia in 18.75% and Grade 3 thrombocytopenia in 6.25% of cases. This stands in stark contrast to the higher incidence rates, ranging from 50–90%, of Grade III or worse acute hematologic toxicity reported in most other studies of CSI. For comparison, Sugie et al. documented Grade 3 leukopenia in 92% and Grade 3 thrombocytopenia in 42% of their 12-patient cohort, with a median follow-up of 9 months.³⁹ Similarly, Lopez Guerra et al. reported a 58% incidence of Grade 3 hematologic toxicity among 10 patients, with a median follow-up of 40 months.⁴⁴ The predominant factor contributing to the elevated rates of acute hematologic toxicity in these studies was the administration of chemotherapy either prior to or concurrently with CSI, unlike our cohort, which predominantly received concurrent chemotherapy alone. It is well recognized that the inclusion of pre-irradiation or concurrent chemotherapy significantly heightens the risk of severe hematologic toxicity.

Helical tomotherapy can achieve planning with obviating field junctions. Highly conformal treatment can be delivered efficiently without need for discontinuous couch and gantry movements. Helical tomotherapy for CSI is a favourable mode of treatment delivery in terms of target coverage, dose homogeneity and conformality, and sparing of surrounding OARs. Tomotherapy increases cumulative radiation dose of predicted total volume of normal body and targeted tissue leading to acute adverse reactions in serum blood elements. While critical organ tolerances are often not exceeded with tomotherapy but relatively large normal tissue volume may encounter low radiation doses which we will try to further improvise in upcoming patients.

Significantly longer Beam on time raises concern about intrafraction motion and whole-body integral dose. A relatively larger number of monitor units associated with helical tomotherapy could result theoretically in a higher risk of secondary malignancies. Larger cohort and longer follow up will be needed to assess late toxicities.

Funding: The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Competing interests: The authors have no relevant financial or non-financial interests to disclose.

Author’s contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by S.M. and A.P. The first draft of the manuscript was written by S.M. and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Ethics approval: This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by the Ethics Committee of The Gujarat Cancer and Research Institute Institutional Review Committee.

Consent to participate: Informed written consent was obtained from all individual participants included in the study. Written informed consent was obtained from the parents.

Consent for Publication: The authors affirm that human research participants provided informed consent for publication. The authors certify that they have obtained appropriate patient consent form from all participants. In the form, the patients have given their consent for their images and other clinical information to be published in the journal. The patients understands that their name and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Availability of Data and Material: Not applicable

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No competing interests reported.

Table1.xlsx
Table 1: Patient characteristics, treatment details, and clinical outcomes of the study cohort
Table3.xlsx
Table 3: Comparison of PTV-Combine coverage with various studies
Table4.xlsx
Table 4: Comparison of Dmean (Gy) of various OARs
Table5.xlsx
Table 5: Comparison of various dose constraints of OARs: Mean (SD)

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Dosimetric Analysis and Acute Toxicity in Craniospinal Irradiation Using Helical Tomotherapy: Experience from a Tertiary Cancer Center

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