Gemma Busuttil
Crown Princess Mary Cancer Centre, Westmead
Radiation Therapist

Jennifer Chard
Radiation Oncologist
Crown Princess Mary Cancer Centre, Westmead

Emily Flower
Senior Radiation Oncology Medical Physicist
Crown Princess Mary Cancer Centre, Westmead

Edgar Estoesta
Radiation Oncology Medical Physicist
Crown Princess Mary Cancer Centre, Westmead

Vossco Nguyen
Radiation Therapist
Crown Princess Mary Cancer Centre, Westmead

Salman Zanjani
Radiation Therapist
Crown Princess Mary Cancer Centre, Westmead

Hang Nguyen
Radiation Therapist
Crown Princess Mary Cancer Centre, Westmead

Jessica Beech
Radiation Therapist
Crown Princess Mary Cancer Centre, Westmead

Joseph Bucci
Radiation Oncologist
St George Cancer Care Centre

Verity Ahern
Radiation Oncologist
Crown Princess Mary Cancer Centre, Westmead

Background and Purpose:

In order to avoid the significant morbidities of total cystectomy/prostatectomy and external beam radiotherapy, the Crown Princess Mary Cancer Centre (CPMCC) offers high dose rate (HDR) interstitial brachytherapy for paediatric patients diagnosed with rhabdomyosarcoma originating from the prostate and/or bladder neck region (PB RMS). Organs at risk (OARs) in close proximity to the treated volume including rectum, urethra and acetabular growth plates must be taken into account when optimising the brachytherapy plan. Limited evidence is currently available on radiation tolerances for paediatric OARs, especially for brachytherapy. This study aims to extrapolate OAR planning dose aims from existing evidence and evaluate their feasibility in the clinical setting.

Method:

OAR tolerance doses were compiled from adult brachytherapy, external beam radiotherapy and peadiatric radiotherapy recommendations and guidelines. Dose limits were reduced by 10% for prior systemic chemotherapy in keeping with Children’s Oncology Group protocols and converted to equivalent dose in 2Gy fractions (EQD2) for application to brachytherapy. All volumetric maximum dose values were reduced to D1cc to account for the smaller organ size in paediatrics. On this basis, a table of EQD2 planning dose aims for all pelvic OARs in PB RMS HDR brachytherapy was developed.

Three PB RMS patients were then treated in 2018 with HDR interstitial brachytherapy delivered over 5 fractions to a dose of 32.5Gy (45Gy EQD2) covering 95% of the clinical target volume (CTV). All OARs were contoured on the planning MRI and dose volume histograms generated. The plans were optimized and evaluated against the OAR planning dose aims while maintaining CTV coverage.

Results:

Across the three patients, the majority of OARs achieved doses reasonably close to or under the planning aim. Rectal D1cc EQD2 ranged from 29.1-49.6Gy for a planning aim of 41.2Gy. Uninvolved bladder D1cc achieved 31.4-49.2Gy for 41.2Gy and prostatic urethra D0.1cc received 78.2-116.2Gy for 108Gy. Acetabular absolute maximum doses ranged from 18-21.8Gy for 18Gy and visual inspection of the dose distribution on each plan ensured the growth plate received 18Gy or less.

Conclusion:

The OAR planning dose aims developed are reasonably achievable and feasible while maintaining CTV coverage when optimizing PB RMS HDR interstitial brachytherapy plans. It is acknowledged that these aims are based on limited evidence in the paediatric brachytherapy setting and long term late effects are yet to be seen.


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