Dosimetric and radiobiological comparison of helical tomotherapy, forward-planned intensity-modulated radiotherapy and two-phase conformal plans for radical radiotherapy treatment of head and neck squamous cell carcinomas.

OBJECTIVES: The usual radical radiotherapy treatment prescribed for head and neck squamous cell carcinoma (HNSCC) is 70 Gy (in 2 Gy per fraction equivalent) administered to the high-risk target volume (TV). This can be planned using either a forward-planned photon-electron junction technique (2P) or a single-phase (1P) forward-planned technique developed in-house. Alternatively, intensity-modulated radiotherapy (IMRT) techniques, including helical tomotherapy (HT), allow image-guided inversely planned treatments. This study was designed to compare these three planning techniques with regards to TV coverage and the dose received by organs at risk. METHODS: We compared the dose-volume histograms and conformity indices (CI) of the three planning processes in five patients with HNSCC. The tumour control probability (TCP), normal tissue complication probability (NTCP) and uncomplicated tumour control probability (UCP) were calculated for each of the 15 plans. In addition, we explored the radiobiological rationality of a dose-escalation strategy. RESULTS: The CI for the high-risk clinical TV (CTV1) in the 5 patients were 0.78, 0.76, 0.82, 0.72 and 0.81 when HT was used; 0.58, 0.56, 0.47, 0.35 and 0.60 for the single-phase forward-planned technique and 0.46, 0.36, 0.29, 0.22 and 0.49 for the two-phase technique. The TCP for CTV1 with HT were 79.2%, 85.2%, 81.1%, 83.0% and 53.0%; for single-phase forward-planned technique, 76.5%, 86.9%, 73.4%, 81.8% and 31.8% and for the two-phase technique, 38.2%, 86.2%, 42.7%, 0.0% and 3.4%. Dose escalation using HT confirmed the radiobiological advantage in terms of TCP. CONCLUSION: TCP for the single-phase plans was comparable to that of HT plans, whereas that for the two-phase technique was lower. Centres that cannot provide IMRT for the radical treatment of all patients could implement the single-phase technique as standard to attain comparable TCP. However, IMRT produced better UCP, thereby enabling the exploration of dose escalation.

Squamous cell carcinoma of the cervix: report of an unusual case of bicornuate bicollis uterus treated with bilateral intracavity brachytherapy.

We report a case of congenital abnormality of bicornuate bicollis uterus in a patient who developed FIGO (International Federation of Gynecology and Obstetrics) stage IIB invasive carcinoma of the cervix in 2006. She was managed with radical concurrent chemoradiotherapy using an external photon beam of 50 Gy in 25 fractions and a weekly infusion of cisplatin, followed by low dose rate intracavity brachytherapy of 18 Gy to Manchester point A over two fractions. Intra-uterine afterloading brachytherapy catheters were inserted into both uterine cavities. Treatment was well tolerated with manageable acute toxicities. Complete response was achieved with therapy. The patient remains well on follow up with no clinical evidence of disease recurrence two years after initial treatment.

The IPEM code of practice for determination of the reference air kerma rate for HDR (192)Ir brachytherapy sources based on the NPL air kerma standard.

This paper contains the recommendations of the high dose rate (HDR) brachytherapy working party of the UK Institute of Physics and Engineering in Medicine (IPEM). The recommendations consist of a Code of Practice (COP) for the UK for measuring the reference air kerma rate (RAKR) of HDR (192)Ir brachytherapy sources. In 2004, the National Physical Laboratory (NPL) commissioned a primary standard for the realization of RAKR of HDR (192)Ir brachytherapy sources. This has meant that it is now possible to calibrate ionization chambers directly traceable to an air kerma standard using an (192)Ir source (Sander and Nutbrown 2006 NPL Report DQL-RD 004 (Teddington: NPL) http://publications.npl.co.uk). In order to use the source specification in terms of either RAKR, Κ(R) (ICRU 1985 ICRU Report No 38 (Washington, DC: ICRU); ICRU 1997 ICRU Report No 58 (Bethesda, MD: ICRU)), or air kerma strength, S(K) (Nath et al 1995 Med. Phys. 22 209-34), it has been necessary to develop algorithms that can calculate the dose at any point around brachytherapy sources within the patient tissues. The AAPM TG-43 protocol (Nath et al 1995 Med. Phys. 22 209-34) and the 2004 update TG-43U1 (Rivard et al 2004 Med. Phys. 31 633-74) have been developed more fully than any other protocol and are widely used in commercial treatment planning systems. Since the TG-43 formalism uses the quantity air kerma strength, whereas this COP uses RAKR, a unit conversion from RAKR to air kerma strength was included in the appendix to this COP. It is recommended that the measured RAKR determined with a calibrated well chamber traceable to the NPL (192)Ir primary standard is used in the treatment planning system. The measurement uncertainty in the source calibration based on the system described in this COP has been reduced considerably compared to other methods based on interpolation techniques.

Technical note: check software for use with trans-rectal ultrasound guided I-125 seed prostate implants.

In-house software has been developed to enable the dose distribution of a commercial treatment planning system to be verified prior to permanent trans-rectal ultrasound guided I-125 seed implantation of the prostate. Printouts enabling the required loading pattern to be communicated to other staff groups are also generated. This software can be obtained via the authors and the AAPM software exchange (URL: http://aapm.org/medphys/resources/software/index.htm).

The effect of secondary electron spread on the penumbra in high energy photon beam therapy.

In photon beam therapy, the geometric penumbra width is determined by the source-size and the collimator design. The width of the physical (i.e. dosimetric) penumbra involves an additional contribution from secondary electron spread. Using a suitably defined measure of penumbra width, the separate widths due to photons and secondary electrons are additive in quadrature. Secondary electron spread functions were measured using photographic dosimetry for 60Co gamma rays and for 4, 8 and 16 MV X rays. The results suggest that, under typical treatment conditions, secondary electron spread may be the predominant contributor to the penumbra at effective generating voltages exceeding 10 MV. The implications for selection of beam energy in precision small-field radiotherapy are discussed briefly.